IE20160092A1

IE20160092A1 - Roof foil

Info

Publication number: IE20160092A1
Application number: IE20160092A
Authority: IE
Inventors: Reed Paul
Original assignee: Marley Eternit Ltd
Priority date: 2015-04-13
Filing date: 2016-04-01
Publication date: 2016-10-19
Also published as: GB2537368A; IE86781B1; GB201506227D0; GB2539076A; GB2539076B

Abstract

A method for covering a roof structure comprising at least the successive steps of: providing a roof structure comprising at least a first and a second rafter; Providing a foil comprising at least one set of at least two adjacent position indication means being spaced by a distance S of at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm in between the rafters fixing said foil on said first rafter through said first of position indication means, spreading said foil until a second position indication means appears, fixing said foil on said second rafter, thereby creating a foil hanging in between said rafters. <Figure 6>

Description

Method for covering a roof structure with foil Technical field of the invention The present invention relates to a method for covering a roof structure 5 with a foil. Particularly, the invention relates to a method for covering a roof structure with a foil, said roof structure comprising no boarding on the top of the rafters and to a foil comprising position indication means.

Background of the invention Whereas in some areas typically panel material is used as a sub-roof 10 under tiles, in other areas use is made of a foil to provide for weather protection. More particularly, a foil is positioned underneath roof tiles, also known as a ‘roof underlay’, the foil being capable of making roofs impermeable to water and permeable or impermeable to water vapour. Typically such foils also should permit a good ventilation of the structure in order to avoid the 15 occurrence of trapped moisture from condensation or small amounts of leakage through the outer covering which may give rise to rotting of elements, e.g. wooden elements, of the roof. This is more particularly the case of a cold roof, and even more particularly when combined with vapour permeable underlays. A “cold” roof construction is one where the insulation is laid below 20 the roof deck and the associated joints (i.e. over a normal flat ceiling). This then means that everything above the insulation, such as the rafters, is colder than the living place. Warm, moisture-loaded air can permeate up through the ceiling. When it reaches the cold roof space (i.e. the loft) or more particularly the batten space immediately underneath the outer covering of tiles or slates, 25 the change in the temperature can mean that condensation will form on timbers, underlay, etc. A roof space ventilation is used to remove this damp air and thus prevent condensation forming. Typically a cold roof is a roof comprising no boarding on the top of the rafters.

The foils also should enable a release of water penetrating the outer 30 covering of roof tiles, slates or other discontinuous elements, or arising from condensation on the underside of the outer covering when this temperature falls below the dewpoint for air held between the foil and outer covering. Tiles and slates are also designed and installed with head and sidelaps to minimise rainwater penetration, but this can still occur under certain weather conditions, for instance wind driven fine snow can be blown between small gaps in a roof tile array. Again this results in a need to provide a drainage path for escape of entrapped moisture down the length of the roof, without ponding behind battens, to eventual discharge at eaves level, if not lost through evaporation.

Further, these foils must also be sufficiently strong to resistant wind uplift forces, with pressure loading generally calculated to be about 1/3 of the total across the roof structure. By providing a windproof layer, the underlay foil also helps ensure that insulation materials maintain maximum thermal efficiency through limiting air infiltration.

Furthermore, the method at present used for covering a roof structure with a foil is very tedious and labour-intensive. Correct positioning of the foil all along the roof structure in order to obtain a repetitive correctly dangling foil between two adjacent rafters requires a constant control of the dangling foil construction made during the fixing of said foil on the rafters. If the foil is left too loose, and there is substantially more than 30mm of drape between the rafters, the foil is free to lift a significant height through uplift by air pressure during storms. If too great, this uplift can dislodge roof tiles and other discontinuous roofing elements, causing damage to the roof assembly. For this reason, the drape typically is to be limited to not more than 30mm, such as only 25 or even only 10mm.

US20100024325 discloses a sheet with tie-straps extending across a width of the sheet and connected to the sheet at multiple locations. The connection locations are to be installed on the roof purlins, such that between these purlins, the sheet drapes.

These sheets have the disadvantage that they can only overlap with a second sheet along the direction of the tie-straps over a distance being equal to the distance between the tie-strap and the edge of the sheet. As such, the roofer has very little to no flexibility in choosing the most appropriate overlap along the direction of the tie-straps, to have several overlapping sheets meeting the height of the roof.

