This invention relates to struts for transmitting loads from a roof member to an attic floor structure of a building and to attic floor structures. Conventional attic roof trusses are of triangular-frame construction, comprising two divergent rafters with an internal cross-tie at or above attic ceiling height, and a base cross-tie at or below attic floor level. In conventional house construction the supporting walls are constructed of masonry or timber framing. They are designed to support the vertical loads imposed by the roof structure, but they are not capable of resisting horizontal loads. The design of conventional attic roof trusses therefore requires a tie at attic floor level to resist the horizontal outward thrust loads generated by the weight of roofing materials, internal room finishes and imposed external loads due to snow and wind. Current design solutions to produce "Room" in the roof/ "Attic rooms" are subject to the following practical considerations : 1. Transport restrictions restrict prefabricated roof truss height to 4.2m. 2. Health and Safety Executive requires fall arrest systems to be provided below truss erection operations. 3. Other works and trades cannot progress below the roof until the roof is made weatherproof. 4. To complete the "structural triangle" a floor joint member must line up with be connected to every roof truss. This means openings in both the roof and floor openings have to be framed so that t hey either line through or precisely or completely avoid each other. 5. Full fabricated trusses take up a large amount of site storage area. 6. Cranes are required to hoist full roof trusss to roof level, attracting costs and health & safety considerations. There is also a risk to damage to the trusses from slings and chains. Attic rooms are being increasingly incorporated into new homes to increase the available living space without increasing land occupation. Conventional attic trusses are factory made and transported complete to site. This can create access problems and a requirement for heavy lifting equipment. Using conventional attic trusses the attic floor cannot be constructed until the trusses are installed. This leads to a requirement for safety equipment and procedures to minimize the risks of workmen falling. This generally adds to installation times and costs. The "Spatial" Roof™ bearing and design methodology which forms the subject of the present invention has a beneficial influence on all of the six items described above. The Spatial Roof bearing allows the spare capacity of the floor framing to support vertical and horizontal loads from the roof, without adding to the loads on the roof structure, as the floor loads remain independent of the roof truss. The current Spatial Roof™ design methodology currently uses the whole floor construction as a "Tie-member" . This invention provides a strut for transmitting a load from a roof truss or rafter to a floor structure of a building, the strut having a rigid attachment to the truss or rafter and depending generally vertically downwardly therefrom, and means acting between a lower end of the strut and the floor structure which can transmit horizontal force from the truss into the floor structure under the truss but not vertical force from the floor to the truss. The principle of the invention is to design and construct an independent attic floor before the roof structure is assembled. The design of the floor is modified to accept all horizontal thrust loads from the roof. The design of the floor can also be modified to accept controlled vertical loads (including zero load) from the roof structure. Generally the greater part of the vertical roof load will be directly applied to the supporting walls. The"floor members generally run in the same direction as the roof trusses. Where floor members run perpendicular to the roof trusses straps and ties are required to contain the outward horizontal thrust. The floor members encompass the following: Engineered composite 1-joists; metal open web joists; rectangular section timber; composite glue and timber veneer/strandes sections (glulam, laminated veneer lumber, parallel strand lumber, oriented strand lumber, laminated strand lumber and similar) ; metal floor joists; concrete joists or planks; or other composite floor systems. The interface between the roof structure and the floor is a system of proprietary engineered members which the roof truss reacts against to transfer horizontal load into the attic floor. The point of contact between the roof truss and the member is a proprietary bearing which allows controlled vertical movement, which in turn, limits the imposition of vertical loads into the floor (ranging from zero up to a permissible value) . The greater part of the vertical roof load is transmitted directly through a proprietary engineered member to the supporting walls. The arrangement of the invention enables the use of a multipart truss that can be easily assembled and connected on site using generally available carpentry skills. The arrangement does not require the roof trusses to have an integral tie at