WO1995030808A1

WO1995030808A1 - Building elements

Info

Publication number: WO1995030808A1
Application number: PCT/GB1995/001031
Authority: WO
Inventors: William John Bernard Ollis; William Henry Ollis
Original assignee: William John Bernard Ollis; William Henry Ollis
Priority date: 1994-05-05
Filing date: 1995-05-05
Publication date: 1995-11-16
Also published as: AU2450495A; EP0758423A1; GB9408884D0

Abstract

A prefabricated composite beam formed from two strips of timber (1, 2) are joined by metal connectors (4), preferably constituted by a rod having helical fins (15), retaining a block (18) of mineral wool in compression between the timber strips. The block of mineral wool is somewhat elastic and exerts outwardly directed forces shown by the arrow (18) upon the faces of the strips (1, 2) with which it is in contact. The mineral wool provides shear resistance and minimises relative movement upon a shearing force, being applied between the timber strips, for instance a bending force imposed upon the beam. The metal connectors (4) may be positioned at an angle to the longitudinal axis of the beam, to provide additional shear resistance. Several beams may be joined together by a flexible web (23) which allows the assembly to be compacted for transport and storage and extended before use so that the flexible web is stretched out to a suitable distance for use as rafters, suspended ceiling beams or floors. A kit including mineral fibre insulation for inserting between the beams is described.

Description

BUILDING ELEMENTS This invention involves the fabrication of elongate elements which are girder type beams having parallel longitudinal timber strips spaced apart with connections along their length provided by metal rods, which are useful to form roofs, walls and suspended floors. The beams preferably include insulating material which provides resistance to shear between the two timber strips. The beams are preferably linked together by a flexible web, and such assemblies may be provided in kit form with insulating material suitable for filling space between the beams when the flexible web is stretched out.

Conventional built-up timber girder-beams or joists are normally made with continuous plywood webs connecting wooden flange pieces or with substantial triangulated lattice members connecting top and bottom timber strips. It is difficult effectively to stuff insulation between the strips, in the latter case, and in the former case there can be a considerable amount of cold-bridging and sound transmission through the plywood webs. Cold-bridging can lead to undesirable interior condensation.

Composite elements including insulating material, and which are in the form of elongate elements, are known. For instance in DE-A-3017332 a girder type beam comprises two spaced apart elongate strips with insulation therebetween and with continuous slat provided across each side of the beam. In US-A-4224774, a composite load bearing structural element, which is a stud wall, includes wooden strips with a core of mineral fibre positioned between them and adhesively secured to adjacent faces of the mineral strips forming the stud elements. The stud elements are attached in the normal fashion to the wall panel. The mineral fibre provides inadequate load bearing capacity. In SE-A-8008184-7 two longitudinal strips have longitudinal slots along each edge, for receiving perpendicularly arranged panels. A block of insulation is incorporated into the volume between the strips and panels before the panels are slotted into place. In DK-A-151719, two elongate metal plates having flanges along their longitudinal edges have a block of insulation there between and are retained in compression by straps passing circumferentially around the periphery of the composite element. In one embodiment central flanges running along the inwardly directed faces of the metal strips provide a slot for reception of a perpendicular continuous elongate plate member to form an I-beam construction. All these constructions result in cold bridging through the continuous plates.

In DK-A-888-79 a longitudinally extending zigzag strip is used to support a strip above a layer of concrete. The peaks of the zigzag plate are attached to the strip and the troughs are attached to the concrete.

In DK 150650 perpendicularly arranged metal rods join two vertical strips which have insulating material between. In NO 159301 metal bolts are used to join two parallel spaced members. The bolts are cranked, that is the ends are straight and have parallel, but spaced apart axes, and are joined together by a diagonally arranged straight central portion. The straight ends pass through bores formed in the strips substantially perpendicular to the face of the strip. The bolts are retained by nuts bearing against the outer face of the strip with washers bearing against the inner face of the strip or by friction in the bore. A pair of adjacent cranked bolts join the two strips such that the cranked central portions are angled in equal and opposite directions to the perpendicular to the plane bisecting the two strips. In fabricating the assembly it is clear that the rods must, in a first step, all be connected to one element by passage of the first ends through its inward face. When all the rods have been located in the first strip, the second strip can be located over the free ends of the bolts, the free ends thus passing through the inner face of the second strip. Fabrication of the strips is relatively complicated and location of all the second ends in the second strip is difficult and apparently requires provision of a separate positioning member joining the ends to one another. It would not be possible to incorporate single blocks of insulation in the product.