Summary of the invention It is an object of the present invention to provide a foil suitable for covering a roof structure for weather protection and a method for covering a roof structure with such a foil allowing efficient and accurate mounting of the foil such that it provides good ventilation in the roof construction. It is an object of the present invention to provide foils which allow the roofer to select the most appropriate overlap of foils, to have the set of foils to cover the roof substructure in the most efficient and appropriate way.

The above object is obtained by a foil and method for covering a roof structure with such a foil according to embodiments of the present invention.

According to a first aspect of the invention, a method for covering a roof structure with a foil is provided, the roof structure comprises no boarding on the top of the rafters. The method comprises at least the successive steps of: a) Providing a roof structure comprising at least a first and a second rafter, the rafters being adjacent one to the other and being spaced by a distance D; b) Providing a foil comprising at least one set of at least two adjacent position indication means, the position indication means being spaced by a distance S of at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm in between the rafters, the position indication means being selected from the group consisting of a printed, drawn or painted marker on the upper or lower face of the foil, a thickening of the foil, a thinning of the foil, an embossment of the foil, a blind hole or a hole in the foil; c) positioning the foil on the top of a first rafter, the center of a first position indication means being superimposed on the first rafter, d) fixing the foil on the first rafter through the first of position indication means, e) spreading the foil until a second position indication means appears, f) superimposing the center of the second position indication means on the second rafter, g) fixing the foil on the second rafter through the second position indication means, thereby creating a foil hanging in between the rafters.

Distance D refers to the “center-to-center” distance between adjacent rafters.

“A catenary positioned between the two adjacent rafters” refers to the imaginary curve having the shape of a hyperbolic cosine function between two points, each positioned at equal height on two mutually facing sides of the adjacent rafters.

According to some embodiments, the position indication means are printed markers on the upper or lower face of the foil.

According to some embodiments, the position indication means are superimposed to the center of the top of the rafters.

According to some embodiments, the fixing means may comprise at least a means selected from the group consisting of a nail, a screw and clip.

According to some embodiments, the distance S may be at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm but more than 9mm in between the rafters. According to some embodiments, the distance S may be at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of 10mm in between the rafters. Typically the drape is in the range of 9mm to 30mm, such as in the range of 25mm to 10mm, e.g. 25mm, 20mm, 15mm or 10mm.

According to some embodiments, the foil may comprise at least a first and a second set of at least two adjacent position indication means, the first two adjacent position indication means being spaced by a distance S1 of at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm, the second two adjacent position indication means being spaced by a distance S2 of at least 0,9 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm, S1 being different from S2, the first and second set of at least two adjacent indication means being selected from the group consisting of a printed, drawn or painted marker on the upper or lower face of the foil, a thickening of the foil, a thinning of the foil, an embossment of the foil a blind hole or a hole in the foil.

According to some embodiments, the drape provided by S1 or S2 may be 10mm. S1 or S2 may be chosen such that at leat one of the drapes provided is a drape of 10mm.

According to some embodiments, the at least two adjacent rafters may be spaced by a distance of approximately 600mm, the distance S between the adjacent position indication means is at least 0,9 times the length expressed by 2*a* sinh(x/a) + z mm, whereby “a” is a figure in the range of 4510 to 1500 and “x” represents the distance expressed in millimetres between two mutually facing sides of two adjacent rafters and “z” represents the sum of the half of the widths of each adjacent rafter in millimeters. More preferred, the constant a may be between 4500 and 4502, such as 4501, causing a drape of about 10mm for a distance expressed in millimetres between two adjacent rafters spaced by about 600mm. The constant a may be between 1804 and 1805, such as 1841.15, causing a drape of about 25mm for a distance expressed in millimetres between two adjacent rafters spaced by about 600mm. The constant a may be between 1504 and 1505, such as 1504.97, causing a drape of about 30mm for a distance expressed in millimetres between two adjacent rafters spaced by about 600mm.

According to some embodiments, the distance S may be between 1 and 1,5 times the length of a catenary positioned between the two adjacent rafters and forming a drape of less than or equal to 30mm.

According to some embodiments, the distance S may be at least 0,9 times the sum of 600.45 mm and z, where z represents the sum of the half of the width of each adjacent rafter in mm.