floor level. Preferably a low friction sliding joint is provided on the lower end of the strut to minimise resistance to vertical movement of the strut with respect to the abutment on the floor structure with which it is engageable. For example the low friction sliding joint may comprise a low friction bearing layer applied to said horizontally directed face of the downwardly extending strut and a second low friction bearing layer to act between the first bearing layer on the dependent member and the abutment on the floor to minimise friction in a vertical direction between the strut and floor structure. In a particular arrangement according to the invention a strip of flexible material has a pair of low friction bearing layers located thereon, the strip being secured to the lower end of the vertical face of the dependent member such that the strip can be folded into a U to bring the bearing layers into contact with one another between the strut and the abutment on the floor structure to provide a low friction sliding engagement between the bearing layers to minimise any vertical load imposed by the strut on the floor. In a further arrangement, the low friction engagement between the strut and an abutment on the floor structure • comprises a resilient shear block mounted on the lower end of the dependent member for engagement with an abutment on the floor structure. Any of the above arrangements may be combined with a floor structure comprising joists extending between the inner walls of the building, boards secured on the joists to provide a floor covering and frame members are secured to the floor to provide vertical abutment faces with which the vertical faces of the downwardly dependent are engageable to impart horizontal loads from the roof members into the floor structure. In the latter case the floor boards extend over and are mounted on wall plates at the upper ends of the walls of the building structure, outer frame members extend along and are secured to the floor structure above the wall plates, inner frame members extend over the floor parallel to the outer members and are spaced inwardly therefrom and are secured to the floor and further frame members extend between the inner and outer frame members to brace the inner members, the inner members have inwardly directed vertical abutment faces to be engaged by the struts extending downwardly from the roof member to transmit horizontal load from the roof members into the frame members. The horizontal struts which brace the members depending from the rafters may rest on the outer frame members to transmit vertical load from the rafters into the wall plates and hence the walls of the building structure. The following is a description of some specific embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is an elevation view of an upper part of a building including upper parts of the building walls, a floor structure extending between the walls and a roof truss to form part of the roof structure of the building; Figure 2 is a detailed view of the left-hand portion of the building structure as shown in Figure 1; Figure 3 is a diagram illustrating the connection between a roof structure and floor in detail showing the application of the relevant forces; Figure 4 is a detailed view of a connection between a member of the roof truss and the floor structure for transmitting horizontal forces into the floor structure; Figure 5 is a similar view to Figure 4 showing the directions in which the forces act; Figure 6 is a perspective view of the joist forming of the floor structure; Figure 7 is a similar view to Figure 6 showing floor deck panels laid on the joists and strap members secured to ,the floor for transmitting horizontal forces into the floor; Figure 8 is a similar view to Figure 7 showing a complete framework mounted on the floor for transmitting horizontal forces into the floor structure from the roof trusses; Figure 9 is a similar view to Figure 8 showing the directions of the reactions imposed on the floor structure to the loading of the roof trusses; Figure 10 is the floor structure of Figure 8 with a number of roof trusses mounted in situ; Figure 10 is a similar view to Figure 9 showing the application of forces by the elements of the roof trusses to the floor structure; Figure 11 is a diagrammatic view of an example of a low friction joint for engaging between components of the roof trusses and the floor structure to transmit horizontal loads into the floor structure; Figure 12 illustrates the low friction joint of Figure 11 secured to a component of the roof truss; Figure 13 is a similar view to Figure 12 showing the low friction element in a folded condition ready for engagement with the floor structure; Figure 14 shows the low friction component mounted on the roof truss and in engagement with the floor structure; Figures 15 and 16 show an alternative flow resistant coupling for engagement between a roof truss and the floor structure; Figure 17 is a similar view to Figure 1 showing an alternative roof construction embodying dormer windows; and Figures 18 to 44 show further arrangements.

