1 An Engagement Structure This application is a divisional of Australian Patent Application No. 2009202866, the entire contents of which are incorporated herein by reference. 5 Field of the Invention This invention relates to an engagement structure for securing together overlapping upper and lower profiled roof sheets. The invention is particularly, though not exclusively, suitable for use with a sealing device for use in sealing 10 between overlapping profiled sheets such as roofing sheets. Background of the Invention The sealing device has particular use in joining profiled steel roofing sheets of a long span roof. The term "long span" refers to an industrial roof that may, for 15 example, be up to 80 metres long and have a fall to enable rainwater run-off which can be as low as 10. The roofing sheets are held in place by being engaged with roofing clips mounted on a supporting roof frame over which the sheets are installed. The engagement is to be such as to allow for thermal expansion of the sheets relative to the roof frame and clips. In many systems, 20 the sheets are pushed onto the clips so as to secure a head portion of each clip above a re-entrant surface or shoulder within a rib defined by the profiled sheet. The length of roofing sheets that cover a long span can be of a relatively short, 25 transportable length, such as from 12 to 20 metres. Alternatively, the sheets can be of a relatively long continuous length of up to about 80 metres if the sheets are manufactured on site. A waterproof step joint is necessary where shorter length sheets join, but the need for such intermediate joints is avoided by using such long continuous length sheets. However, use of the long 30 continuous lengths incurs the costs of transporting a sheet roll former to manufacture the sheet on site and of the necessary handling equipment for moving the long continuous sheet onto a roof frame and into place. Those costs are high, and can be prohibitive for some roofing requirements. 857307 speci (clean) 10 July 09 2 As intimated, using short, transportable sheets necessitates a waterproof step joint where the ends of longitudinally successive sheets overlap. That is, a seal is required where the lower end of one sheet overlaps with the upper end 5 of another sheet below the one sheet. Also, there typically needs to be two purlins below and supporting each step joint. The height of a typical step joint is visually significant as it can be as great as about 100mm and, hence, it can be aesthetically undesirable. The step, in addition to providing a waterproof joint, is to function as an expansion joint. That is, the joint is to allow for 10 thermal expansion and contraction of the sheets caused by ambient temperature cycles. Summary of the Invention The present invention provides an engagement structure for securing together 15 overlapping upper and lower profiled roof sheets comprising ribs spaced apart by a pan or trough section, wherein the ribs have a hollow mushroom shape in cross-section including a head and a stem which narrows to the head, the engagement structure including a cleat structure of a rigid material positionable between the overlapping roof sheets for achieving a snap-fit engagement with 20 one of the ribs of the upper roof sheet and with one of the ribs of the lower roof sheet to thereby secure the overlapping sheets together, the cleat structure including legs converging towards an enlarged head portion to form a mushroom shape. 25 The cleat structure can include a recess defined by a spacing between the legs and an interior of the head portion adapted for receiving the rib of the lower roof sheet. The head section of the cleat structure can be dimensioned so that as the 30 upper roof sheet is pressed down on the cleat structure the head section of the cleat structure initially forces the wall of the rib of the upper roof sheet apart and allows that wall to recover when the head section of the cleat structure is within the head of the rib and behind shoulders of the rib. 857307 speci (clean) 10 July 09 3 The engagement structure can be adapted to provide a step between the ends of the overlapping roofing sheets which is substantially less than 100mm in height. 