Helically flanged rods used as reinforcements and ties are described in EP-A-0171250. These may be used as ties between inner and outer walls of the cavity wall. In one embodiment, inner and outer layers of a timber wall, wherein insulation is provided between the layers, are tied together. The sharpened end of the rod is driven directly through from the outer face of one layer through the insulation into the inner layer on site, that is during construction of the building. The insulation does not provide any load bearing capacity.

A prefabricated elongate composite element according to the invention is formed from a pair of spaced apart timber strips which are retained in parallel orientation to one another by several metal rods, each of which has two straight coaxial ends, one end passing at least partially through one strip and the other end passing at least partially through the other strip, such that relative movement of rods and timber strips is resisted upon imposition of axially (with respect to the rod end) directed forces, in each direction, the element being provided with means to resist longitudinal shear forces applied between the strips.

In the invention each rod is preferably substantially straight along its entire length. It is preferred for the rod to have a constant cross-section. It may, however, be desirable under some circumstances for the rod to have a shoulder inward from each end, which can itself bear against the inner face of the timber strip or may retain a washer which bears against that face. Preferably any shoulder is small enough so that it does not prevent passage of the rod through a block of insulating material in an axial direction.

In the invention the resistance to shear prevents or minimises relative movement between the two timber strips in a direction parallel with their lengths. This provides for good mechanical strength characteristics for the composite elements, comparable with I-beams and superior to a similar dimensioned beam formed entirely of timber of equivalent or even better quality. The invention allows the use of small pieces of wood, economically cut from relatively small tree trunks, such as replantable soft wood forests provide.

In one embodiment of the present invention the means to resist shear forces comprises the attachment at at least one position, suitably at or adjacent to an end of the element, and preferably at both ends of the element of a shear plate which is joined to each timber strip at at least two positions based longitudinally with respect to one another. In this embodiment, there is preferably a shear plate provided at both ends of the elements. The shear plate must be joined to each of the timber strips, at at least two positions which are spaced apart by a suitable distance such as, for instance, at least 50 mm, preferably at least 100 mm, for instance up to 300 mm and preferably in the range 150 to 200 mm. A shear plate may be attached to the outer edges of the timber strips, and preferably on opposing sides thereof or, alternatively, may be retained in a slot formed in the inner face of each strip. In this embodiment of the invention the metal rods may be oriented such that the ends have their axes oriented substantially perpendicularly to the plane bisecting the strips.

In a preferred second embodiment of the invention, which may be used in combination with the first embodiment, the means to resist shear forces comprises the orientation of at least two of the metal rods such that their axes are at mutually opposed angles to the said perpendicular, with the axes of the rod ends lying substantially parallel to the plane joining the longitudinal axes of the timber strips. The provision of angled rods allows the resolution of shear forces down the axes of the rods and into the opposite strip. In this embodiment, it is preferred that several metal rods are angled in one direction to the said perpendicular and several metal rods are angled in the other direction, the number of rods angled in one direction preferably being the same as the number angled in the other direction, and the rods preferably being paired such that for each rod angled in one direction there is a corresponding rod angled in the other direction, the angles to the said perpendicular being, for each pair, substantially equal to one another.

In this preferred embodiment, the rods may be arranged such that alternate rods are angled in the same direction with intermediate rods angled in the opposite direction. Alternatively the rods may be arranged such that all rods located between the centre of the element and one end are angled in one direction and all the rods located between the centre and the other end are angled in the opposite direction. It is preferred, for optimal mechanical characteristics, for the arrangement to be substantially symmetrical around the mid point of the element.