Referring firstly to Figure 1 of the drawings, there is shown an upper part of a building which in this case is a domestic house having walls indicated generally at 10 and a roof structure indicated generally at 11. The walls are of cavity construction comprising an outer wall 12 formed in brick and an inner wall 13 formed in either blocks or timber frame panels with a cavity 14 between the walls. Rim boards 15 are secured by straps (not shown) on the inner block walls 13 and joists 16 in the form of "I" beams extend between the rim boards with the ends of the joists being let into the rim boards as indicated at 17. Structural weatherproof deck boards 18 are laid on the joists and are secured to the joists by nailing or equivalent fixing means so that the joists serve to reinforce the boards. The roof structure of the building is formed by a series of spaced roof trusses 19. Each truss comprises a pair of divergent rafters which extend from an apex joint 21 where the rafters are secured together by plates nailed to the rafters. The joint 21 and spread of the rafters is reinforced by a cross member 22 and struts 20 extending between the cross member and the region of the joint 21 between the rafter. The rafters 20 extend over the walls of the building structure to form eaves indicated at 24. The ends of the rafters are finished by vertical facia boards 25 and horizontal eaves boards 26 which extend between the lower ends of the rafters and the tops of the outer walls 12 of the"building structure. Approximately two-thirds of the way down the rafters 20, there are vertically downwardly extending members 27 which are rigidly secured to the rafters and which stop short of the upper surface of the floor 18 by a distance of the order of a centimetre or so. The members 27 may be formed in TimberStrand™ which is a Laminated Strand Lumber (LSL) . Bridging members 28 extend horizontally between the members 27 and the rafters to form a rigid triangulated structure between the three components. The outwardly facing vertical sides of the members 27 bear against inner frame members 29 mounted on the floor structure. The outer ends of the bridging members 28 rest on outer frame members 30 secured to the floor above the wall plate. Referring now to Figure 3 of the drawings, it will be understood that the weight of roofing material supported on the rafters 20 will cause the rafters to impose a vertical load and a horizontal or spreading load on the structure supporting the rafters. The vertical load is directed into the inner wall 13 of the building via bridging member 28 which sits on the outer frame member 30 on the floor strut and transmits a vertical load directly through the flooring boards 18 on the wall plate 15 which sits directly on the inner block or timber frame wall. This takes care of all the vertical loads imposed by the trusses of the roof structure. Horizontal loads which tend to spread or open the V of the truss are imposed via the downwardly dependent members 27 and' their engagement with the inner frame members 29 on the floor structure. As can be seen in Figure 3, a block of material is interposed between the respective vertical face of the member 27 and abutment face provided by the inner frame member 29 and this block is intended to indicate that the connection between the vertical member 27 and frame member 29 permits horizontal forces to be transmitted into the frame member of the floor structure from the roof truss but only low resistance to vertical movement between the member 27 and floor member 29 is provided so that little or no vertical force is imposed on the floor structure by the roof truss. Details of the floor structure will now be described with reference to Figures 6 to 9. As can be seen, the rim boards 15 are connected by a series of parallel "I" beam section joists 16. Weather resistant boards 18 are secured to the joists and the rim boards as shown in Figure 7 and frame structures are mounted along either side of the floor comprising outer frame members 30 laid along the outer edges of the floor directly over the wall plates 15 and inner frame members 29 extending parallel to the outer members 30. Strut members 35 extend the inner and outer frame members to provide additional bracing between the members. The frame members are secured to the floor and the joists beneath the floor using conventional nailing or other suitable fixings. In Figure 9 the outline arrows show the directions in which forces are imposed on the floor structure by the roof trusses and the black arrows show the reaction forces which resist the loads imposed on the floor structure. Figures 9 and 10 are perspective views of the floor structure and a series of roof trusses in place on the floor structure. The forces imposed by the members of the roof structure on the floor structure are shown by the arrows on Figure 10. As indicated earlier, a feature of the connection between the roof truss structure and the floor structure is the"~very low vertical resistance in the joint where the vertical members 27 engage the inner floor member to transmit horizontal loads to the floor structure. To provide a low friction joint between the vertical members 28 and inner floor members 29 a separate sliding joint is provided the construction of which is illustrated in Figures 11 to 14. In Figure 11, a strip of silicon rubber 40 is shown to which a thick block of low friction PTFE material 41 is and a second shallower block of low friction PTFE material 42 is bonded spaced from block 41. Block 41 is provided with a countersunk screw opening 43 and the joint is attached to the lower vertical face of member 27 of the screw 44. The thin block 42 of PTFE lies vertically above the thicker block 41. The strip is then folded over to bring the thin block 42 into engagement with the thick block - li ¬