5 The cleat structure can be formed of a metal and is adapted for securing together the overlapping upper and lower profiled roof sheets with a sealing device positioned between the sheets to form a seal at the ribs of the sheets and to form a seal between the pan or trough sections of the upper and lower sheets laterally from the ribs. 10 Brief Description of the Figures In order that the invention may more readily be understood, reference now is directed to the accompanying drawings, in which: 15 Figure 1 is a perspective view showing part of a roof frame with overlapping roof sheets of a roofing system; Figure 2 is similar to Figure 1, but shows enlarged detail of a sealing device between overlapping ends of the roof sheets; 20 Figure 3 shows, in a partially exploded end elevation, enlarged detail of a connection in the system of Figures 1 and 2, illustrating a first embodiment of a sealing device; 25 Figure 4 shows a perspective exploded view of the connection of Figure 3; Figure 5 is a perspective view of a second embodiment of a sealing device; 30 Figure 6 is an exploded perspective view of the device of Figure 5; Figure 7 is a perspective view of a third embodiment of a sealing device; 857307 speci (clean) 10 July 09 4 Figure 8 is an exploded perspective view of a modified form of the device of Figure 7; 5 Figure 9 is a perspective view of a fourth embodiment of a sealing device; Figure 10 is a side elevation of the device of Figure 9; 10 Figure 11 is an exploded perspective view of the device of Figure 9; and Figure 12 is a perspective view of a fifth embodiment of a sealing device. 15 Detailed Description The views of Figures 1 and 2 show two longitudinally in-line roof sheets R1 and R2, shown truncated in Figure 1, and very much shortened in Figure 2 to show essentially only that part of sheets R1 and R2 at and adjacent to their overlap. The sheets R1 and R2 are mounted in relation to a C-section purlin P 20 comprising part of a roof frame (not otherwise illustrated). The roofing of which sheets R1 and R2 form part is inclined downwardly to the left so that rainwater will drain to sheet R1 from sheet R2. The upper end of sheet R1 is secured on purlin P by a connector C. A strap S 25 of connector C is secured along the top surface of purlin P, while a number of uniformly spaced clips C1, C2 and C3 are mounted on the strap S. The connector C shown in Figure 1 is provided for a second sheet R1 to be laterally adjacent the sheet R1 shown, with the laterally adjacent sheets having inter-fitting side margins. The second sheet R1 would be installed before 30 sheet R2, but has been omitted for ease of illustration. The lower end of sheet R2 overlaps with the upper end of sheet R1 and a sealing device D, shown more clearly in Figure 2, provides sealing between 857307 speci (clean) 10 July 09 5 the overlapping ends of sheets R1 and R2. Below purlin P along sheet R1, sheet R1 is similarly connected to other purlins by respective connectors C while, above purlin P along sheet R2, respective connectors C will connect sheet R2 to other purlins. Also, unless sheet R1 is the lowermost sheet or 5 sheet R2 is an uppermost sheet, its other end will similarly overlap with a further sheet. Each sheet R1 and R2 has laterally spaced ribs 10 spaced from each other by a pan or trough 12. The side margins of laterally adjacent sheets are profiled 10 and inter-fit to form a composite rib (not shown) of the same form as ribs 10. The spacing between ribs 10 and between the composite ribs and the closest rib 10 is uniform and corresponds to the spacing between clips C1 to C3 of connector C. 15 As shown in Figure 3, the sheet R1 is able to be pressed down onto the connector C to locate, and lock, each of clips C1 to C3 in a respective channel or groove 14, shown in Figure 4 as defined by the respective ribs 10 and the composite rib. As can be appreciated most readily from Figure 4, the ribs 10 are of a mushroom shape as seen in end elevation. The ribs 10 have side 20 walls 6 in which the groove 14 tapers inwardly towards a head 18 within which the groove 14 has re-entrant shoulders 20. Each of the clips C1 to C3 is of a complementary form to groove 14, in having a stem portion 22 which tapers inwardly away from strap S to an enlarged head 24. As sheet R1 is pressed down onto connector C, the head 24 of each of clips C1 to C3 enters a 25 respective groove 14 and forces apart the side walls 16 of the rib 10. The head 24 of the clip is able to enter the head 18, after which the walls 16 recover to position shoulders 20 behind head 18 and thereby hold the rib 10 captive on the clip C. With sheet R1 locked down on purlin P by connector C, the sealing device D then is able to be pressed down onto, and engaged with, 30 sheet R1. The sheet