It is preferred that the element of the invention, whether the first or the second embodiment mentioned above, has a block of heat and/or sound insulating and/or fire resistant material located between the two timber strips and preferably substantially completely filling the space between the strips, the block preferably being held in compression by inwardly directed forces exerted by the strips on the block. The material preferably comprises mineral wool, most preferably rockwool.

Where the block of material is held between the timber strips in compression, that is such that each of the timber strips exert inwardly directed forces upon the face of the block with which it is in contact, whilst the elasticity of the block of material results in it exerting an outwardly directed force on the inner faces of the timber strips, the block itself may contribute to the provision of means to resist shear forces. For this purpose it is preferred for the block to be formed of mineral wool, preferably rockwool having a density of at least 100 kg/m . The density is at least 150 kg/m for instance about 180 kg/m . The material is of a type which provides high compression strength, for instance such that, to compress the material by 10% the force necessary is more than 50 kPa, for instance about 100 kPa.

Where mineral wool is retained between the timber strips, it is preferred for each metal rod to pass entirely through the block of mineral wool.

The metal rods can be described as "stand-off" metal connectors and are suitable for carrying both compressive and tensile forces across the gap between the two strips. Such rods may conveniently be helically flanged rods as described in EP-A-0,172,250. The helical rods generally have constant cross-section and comprise a longitudinal core with radial fins which extend from the core and helically along the length of the rod. The shear plates are preferably provided at the ends of the element, since the resultants of longitudinal shearing forces induced by bending of the element tend to become more concentrated at the ends of the element. The plates can also be used to provide connecting means for other components of the building, and thus transfer the collected loadings onto the bearing surfaces of a supporting structure or else provide connector means for joining main structural members at angles to one another, for instance when used to make portal frames. Where the element includes shear plates, the rods may be spaced apart by relatively large distances, for instance up to 500 mm or more. Where no shear plates are included to provide resistance to shear, the metal rods tend to be spaced more closely, for instance in the range 100 to 500 mm, preferably between 200 and 400 mm, suitably around 300 mm apart. The timber strips are preferably substantially equal in size to one another. The cross-sectional area of each strip is generally in the range 10 to 100 cm , preferably in the range 20 to 75 cm , for instance suitably around 35- 40cm . The strip may be square or may be rectangular, with the longer side of the rectangle being in the range 1.2 to 3 times the length of the shorter side, conveniently around 1.5 times.

The space between the two strips may be in the range 30 to 200 mm, preferably in the range 50 to 150 mm, for instance around 100 mm. When insulation material is compressed between the two timber strips, it is generally compressed by 1 to 10% of its thickness. It is found convenient for the pressure applied to the strips to compress the mineral wool to be in the range 20 to 200 kPa, preferably in the range 50 to 150 kPa, for instance around 100 kPa.

The composite element according to the invention is of use in various load bearing capacities in buildings. Most suitably it can be used as rafters or as suspended ceilings, as well as for use in walls and floors. The length of the element depends upon its ultimate use and is typically in the range 1.5 to 15 m, for instance in the range 4 to 10 m. A suitable standard length is around 5.5 m. Where the metal rods are all substantially straight and are to be oriented substantially perpendicular to the plane bisecting the timber strips, the element may be fabricated by inserting the first ends of all of the metal rods into the first timber strip through the face ultimately to form the inner face of the strip, and then to insert the second ends of the rods simultaneously into the second timber strip through its inner face. Where the element is fabricated in this order, and where the element includes a block of heat and/or sound insulating and/or fire resistant material, the block may be positioned against the first timber strip prior to insertion of any of the metal rods, though preferably, after all the metal rods have been inserted into the first timber strip, the second ends of the rods are pushed simultaneously through the block of material, out through the other side and are subsequently inserted into the second timber strip. For this method of fabrication it is preferred for the rods to have a constant cross-section, or for any shoulder near the second ends to be small enough to allow passage of the rods entirely through the block.