as shown in Figure 13 a temporary tie or wrap is provided to hold the joint in that form ready for installation. When the truss is installed, the tie and wrap holding the joint at the lower end of member 27 are removed and the rubber strip 40 is bonded to frame member 29 to hold the joint in place between the frame member and vertical member 27. The interacting PTFE blocks provide a low friction vertical joint whilst transmitting the full horizontal load as described earlier. Figures 15 and 16 show an alternative arrangement in which a resilient block is mounted between the member 27 and inner frame member 28 which is adapted to provide low resistance to vertical movement whilst transmitting horizontal force. Figure 17 shows a similar view to Figure 1 for a roof structure having attic or dormer windows on either side. ~~ It will be appreciated that all the arrangements described above provide a considerable clear space in the void in the roof structure which can readily be used as living space. The roof truss can be manufactured and delivered in one piece although the constructions lends itself to being formed in two or more parts which is particularly convenient where a very high roof is required which could cause difficulties in transporting assembled roof trusses to site because of height restrictions resulting from bridges and other similar obstacles. The roof truss can readily be formed in two components which are connected together at the apex of the truss on site and can be connected together at roof level if necessary. Figures 18 to 23 show a further form of connection between the lower end of strut member 27 and frame member 29. The joint comprises a pair of plates formed from plastic metal PTFE or other suitable material comprising a bearing plate 50 having a pair of vertically extending side by side dovetail section grooves 51 for securing the bearing plate to the side of the member 27 using screws 52. A second plate 53 has dovetail section projections 54 which engage in the grooves 51 so that the plate 53 is constrained to slide vertically with respect to plate 53. The outwardly projecting side of the plate has horizontally extending serrations 55 to bite into the surface of the frame member 29 to maintain the plate 53 in situ on the frame member. The vertical sliding joint between the plates allows the strut member 27 to move vertically with the truss from which it depends whilst transmitting horizontal force from the truss via the strut into the frame member 29 of the floor structure below. The floor can deflect vertically with loading on the floor and because of the vertical sliding connection between the floor member 29 and the strut member 27, vertical forces are not imposed on the strut and therefore on the roof truss by the floor. Figure 24 shows a typical roof construction incorporating the truss having struts in accordance with the invention imparting horizontal forces into the floor structure below without imparting vertical force from the floor structure into the groove truss. Figure 25 shows a typical floor plan. Figures 26 and 27 illustrate the vertical and horizontal loading on a roof structure and the consequential defamation of the roof structure highly exaggerated for the purposes of illustration. Figures 28 and 29 shows yet a further arrangement for interconnecting a vertical strut from a roof truss to the floor structure below comprising a metal strap connector 70 having a bracket 71 at one end and a plate 72 at the other end. The strut 27 has a horizontally extending cross member 73 secured between the lower end of the strut and an adjacent part of the roof truss by conventional nail plates 74. The bracket 71 of the metal strap connector is secured to the cross member 73 and the plate of the strap is secured by nailing to the floor below. The strap 70 lies at a shallow angle to the horizontal between the cross member and floor to transmit horizontal forces from the strut 27/cross member 73 to the floor whilst allowing vertical movement of the floor below the truss with respect to the strut without imposing a force on the strut . Figure 30 shows yet a further arrangement in which the lower end of the strut 27 acts on a block 90 on the floor structure through a resilient shear bearing 80 which may be formed in rubber, plastic, foam metal with optional reinforcement embodied in the bearing. The block is designed to be stiff and strong in compression but flexible and weak in shear so that vertical shear forces are not transmitted from the floor to the strut whereas horizontal compression forces from the strut to the floor are transmitted. The block is glued, nailed or screwed to the strut of the timber. Figure 31 shows yet a further arrangement in which the extruded bearing of rubber, plastic foam metal has a hollow or resilient fill which may be a liquid gel or granular material. The block 90 is secured to the floor structure by glue, nailing or screwing and has a serrated contact surface 91 with which the side of the strut 27 engages. As before, the block transmits horizontal forces from the strut to the floor structure but does not transmit vertical forces from the floor structure to the strut. Figure 32 shows yet a further arrangement in which a roller shear bearing indicated at 100 acts between the lower end of the strut and the floor structure. The roller shear bearing may be formed in hard rubber, plastic or metal and may have a cylindrical or spherical rollers which provide vertical movement of the bearing and which are captured by bearing plates. The parts of the bearing are secured to the strut and floor structure by glue, nail or screws. Figure 33 shows yet a further arrangement in which smooth low friction plates 110, 111 are secured to opposing surfaces of the lower end of the strut 27 and floor structure to provide a vertically sliding connection with can transmit horizontal force. Figures 34 to 36 show a bracket 120 having vertical slots 121 for fixing to be secured to the vertical strut and floor structure, the vertical slots in the bracket permitting vertical movement of the floor structure with respect to the strut whilst allowing horizontal forces to be transmitted to the floor structure by the strut . Figures 37 to 39 illustrate application of forces to the roof structure. Figures 40 and 41 illustrate a wing girder structure 130, 131 applied to the upper perimeter of the building to receive the roof. The girder may be formed using any material such as timber, panel products or steel. Figure 41 shows a girder reaction tied to the opposing girder using straps, tie rods or cables. Alternatively the wing girder can be fixed to the side walls of the building as shown in Figures 42 to 44. Advantages and benefits of using a wing girder are as follows: 1. The floor joints can run back to front or side to side as the floor is bypassed by the tie load. 2. Openings in the floor do not need to be considered. 3. The floor decking fixing does not need to be checked for diaphragm action.