R2 then is able to be pressed down onto, and engaged by the sealing device D. 857307 speci (clean) 10 July 09 6 As can be seen in Figures 3 and 4, the device D has an elongate body sections B which have upper and lower surfaces 26 and 28, and respective sides 29 which extend between surfaces 26 and 28. Also, at spaced centres corresponding to the spacing between centres for ribs 10, the device D has 5 upstanding engagement portions or formations F. The formations F provide raised parts 26a of the upper surface, and recessed parts 28a of the lower surface, of body B. As can be appreciated from Figure 4, the lower surface 28 of the body B of 10 device D as viewed in side elevation is substantially the complement of the upper surface of sheet R1, while the upper surface 26 is substantially the complement of the lower surface of sheet R2. Thus, as shown in Figure 4, the engagement formations F have a mushroom-like shape in side elevation, provided by legs 30 which converge, away from body B, to an enlarged head 15 section 32. The recessed part 28a defines a recess defined by the spacing between legs 30 and the interior of head section 32, while the raised part 26a of surface 26 is similar in form to a rib 10 and is defined by the outer surface of legs 30 and the exterior of head section 32. As a consequence, the device D is able to be pressed down onto lower sheet R1 to enable the head 18 of each 20 rib 10 of sheet R1 to be received in a recessed part 28a within a respective formation F. The head 18 of rib 10 is able to pass between legs 30 to locate in head section 32 of formation F. Thus, device D is able to be engaged on sheet R1 in a manner analogous to the securement of sheet R1 on purlin P by connector C. As device D is pressed onto sheet R1, the head 18 of each rib 25 10 resiliently deforms the legs 30, to enable the head 18 to be received in the enlarged head section 32, after which the legs recover to contact walls 26 of rib 10. Similarly, as also shown in Figure 4, sheet R2 is able to be pressed onto 30 device D to enable each head section 32 of each formation F to be received within a groove 14 of a respective rib 10 of sheet R2. For this, the walls 26 of the ribs 10 of sheet R2 pass down over and resiliently compress the head section 32, until the head section 32 is received into the head 18 of the rib and 857307 speci (clean) 10 July 09 7 recovers to locate behind shoulders 20. Thus, sheet R2 is able to be engaged on device D in a manner analogous to the securement of sheet R1 on purlin P by connector C, such that sheet R2 is in overlapping relation to sheet R2. To a degree, device D is secured on sheet R1 once sheet R2 is installed. However, 5 sheets R1 and R2 are secured in relation to each other by connector C securing sheet R1 to one side of device D and a further connector located to the other side of device D and securing sheet R2. With the device D engaged on sheet R1, the re-entrant shoulders 34 defined in 10 each head section 32 of formation F locate under the head 18 of a respective rib 10 of sheet R1. This acts to resist device D from lifting from sheet R1. Also, with sheet R2 secured on device D, the re-entrant shoulders 20 in each head 18 of the ribs 10 of sheet R2 locate under the head section 32 of the respective formation F. This similarly acts to resist sheet R2 from lifting from 15 device D. Also, in being constrained on a rib 10 of sheet R1 by location within a rib 10 of sheet R2, deformation of formations F is resisted, securing the assembly of sheets R1 and R2 and device D therebetween. Despite this, it is respective connectors C which secure the overlapping ends of sheets R1 and R2 in relation to each other. 20 Figures 3 and 4 are provided primarily for the purpose of illustrating how a seal between the overlapping ends of sheets R1 and R2 is provided by the sealing device D. However, it also will be noted that device D is strengthened at each formation F by what, in Figures 5 and 6, is referred to as cleat structure having 25 a respective part at each side of body B. The cleat structures are stiff or rigid and will be understood from description of other embodiments subsequently detailed herein. As with some of those embodiments, the parts of each cleat structure are inter-connected by pins extending laterally through body B. Also, abutting ends of successive devices D can be inter-connected by such a cleat 30 structure. The device D may have a body B of a length corresponding substantially to the width of the roofing sheets with which it is to be used. Alternatively, the body B 857307 speci (clean) 10 July 09 8 may be built up of successive end to end abutting modules which are secured together by a cleat structure. Figures 5 and 6 show a sealing device Dl according to a second embodiment. 