In a preferred method of fabricating a composite element, which is applicable to any embodiment where the rods are substantially straight and are such that the rods each have substantially constant cross-section with the rods being oriented either perpendicular to the plane bisecting the strips or at an angle thereto, the first end of each rod is inserted through the second strip by passing through its outer face and out through its inner face, across the space between the strips and then into the first strip through its inner face. The pins are generally inserted sequentially. In this embodiment, where a block of insulating material is included between the strips of timber, the block must be located between the timber strips prior to insertion of the first metal rod. Where the insulation is formed of mineral wool, it is preferred for a co pressive force to be applied between the timber strips to compress the mineral wool by an amount of between 1 and

10% of its total thickness. Since the mineral wool has some elasticity, the metal rods retain some compressive energy in the block of mineral wool upon being inserted.

Where the elements are to be used in a building such that they are positioned parallel to one another and are spaced apart, it has been found convenient to provide the elements in the form of an assembly in which they are attached to one another by a web member formed of flexible but substantially inelastic material which is attached to each of the elements and in which each element is connected at at least two positions along its length to the at least one web member, such that when the flexible web member is stretched out, the elements are parallel to one another and spaced apart by equal distances.

The idea behind the assembly may be used with other types of timber beams, which are not necessarily composed of elements according to the first aspect of this invention. Such an assembly forms a second aspect of the present invention. The assembly may be stored and transported to a building site in a closed concertina conformation in which the elements or beams are as close together as possible, to take up a minimal total space. At the point of use the assembly is opened out so that the flexible web is stretched out to its fullest extent. The beams or elongate elements are, in this conformation, spaced apart by appropriate distances for incorporating into the building. Suitable standard separations are in the range 300 to 1500 mm, preferably about 600, 900 or 1200 mm. If the web member or web members which are joined at one position to one beam are joined to the adjacent beam at longitudinally spaced positions, then the web can minimise relative movement of the beams or elements when in position.

The flexible web may be formed of several lengths of material, each length being joined to each elongate element. Preferably a series of such lengths would be arranged parallel to one another, optionally perpendicular to the elongate elements or angled to the perpendicular. Preferably a second length of material or series of lengths is joined to each elongate element whilst crossing the first series of elements. In this way relative movement of the beams when the assembly is opened out is minimised. Such lengths may be formed of conventional strapping material, for instance as is used in packaging operations. Preferably the flexible web comprises a sheet member extending substantially from one set of ends of the elongate elements to the other set of ends and from one side of the assembly to the other. It is preferred for a sheet to be moisture and water resistant and it may be wholly moisture and vapour impermeable. Preferably, however, it is permeable at least to water vapour, so that the opened out assembly, when positioned in the building, is breathable which avoids moisture build-up within the building which would otherwise result in condensation. It may, for instance, be formed of netting or of a web of non- woven or woven material. Suitable vapour-impermeable sheet materials are, for instance thermoplastic polymer films such as of polyethylene or polypropylene or building paper (a bitumen coated product) .

According to a further aspect of this invention, there is provided a kit comprising the assembly as defined above and slabs of sound and/or heat insulating and/or fire resistant material of suitable dimensions for positioning within the spaces between the elongate elements when the assembly is in its opened out form. The slab of material is preferably formed of mineral wool, most preferably of rockwool. A suitable density for the rockwool is in the range 25-50 kg/m , most preferably in the range 30 to 45 kg/m³.

In a most preferred aspect of the kit, the slab of insulating material is elongate and has a width slightly greater than the distance between the elongate elements in the opened out conformation of the assembly. The slab is provided with compressibility such that it can be widthways compressed, preferably elastically, to fit between the elements when the assembly is in position in the building. Such elastically compressible slabs and their production is described in EP-A-0436681 and DE-A-3203622. The heat and/or sound insulating and/or fire resistant materials used in both the elongate elements and the kit of the invention are preferably moisture resistant, whilst being vapour permeable. This again provides breathability and minimises interstitial condensation, that is condensation within the insulating material itself. The materials preferably have fire resistance meeting the requirements of British Standard BS 476 part 4.

According to a further aspect of the invention a prefabricated structural roof element comprises two elongate rafters arranged in the same vertical plane and sloping upwards to meet at an apex, the elements being joined to one another at or adjacent the upper ends by an apex support member, and each rafter having wall attachment means at or adjacent the lower ends of the respective rafters, wherein the roof element has no other member joining the rafters to one another to resolve horizontal forces. The roof element is used in the construction of a building in which a main beam provides vertical support to the rafter at the apex thereof and comprises vertical wall members joined to the wall attachment means to provide vertical support at each of the lower rafter ends, and in this construction no additional member is used to join the rafters to one another to resolve horizontal forces exerted by the rafters.