According to some embodiments, the foil may be rolled up, the position indication means are distanced from each other over at least the distance S along a direction of unrolling (u) of the rolled up foil.

According to some embodiments, the foil may comprise at least a polymeric material selected from the group consisting of high-density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP)* polyethylene terephthalate (PET), polystyrene (PS), polyamide (PA), Ethylene Propylene diene monomer (EPDM).

According to some embodiments, the foil may be a multilayer sheet.

According to some embodiments, the said two adjacent position indication means being located on the upper surface or on the lower surface of the foil. According to a second aspect of the invention, a roof structure of a building is provided, which roof structure is provided using the method according to the first aspect of the invention.

According to some embodiments, the distance between the imaginary line between the tops of the two adjacent rafters and the bottom of the foil hanging between the rafters may be equal to 10 mm, 25mm or 30mm.

It is an advantage of embodiments of the present invention that easy and correctly mounting of the foil can be performed.

It is an advantage of embodiments of the present invention that an easy control of the proper mounting/positioning of the foil can be obtained.

The method permits to obtain cold roofs having a homogeneous ventilation preventing condensation forming.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Brief description of the drawings Fig. 1 shows a horizontal cross-section of an embodiment of the foil used in an embodiment according to the invention.

Fig. 2 illustrates a horizontal cross-section of the foil according used in an embodiment according to the invention .

Fig. 3 is a vertical view of an embodiment of the foil used in an embodiment according to the invention.

Fig. 4 is a vertical view of another embodiment of the foil used in an embodiment according to the invention.

Fig. 5 illustrates a diagrammatic illustration of the method according to the invention.

Fig. 6 shows a horizontal cross-section of a roof on which a foil is fixed according to an embodiment of the method according to the invention.

Fig. 7 shows a horizontal cross-section of a roof on which a foil according to the invention is fixed.

In the different figures, the same reference signs refer to the same or analogous elements.

Description of illustrative embodiments The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression a foil comprising means A and B” should not be limited to foils consisting only of components A and B. It means that with respect to the present invention, the only relevant means of the foil are A and B.

The expression “upper surface of the foil” means the surface of the foil that is in the opposite of the top of the rafters.

The expression “lower surface of the foil” means the surface of the foil contacting the top of the rafters.

The expressions “center of said top of said first rafter” or center of said top of a second rafter” means the line equidistant to the two longest sides of the tops of said first and second rafters.

The expressions “center of the first position indication means” or center of the second position indication means” means the line which is the axis of symmetry orthogonal of the position indication means.

The expression “top of the rafters” means the superior face of the rafters viewed from the sky, once the rafters are mounted on the roof.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

The present invention relates to the use of a foil suitable for and intended for covering a roof structure for weather protection. Such a roof structure typically comprising at least two adjacent rafters, but in practice a plurality of rafters. It is a standard that such rafters are being spaced by a certain center-to-center distance, typically 600mm. Although the invention is claimed and explained with reference to such a specific interdistance between the rafters, it will be clear that equivalently herewith a similar foil and method for covering can be obtained if other interdistances between the rafters are used for the roof construction. According to embodiments of the present invention, the foil comprises at least adjacent position indication means which are spaced by a distance s of at least 0,9 times the cord length of a cord hanging between the two adjacent rafters and forming a drape of the desired depth, e.g. 30mm, 25mm or 10 mm in between the rafters. The latter creates upon fixing the foil on the roof structure at the position indication means, ventilation between the foil and a further roof covering material when the further roof covering material is applied. The position indication means may be any suitable means for indicating the fixing position for the foil on the rafter.

The position indication means may occur at regularly interspaced distances, e.g. with an interdistance s. The position indication means thus may be repeated over the full length of the foil, so that these can be used independently of the number of rafters that are present on a specific roof.

The terms “two adjacent rafters” means that the said rafters being successive rafters.

The foil may be in folded or rolled form or may already be in in unfolded or unrolled form. In some embodiments, the foil may be provided as a rolled up foil, allowing easily spreading, positioning and fixing on the rafters, e.g. revealing the position indication means along the unrolling direction following the unrolling of the foil over the rafters.

The position indication means may be selected from the group consisting of a marker, a thickening of the foil, a thinning of the foil, a protrusion, an embossment, a blind hole and a hole, preferably selected from the group consisting of mark and thickening of the foil, more preferably the mark is a painted or printed mark.