5 The device Dl has an elongate body B1 which has upper and lower surfaces 126 and 128 and respective outwardly facing sides 130. The body B1 is made up of three layers which comprise side layers 40 and an intermediate layer 42 between layers 40. The layers 40 and 42 have a similar form except that layer 42 is slightly oversized so as to stand proud of layers 40 along the upper and 10 lower surfaces 126 and 128, and at the ends of device Dl. At uniform distances along body B1, the device Dl has engagement portions or formations Fl. The device Dl has two complete formations Fl intermediate of its ends. Also, at each end, device Dl has a respective half formation Fla 15 which is able to form a complete formation Fl with a half formation of another device Dl. In side elevation, each complete formation Fl, and each formation Fl which results from end to end abutting devices Dl, has a shape at upper surface 126 which corresponds substantially to the shape in cross-section of a rib 10 of sheets R1 and R2 and, hence, is substantially the complement of the 20 shape in cross-section of the channel or groove 14 of a rib 10. Also, in side elevation each complete formation Fl, and each formation Fl at such abutting ends, has a shape at lower surface 128 which corresponds substantially to the shape in cross-section of a channel or groove 14 of a rib 10 and is the complement of the shape in cross-section of a rib 10. Thus, each formation Fl 25 has upwardly converging legs 130 and a hollow head section 132, while each half formation Fla has one leg 130a and a part 132a of a head section 132. At each complete formation Fl, there is a cleat structure which is made of a relatively rigid material and has a respective part 46 and 47 at each side of the 30 body B1. The parts 46 and 47 each have a form similar to that of the formation Fl in that they have legs 46a and 47a which converge towards an enlarged head portion 46b and 47b. The part 46 has projections 46c which are inserted through holes formed in formation Fl and into holes 47c formed in part 47 in 857307 speci (clean) 10 July 09 9 which they are secured; there being two projections 46c extending from head portion 46b and one projection 46c from the end of each leg 46a remote from head section 46b. The parts therefore are secured in assembly on body B1. The legs 46a and 47a of parts 46 and 47 converge to the enlarged head 5 portions 46b and 47b so that, like the formation Fl and the ribs 10 of sheets R1 and R2, the parts 46 and 47 are somewhat mushroom shaped. Each layer 40 of body B1 may be secured to a respective face of layer 42 by adhesive bonding. They alternatively, or additionally, may be secured by pins 10 43 shown in Figure 6 which are inserted laterally through body B1, with the pins being retained by having an enlargement at each end. The layers also are held by the engagement, between parts 46 and 47, provided by projections 46c of part 46 engaging in the holes 47c of part 47. 15 The layers 40 and 42 of device Dl may be formed of a first resilient material, such as a foam plastics material, with each layer 40 being of a stiffer resilient material, such as a more rigid foam and, hence, less readily compressible than the material of layer 42. The parts 46 and 47 of structure 44 may be of a flexible, but relatively stiff engineering plastics material so that parts 46 and 47 20 reinforce the formation F and limit the extent to which they are compressible. The head section 49 of each of parts 46 and 47 is dimensioned so that, as sheet R2 is pressed down on device Dl, section 49 initially forces the walls 26 of the ribs 10 of sheet R2 apart, and allows the walls to recover when section 49 is within head 18 of the rib 10, behind the shoulders 20. 25 One of the half formations Fla also has a cleat structure 44 which is substantially the same as the structure 44 at the complete formations Fl. However, the projections 46c of part 46 securing parts 46 and 47 together extend through only the half formation, from head section 46b and to head 30 section 47b. Also, the half formation Fla also includes a U-shaped member 50, having parallel arms 50a joined by a web section 50b. The arms 50a correspond to projections 46c in form and function. The arms 50a are spaced by portion 50b so that each arm is able to extend through the end of each leg 857307 speci (clean) 10 July 09 10 46a remote from head section 46b to the other cleat part 47. Member 50 is fitted and secured when two devices Dl are in abutting end to end relationship, with each arm 50a extending through a half formation Fla of a respective device Dl, to secure the devices Dl in that relationship. 