The roof element may comprise part of a portal frame, in which a wall beam member is joined to the wall attachment means at at least one and preferably both lower rafter ends.

In general the type of construction described is designed to maximize cost-benefits from efficient production methods in sawmills, fabrication workshops and on building sites as well as in connection with transport costs. It is also designed to provide improved building performances in relation to thermal conservation, comfort, avoidance of internal condensation and noise transmission.

The type of construction described will have special and additional benefits in relation to the economical use of enclosed space in dwellings designed to provide habitable rooms partially or wholly within pitched roof structures. If such girder-beams are used as rafters supported at roof ridges, by main beams, central purlins or load-bearing spine partitions, the use of horizontal ties between the feet of the rafters to stop the roof spreading and sagging can be eliminated so that the whole of the internal roof space can be left unobstructed and readily used.

The elongate element of the present invention has been found to have greater stiffness than an assembly of two equivalent pieces of timber, not joined together and higher than the stiffness of a beam having the same dimensions, but formed of solid seasoned timber. The elements provide an economical component with excellent strength characteristics and excellent heat, sound insulating and fire resistance properties of great utility in various aspects of construction.

The invention is further illustrated in the accompanying drawings in which:

Figure 1 shows a side elevation of a longitudinal element according to the first embodiment of the first aspect of the present invention;

Figure 2 shows a side elevation of an element according to the second embodiment of the first aspect of the invention; Figure 3 shows a side elevation of another element according to the second embodiment of the first aspect of the invention;

Figure 4 shows an isometric view of a shear plate to be used in the first aspect of the invention; Figure 5 shows an isometric view of a second type of shear plate used in the first aspect of the invention;

Figure 6 shows a section, perpendicular to the axis, through an elongate element according to the first aspect of the invention; Figure 7 shows a section perpendicular to the axis through an embodiment of the third embodiment of the first aspect of the invention; i3

Figure 8 is a perspective view of a pitched roof formed from elongate elements of the invention;

Figure 9 is a section through an assembly according to the present invention in relatively compact form for storage or transportation;

Figure 10 is a cross-section through the assembly shown in Figure 9 with the assembly partially opened out; Figure 11 shows a diagrammatical representation of a roof element according to the prior.art; Figure 12 shows a side elevation of a corner joint between a roof element of the third embodiment of the first aspect of the present invention and a wall element;

Figure 13 shows a diagrammatic representation of the internal space under a roof constructed from the roof elements of the present invention; and

Figure 14 shows in side elevation a portion of a portal frame according to the present invention.

Figure 1 shows a form of elongate element where the metal rods are all at right angles to the timber strips and the longitudinal shearing forces, due to bending of the element, are resolved/resisted almost entirely at the ends.

Figure 2 shows an alternative form where the rods are slightly angled to different degrees, towards the resultant lines of force, when compressive forces between the timber strips interact with shearing/sliding forces acting longitudinally as between the chords (timber strips) . In this way the shear forces to be resolved at the ends can be somewhat reduced.

Figure 3 shows another form where individual rods are set at alternate angles some distance apart from each other. In this way the longitudinal shear forces to be dealt with at the ends may be further reduced.

Figures 4 and 5 shows two alternative forms of end shear plates made of plywood or similar material. Figure 6 shows one example of "stand-off" metal connecting rod, with points and seating shoulders at both ends, which could be used in connection with presses. Figure 7 illustrates the incorporation of pre- compressed semi-structural fibrous insulation strips between top and bottom chords of a spanning member and the use of helically flanged rods. Figure 8 indicates how an effectively continuous thick layer of insulation material can be incorporated between spaced apart upper and lower timber members of a pitched roof structure.