According to a particular embodiment, said position indication means has a width lower than or equal to the width of the rafter on which said position indication means is superimposed.

According to an embodiment, said position means has a length smaller to or equal than the width of said foil. Preferably, said position indication means may be in form of a continuous line or a dotted line in the cross direction of the foil with respect to the unrolling direction of the rolled foil. When the position indication means is in the form of a dotted line, said dotted line comprises at least one element selected from the group consisting of lines, dots, circles, ellipses, squares, rectangles.

According to a particular embodiment said foil comprises at least a polymeric material selected from the group consisting of high-density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyamide (PA). The polymeric sheets most typically are non-woven fabrics such as single layer flash-spun, heat stabilised HDPE or multilayer microfibre PP. Reinforced bitumen membranes may be used.

According to a particular embodiment; the foil is a multilayer sheet. Preferably, said multilayer sheet comprises a reinforced sheet, an adhesive sheet ora polymeric sheet.

In embodiments of the present invention, the spacing between the subsequent position indication means is based on the fact that for a foil, a specific drape between the subsequent rafters is required for allowing appropriate ventilation. The shape followed by the foil hanging in between the rafters can be mathematically described or approximated (when considering the shape in cross-section perpendicular to the rafters) by the cord formula expressed as follows : y = a*cosh(x/a) where y represents the vertical position, given a certain horizontal position x, and where a represents a constant. For the situation of rafters being positioned approximately 600mm spaced from each other, the constant a may e.g. be between 4510 and 1500.

The corresponding interdistance between subsequent position indication means can, based on the above, be expressed as S = 2a sinh(x/a) + z whereby S is the interdistance between subsequent position indication means, a is the constant as occurring in the equation describing the x,y position of the cord, x is the interdistance between two subsequent rafters, i.e. the free space between the rafters and not the spacing between the centerpoints of the rafters, and z represents the sum of the half of the width of each of said adjacent rafter in mm.

For the example of a spacing between the rafters spaced by about 600mm, the distance s between the adjacent position indication means is at least 0,9 times the length expressed by 2a* sinh(x/a) + z wherein a may range between 4510 and 1500. This corresponds with a distance s of at least 0,9 times the sum of the cord length in mm and z, where z represents the sum of the half of the width of each adjacent rafter in mm. This cord length varies between about 600,4 and 604mm.

An aspect the present invention concerns a method for covering a roof structure, i.e. covering the roof structure with a foil to avoid water damage e.g. caused by weather conditions. The method is especially suitable for a roof structure with a set of equidistant rafters. The roof structure thereby is a roof structure whereby no boarding is used on top of the rafters, but a foil is used without additional support. A foils and at least a pair or rafters as set out above are provided. The method according to embodiments further may comprise at least the following steps : a) Positioning the foil forming a coil on the top of a first rafter, whereby the center of a first position indication means is superimposed on the center of said top of a first rafter, said first position indication means being located on the upper surface of the foil or on the lower surface of the coil, preferably on the upper surface of the foil, b) fixing the foil on said first rafter through said first of positioning means, c) spreading the foil until a second position indication means appears, said second position indication means being located on the upper surface of the foil or on the lower surface of the foil, preferably on the upper surface of the foil. Such spreading may be unrolling a rolled up foil, d) The center of the second position indication means is superimposed on the center of the top of a second rafter, said second rafter being adjacent to the first rafter, e) Fixing the foil on said top of a second rafter through said second positioning means, thereby creating a foil hanging in between said rafters for forming a ventilation region between the foil and a further roof covering material, when the further roof covering material is applied.

The position indication means can be selected from the group consisting of a printed, painted or drawn mark on the upper face of the foil and/or on the lower face of the foil, a thickening of the foil, a thinning of the foil, an embossment of the foil.

According to a particular embodiment, the fixing means comprises at least a fixing means selected from the group consisting of nail, screw and clip.

A further aspect of the present invention relates to a roof structure of a building obtained according the method according to the invention. In a preferred embodiment of the preceding one, the roof structure is such that the distance between the imaginary line between the tops of the two adjacent rafters and the bottom of the inverted arch structure is equal to 10 cm.

The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the technical teaching of the invention, the invention being limited only by the terms of the appended claims.