5 Figures 7 and 8 show respective devices D2 which are sufficiently similar to enable them initially to be described in the same terms. However their differences also will be discussed later herein. Also, the devices D2 are similar in overall form and function to device Dl of Figures 5 and 6, and the initial 10 description of them first will deal with the features by which they each differ from device Dl. The principal difference between each device D2 and the device Dl is that the devices D2 are made up of repeated modules, of which three are illustrated 15 and distinguished as modules Ml, M2 and M3. However, the number of modules can be varied as required. Also, as provided for use, the devices D2 can be pre-assembled, as shown in Figure 7, in a required standard length. In the devices D2, each of modules Ml, M2 and M3 is of the same form and 20 includes a respective unit U forming part of the overall body B2, and a respective cleat structure 144. However, the structure 144 is of unitary or integral construction, with parts 146 and 147 connected by a bridging member 52. The formations F2 are created by the modules Ml, M2 and M3 abutting in end to end relation in a manner resulting in the member 52 being enclosed 25 between the abutting ends of successive modules, within the head section of a formation F2. As shown in Figure 8, the half formations to produce the formations F2 are recessed at 132a to accommodate member 52. The unitary structures 144 of each device D2 receives the end formations of 30 abutting end to end units U between a respective half of the opposed parts 146 and 147. The units U then are secured by screws or rivets 53 which pass through one of parts 146, 147 and through holes 54 in the unit U to the other one of parts 146, 147. 857307 speci (clean) 10 July 09 11 The devices D2 differ from each other principally in the material and fabrication of the respective structures 144. In the case of Figure 7, the structures 144 are formed of stiff, but resilient steel, such as a spring steel, shaped and folded 5 to the required configuration. In the case of Figure 8, the structures 144 are made of a stiff, but resilient engineering plastics material, such as polypropylene, polystyrene acrylonitrile or a polyamide, such as by injection moulding. 10 In relation to each of Figures 7 and 8, it is to be appreciated that each of modules M1, M2 and M3, comprising a part of the overall body B2 in the form of unit U, plus a cleat structure 144, itself is a sealing device according to the invention. That is, the overall sealing device D2 according to the invention is an assembly of end to end connected smaller sealing devices each according 15 to the invention. Figures 9 to 11 show a device D3 which is similar to device D1 of Figures 7 and 8. The principal difference is that of the parts 246 and 247 of structure 244 are secured in assembly by an integral bridging member 152, which 20 extends between the head sections 246b and 247b. Also, parts 246 and 247 are secured on body B3 by U-shaped members 250 similar to members 50 shown in 5 and 6. A respective pair of members 250 is used for each structure 244. However, the arms 250a are joined by web section 250b, and one arm 250a of each member 250 extends between the head sections 246b and 247b 25 of parts 246 and 247, while the other arm 250a extends between the ends of legs 246a and 247a remote from sections 246b and 247b. At one end face, device D3 is provided with a recess 232a. The other end face is provided with a protrusion (not shown) which has a shape 30 complementary to that of recess 232a. This facilitates close inter-fitting between abutting ends of a number of devices D3 assembled end to end. 857307 speci (clean) 10 July 09 12 Figure 12 shows a device D4 which is similar to device D1 of Figure 5. However, device D4 is of unitary, integrally moulded form, having opposite ends of complementary form-enabling a number of devices D4 to be assembled end to end. 5 Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention. 857307 speci (clean) 10 July 09