Figures 9 and 10 show how prefabricated structural members may be linked together by folded membranes or netting, transported compactly and spread out on site, with compressible insulation packed into the spaces between them, so that they end up correctly positioned at required centres. Figure 13 shows how a pitched roof may be formed with a ridge beam or other central structural support so that there is no outward thrust at the outer wall supports. In this way the whole roof space can be inhabited. The drawing shows how the tops of the outer walls may consequently be lowered, without adversely affecting headroom; compared with the situation in a conventional trussed-rafter roof, with tie members at ceiling level and obstructed attic space, as shown in Figure 11.

Figure 12 shows how local end shear plates, of plywood or similar material, may be used to connect up with a similar timber-framed wall construction set at an angle and also made in accordance with the invention. Stiff knee-type joints so formed between the elements can result in further reductions of tendencies for the roof members to deflect; this usually being the critical structural design consideration in this building context.

In Figure 1, a strip of timber 1 is fixed to an opposite strip of timber 2, which is some distance away from it, via a space 3, by means of connecting pins 4, which are spaced at intervals along the lengths and are set at right angles to the strips. At the ends 5, local shear plates 6 are fixed to resist concentrated shear forces. The assembled component constitutes a spanning structural member. In Figure 2, the same components and features are present except that the connecting pins 4 are set at angles slightly and progressively diverging from 90 degrees, from the centre of the structural member outwards. In Figure 3, the same components and feature as those in Figures 1 & 2 are present, except that the connecting pins are set at alternating angles along the lengths.

In Figure 4, an end shear plate 7 of timber is set in mortice slots 8 and transfixed with metal pins or nails 9 or wooden dowels 10, for example. In Figure 5, a pair of end shear plates 11 made of plywood or similar material are fixed to the sides, preferably with glue and nails 9.

In Figure 6, a connecting pin 4 has at each end a spike 12 and shoulder seating 13. Such connectors may be driven into one strip of timber 2 and the other strip 1 may be pressed onto the opposite of such connectors, in the direction of the broad arrows 14. The shoulder seatings 13 may consist of slip-on washers bearing on much smaller metal projections of the pins.

In Figure 7, a connecting pin 4 with helical fins 15 and a sharp point 16 is driven right through a strip of timber 1 and onwards through a strip of mineral wool, fibrous insulation material 17 which is being pre-compressed by loads applied to the top strip at positions indicated by the broad arrows 14 which causes compressive reactions as indicated by the dotted broad arrows 18, which will, in use, enhance frictional resistance to longitudinal shear/slip. Having penetrated through the compressed insulation, the connecting pin then penetrates into the opposite timber strip 2 holding all the composited components together with the desired prestress securely built in.

In Figure 8, part of a composite elongate element 19, in this case acting as a roof rafter, is shown in the foreground with insulation material, which may or may not be precompressed, shown fully penetrating the web space 20 between the upper and lower timber strips. The top of the upper timber strip 1, together with the tops 21 of the rafters shown beyond stand above the top surface of the thick and effectively continuous layer of insulation extending between 22 and, in effect, through the web spaces of the composite rafters. The tops 21 can therefore have a waterproof membrane (eg roofing felt) draped over and nailed to them and can have tile or slate battens nailed to them in the conventional way of completing a roof. It is preferred for the felt to be a moisture vapour permeable type felt.

In Figure 9, four composite elongate elements 19, to serve as rafters, are joined together by a foldable web 23, nailed to their undersides through thin timber battens 24 and they can conveniently be strapped together (by means not shown) compactly for economical transportation.

In Figure 10, is shown a similar linked set of structural roof members as though being spread out in a building under construction. The flexible web, if continuous and moisture resistant can act as a useful weather shield during construction when the assembly is used as part of a roof or ceiling construction, in that it may shield workers and the internal space from rain. It is useful in providing safely, preventing tools or roofers falling through. In the middle of the drawing a slab of low density fibrous insulation material 25, cut (in situ or supplied as such) to a width slightly greater than the space available for it to be pushed into, is shown being pressed into such a space in the direction of the broad arrow 44. To the right of the drawing, such a slab is shown fully pressed in, so that its underside rests on the membrane 23, now stretched to its dimensional limits and being held there in its final required position by the compressive pressure applied in the directions indicated by the broad dotted arrows 18. Since it is substantially inelastic, that is does not stretch significantly upon imposition of the forces likely to be encountered during use, the web provides a useful means for ensuring proper spacing of the beams.