Referring to Figure 1: Figure 1 illustrates an embodiment of the rolled foil 1, said foil 10 comprising position means 11, said position means 11 being separated by a distance S (disregarding the thickness of the rafter for the distance s) and comprising two protrusions 110 separated by a distance r, said distance r being equal to the width of the position indication means 11. u represents the unrolling direction of said rolled foil 1.

Referring to Figure 2: Figure 2 illustrates an embodiment of the rolled foil 1 according to the invention, said foil 10 comprising position means 11, said position means 11 being separated by a distance s (disregarding the thickness of the rafter for the distance s) and being constituted by a thickening of the foil 10, said position 5 indication means 11 having a width equal to r. u represents the unrolling direction of said rolled foil 1.

Referring to figure 3 and 4: Figure 3 and 4 show is a vertical view of the upper face of an embodiment of the foil 10, said foil 10 comprising position indication means 11 separated by a 10 distance s (disregarding the thickness of the rafter for the distance s). Said position indication means 11 may be in form of continuous line (figure 3) or dotted line (figure 5) in the cross direction of the foil 10 with respect to the unrolling direction of the rolled foil. When the position indication means is in the form of dotted line, said dotted line comprises at least one element 15 selected from lines, dotted, circles, ellipses, squares, rectangles.

Referring to figure 5: Figure 5 illustrates a diagrammatic illustration of the method for covering a roof structure 2 comprising no boarding on the top of the rafters 3 according to the invention. After having provided the foil and the set of rafters, the method 20 comprising the successive steps : a) positioning the rolled foil 1 forming a coil on the top of a first rafter 3, the center of a first position indication means 11 being superimposed on the center of said top of a first rafter 3, said first position indication means 11 being located on the upper surface of the foil or on the lower surface of the foil 10 ; b) fixing the said foil 10 on said first rafter 3 25 through said first of position indication means 11 ;c) unrolling the said rolled foil until a second position indication means 11 appears, said second position indication means 11 being located on the upper surface of the foil or on the lower surface of the foil 10 ; d) the center of the second position indication means 11 is superimposed on the center of the top of a second rafter 3, said 30 second rafter 3 being adjacent to the first rafter 3 ; e) fixing with at least a fixing means the said rolled foil 1 on said top of a second rafter 3 through said second position indication means 11 , thereby forming an inverted arch structure 4.

Referring to figures 6 and 7: Figure 6 shows a horizontal cross-section of a roof structure 2 comprising at least two adjacent rafters 3 on which a foil 10 is fixed starting from a rolled foil 1, said roof structure 2 having no boarding. The rolled foil 1 is unrolled in such a way that the position indication means are located on the upper face of the foil starting from the rafters 3. The foil 10 drags between two adjacent rafters 3 forming an inverted arch structure 4.

Figure 7 shows a horizontal cross-section of a roof structure 2 comprising at least two rafters 3 on which a foil 10 is fixed, said roof structure 2 having no boarding. The foil 10 drags between two adjacent rafters 3 forming an inverted arch structure 4. Said inverted arch structure 4 having the same length over all said roof structure 2 and leading to a good and homogeneous ventilation of said roof structure 2.

As an example, a foil with three sets of at least two adjacent position indication means is provided, for providing a roof underlay on a roof with rafters separated on a distance of about 600mm center to center, and the rafters having a width of z mm.

For the first set of adjacent position indication means, these means are spaced by a distance s of 600,442 +z. This is the cord length of a cord with functional description 4501* cosh(x/4501), having a length of 2*4501* sinh(x/4501) for x=600 or a length of 600,4442 mm and a drape of 10mm.

For the second set of adjacent position indication means, these are spaced by a distance s of 602,7688 +z. This is the cord length of a cord with functional description 1804,15* cosh(x/1804,15), having a length of 2*1804,15*sinh(x/1804,15) for x=600 or a length of 602,7688 mm and a drape of 25mm.

For the third set of adjacent position indication means, these are spaced by a distance s of 603,9815 +z. This is the cord length of a cord with functional description 1504,97*cosh(x/1504,97), having a length of 2*1504,97*sinh(x/1504,97) for x=600 or a length of 603,9815 mm and a drape of 30mm.

Claims

1 .- A method for covering a roof structure (2) with a foil (1), said roof structure (2) comprising no boarding on the top of the rafters (3), said method comprising at least the successive steps of: a) Providing a roof structure comprising at least a first and a second rafter, said rafters being adjacent one to the other and being spaced by a distance D; b) Providing a foil (10) comprising at least one set of at least two adjacent position indication means (11), said position indication means being spaced by a distance S of at least 0,9 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of less than or equal to 30mm in between the rafters, said position indication means (11) being selected from the group consisting of a printed, drawn or painted marker on the upper or lower face of the foil, a thickening of the foil, a thinning of the foil, an embossment of the foil (10), a blind hole or a hole in the foil (10); c) positioning said foil (1) on the top of a first rafter (3), the center of a first position indication means (11) being superimposed on said first rafter (3), d) fixing said foil (10) on said first rafter (3) through said first of position indication means (11), e) spreading said foil (1) until a second position indication means (11) appears, f) superimposing the center of the second position indication means (11) on said second rafter (3), g) fixing said foil (10) on said second rafter (3) through said second position indication means (11), thereby creating a foil hanging in between said rafters.

2. .- A method for covering a roof structure (2) according to claim 1, wherein the position indication means (11) are printed markers on the upper or lower face of the foil,.

3. .- A method according to any one claims 1 to 2, wherein said position indication means are superimposed to the canter of the top of said rafters.

4. .- A method according to any one claims 1 to 3, wherein the fixing means comprises at least a means selected from the group consisting of a nail, a screw and clip.

5. .- A method according to any one claims 1 to 4, wherein the distance S is at least 0,9 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of less than or equal to 30mm but more than 9mm in between the rafters.

6. .- A method according to any one claims 1 to 5, wherein the distance S is at least 0,9 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of 10mm in between the rafters.

7. .- A method according to any one claims 1 to 6, wherein the foil (1 0) comprising at least a first and a second set of at least two adjacent position indication means (11), the first two adjacent position indication means (11) being spaced by a distance S1 of at least 0,9 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of less than or equal to 30mm, the second two adjacent position indication means (11) being spaced by a distance S2 of at least 0,9 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of less than or equal to 30mm, S1 being different from S2, said first and second set of at least two adjacent indication means being selected from the group consisting of a printed, drawn or painted marker on the upper or lower face of the foil, a thickening of the foil, a thinning of the foil, an embossment of the foil (10) a blind hole or a hole in the foil (10).

8. - A method according to claim 7, wherein S1 or S2 is chosen such that at leat one of the drapes provided is a drape of 10mm.

9. - A method according to any one claims 1 to 8, wherein the at least two adjacent rafters being spaced by a distance of approximately 600mm, wherein the distance S between the adjacent position indication means is at least 0,9 times the length expressed by 2*a* sinh(x/a) + z mm, whereby “a” is a figure in the range of 4510 to 1500 and “x” represents the distance expressed in millimetres between two adjacent rafters and “z” represents the sum of the half of the widths of each adjacent rafter (3) in millimeters.

10. .- A method according to any one claims 1 to 9, wherein said distance S is between 1 and 1,5 times the length of a catenary positioned between the two adjacent rafters (3) and forming a drape of less than or equal to 30mm.

11. .- A method according to any one claims 1 to 10, wherein said distance S is at least 0,9 times the sum of 600.45 mm and z, where z represents the sum of the half of the width of each adjacent rafter (3) in mm.

12. .- A method according to any one claims 1 to 11, wherein the foil (1) is rolled up, and wherein the position indication means (11) are distanced from each other over at least said distance S along a direction of unrolling (u) of said rolled up foil.

13. .- A method according to any one claims 1 to 12, wherein said foil (10) comprises at least a polymeric material selected from the group consisting of high-density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyamide (PA), Ethylene Propylene diene monomer (EPDM).

14. .- A method according to any one claims 1 to 13, wherein said 5 foil (10) is a multilayer sheet.

15. ,- A method according to any one claims 1 to 14, wherein the said two adjacent position indication means (11) being located on the upper surface or on the lower surface of the foil (10).

16. .- A roof structure (2) of a building obtained using the method as defined in any one of claims 1 to 15.

17. .- A roof structure (2) of a building according to claim 16, wherein 15 the distance between the imaginary line between the tops of the two adjacent rafters and the bottom of the foil hanging between the rafters (4) is equal to 10 mm, 25mm or 30mm.