In Figure 13, the tops of the side walls 26 can be somewhat lower than the top of the head of a tall person 27, without reducing the effective usable floor space. The higher ends 28 of the sloping structural roof members are in one plane and can be linked together and they are supported by a ridge beam 29. The lower ends are cut to provide horizontal bearing surfaces to rest on the tops of the outer walls 26. It will be appreciated that the overall resultant loadings onto the ridge beam and the tops of the walls will all be in a vertical direction as indicated by the solid arrowheads 30. There is therefore be no need for a structural tie member between the end bearings at 26, which would in this type of building context severely obstruct available headroom. The thick layer of thermal insulation material 25 is located in the planes of the roof slopes.

In Figure 11, included as a contrasting comparison, is shown a roof constructed with trussed rafters 31, which are currently used widely in the UK and North America. In this case all the downward loadings are, in the ends, converted into compression thrusts down the upper sloping truss members, in the directions indicated by the solid arrowheads 30. It therefore becomes essential to have horizontal tie members at 32. This means that the whole of the roof structure and the tops of the outer walls 26 have to be comfortably above the top of the head of a tall person. In such a roof thermal insulation 25 is normally laid between and/or over ceiling tie members 32. It can be seen that, when overall thicknesses of insulation required exceed depths of tie members 32, it becomes impracticable to lay such insulation without interruption and right through to the eaves location, where trussed rafters 31 rest on supporting walls 26, because of restricted headroom. In Figure 12, pairs of end shear plates 11, made of plywood, are given an extended shape to provide an open pocket into which is inserted the top of a timber-framed wall component 33, which can be securely fixed into this by means of coach bolts 34 or similar timber connectors.

In Figure 14, it can be appreciated that structurally stiff joints between wall and roof elements at 35 and preferably also at 36 will result in mid-span deflections at 37 significantly less than otherwise.

Claims

1. A prefabricated elongate composite element formed from a pair of spaced apart timber strips which are retained in parallel orientation to one another by several metal rods, each of which has two straight coaxial ends, one end passing at least partially through one strip and the other end passing at least partially through the other strip, such that relative movement of rods and timber strips is resisted upon imposition of axially (with respect to the rod end) directed forces, in each direction, the element being provided with means to resist longitudinal shear forces applied between the strips.

2. An element according to claim 1 in. which each rod is substantially straight along its length, preferably having a substantially constant cross-section.

3. A composite element according to claim 1 or claim 2 in which the means to resist shear forces comprises the attachment at at least one position, suitably at or adjacent to an end of the element, and preferably at both ends of the element of a shear plate which is joined to each timber strip at at least two positions based longitudinally with respect to one another.

4. A composite element according to claim 3 in which the metal rods are oriented such that the ends have their longitudinal axes substantially perpendicular to the plane bisecting the strips.

5. A composite element according to any of claims 1-3 in which the means to resist shear forces comprises the orientation of at least two of the metal rods such that their axes are at mutually opposed angles to the said perpendicular, with the axes of the rod ends lying substantially parallel to the plane joining the longitudinal axis of the timber strips.

6. A composite element according to claim 5 in which several metal rods are angled in one direction to the said perpendicular and several metal rods are angled in the direction, the number of rods angled in one direction preferably being the same as the number angled in the other direction, and the rods preferably being paired such that for each rod angled in one direction there is a corresponding rod angled in the other direction, the angles to the said perpendicular being, for each pair, substantially equal to one another.

7. A composite element according to any preceding claim which has a block of heat and/or sound insulating and/or fire resistant material located between the two timber strips and preferably substantially completely filling the space between the strips, the block preferably being held in compression by inwardly directed forces exerted by the strips on the block.

8. A composite element according to claim 7 in which the said material comprises mineral wool, preferably rockwool.

9. A composite element according to any preceding claim in which the means to resist shear forces comprises the provision of an elastic block of mineral wool retained in compression between the timber strips.

10. A composite element according to claim 8 or claim 9 in which the mineral wool block is formed of rockwool having a density of at least 100 kg/m , preferably at least 150 kg/m , more preferably about 180 kg/m .

11. A composite element according to any of claims 8-10 in which each metal rod passes entirely through the block of mineral wool.

12. A composite element according to any preceding claim in which the metal rods each comprise a straight, constant cross-section rod having radially directed fins extending from the core and helically along the rod.

13. An assembly of composite elements according to any preceding claim arranged side by side and joined to one another by at least one flexible substantially inelastic web member which is fixed to each of the elements and in which each element is connected at at least two positions along its length to the at least one web member, such that when the flexible web member is stretched out, the elements are parallel to one another, and spaced apart by an equal distances.

14. A method of fabricating a composite elongate element according to claim 1 in which each metal rod is sequentially inserted into one of the timber strips and then into the other timber strip.

15. A method according to claim 14 for fabricating a composite element according to claim 4 in which the first ends of all the metal rods are inserted into the first timber strip through the inner face and the second ends of the rods are then simultaneously inserted in the second timber strip through its inner face.

16. A method according to claim 15 in which the element comprises a block of heat and/or sound insulating and/or fire resistant material located between the two timber strips, and in which, after the first ends of the rods have all been inserted into the first timber strip, the second ends are inserted through the block of material prior to being inserted in the second strip.

17. A method according to claim 14 in which the rods are straight throughout their length and in which the rod has a substantially constant cross-section, and in which the first end of each rod is inserted through the second strip by passing in through the outer face and out through the inner face of the second strip, across the space between the strips and then into the first strip through its inner face.

18. A method according to claim 17 in which the space between the strips is, before insertion of the first rod, filled with a block of heat and/or sound insulating and/or heat resistant material.

19. A method according to claim 18 in which the block is held under compression by inwardly directed forces exerted by the two wooden strips on the block, preferably at a pressure of at least 20 kPa, more preferably in the range 50 to 200 kPa.

20. A prefabricated assembly formed from several elongate beams arranged side by side and joined to one another by at least one flexible substantially inelastic web member which is fixed to each of the beams and in which each beam is connected at at least two positions along its length to the at least one web member, such that when the flexible web is stretched out, the beams are parallel to one another and spaced apart by equal distances.

21. An assembly according to claim 20 in which at at least one point on each beam which is fixed to a web member, the or each said web member so fixed is also fixed at two longitudinally spaced apart positions on each adjacent beam.

22. An assembly according to claim 21 in which the flexible web comprises a first series of individual strips positioned parallel to one another and each joined to each of the beams, and comprising a second series comprising at least one strip crossing at least some of the first series of strips and joined to all of the beams.

23. An assembly according to claim 20 or claim 21, in which the web is sheet form, preferably being vapour permeable.

24. A kit comprising an assembly according to any of claims 13 and 20 to 23 and slabs of fire resistant and/or heat and/or sound insulating material, the slabs being dimensioned to fit between adjacent beams when the web is stretched out.

25. A kit according to claim 24 in which the slab of material is formed of rockwool, preferably being wider, in relaxed form, than the space between the beams, and being widthways elastically compressible so as to fit between the beams.

26. A kit according to claim 24 or claim 25 in which the beam is an elongate element according to any claims 1 to 13.

27. A prefabricated structural roof element comprising two elongate rafters arranged in the same vertical plane and sloping upwards to meet at an apex, joined to one another at or adjacent to the upper ends by apex support member and having wall attachment means at or adjacent the lower end of each rafter and having no other member joining the rafters to one another to resolve horizontal forces.

28. A roof element according to claim 27 comprising a wall beam member joined to wall attachment means at at least one, and preferably both lower rafter ends.

29. A roof element according to claim 27 or claim 28 in which each rafter comprises an element according to any of claims 1 to 12.

30. Use of a roof element according to any of claims 27 to 29 in the construction of a building comprising a main beam which provides vertical support to each rafter at the apex thereof and vertical wall members joined to wall attachment means to provide vertical support at each of the lower rafter ends and in which no additional member is used to join the rafters to one another to resist outward relative movement of the rafters.