Conventional roof sheets for so-called"rondavels" (houses having a circular outer profile) comprise triangular corrugated sheets, which are arranged towards a centre region of the roof to form a cone, with the sides of the sheets overlapping. A second set of sheets, which are frusto-conical in plan view, is arranged at the base of each triangular sheet.

A disadvantage of the known roof sheets is that a supporting structure of beams and rafters is required to support the sheets. This implies additional cost associated with extra material and additional construction time.

OBJECT OF THE INVENTION It is accordingly an object of the present invention to provide a roof structure; roof sheeting for a roof structure; a method of constructing a roof structure; and a building provided with such a roof structure, with which the aforesaid disadvantage can be overcome or minimised.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a roof structure for a building having a non-rectangular annular circumferential profile and a central region, the roof structure comprising: a plurality of elongate triangular sheets for spanning the area between the circumference and the central region of the building, with each side of each sheet meeting with an adjacent sheet; and each sheet having at least one stiffening formation extending generally in the same direction as the longitudinal axis of the sheet, for supporting the weight of the sheet.

The arrangement of the above roof structure may be such that the use of beams and rafters to support the roof sheet is made redundant due to the provision of the stiffening formations.

The stiffening formations may each comprise an elongate ridge formation.

The elongate ridge formations may extend from the base of each roof sheet partially towards the upper end thereof.

Further according to the invention, the roof structure includes a connecting means for connecting upper ends of the sheets to each other.

The connecting means may comprise two spaced apart members for receiving the upper ends of the roof sheets between them and a fastening means for fastening the two members to each other.

Preferably the spaced apart members are substantially conical.

Further according to the invention, the roof structure includes attachment means to which a lifting means can be connected for lifting the roof structure.

Each edge of each sheet may define a connecting formation for connecting to a complimentary connecting formation defined along an opposite edge of an adjacent sheet.

The connecting formation may comprise an elongate ridge formation and the complementary formation may comprise an elongate groove formation for mating with the ridge formation.

The elongate groove formation may be secured to the elongate ridge formation in mating relationship by crimping them to each other at regular intervals.

According to a second aspect of the invention there is provided a roof sheet comprising an elongate triangular panel having at least one stiffening formation

extending generally in the same direction as the longitudinal axis of the sheet, for supporting the weight of the sheet.

The side edges of the panel may each be provided with a connecting formation for connecting to the side edge of an adjacent panel.

Preferably the connecting formation along one of the side meeting edges may comprise an elongate ridge formation for mating with an elongate groove formation provided along an opposite edge of an adjacent like panel.

Further according to the invention the stiffening formation comprises a triangular ridge formation.

Alternatively the stiffening formation may comprise a plurality of elongate parallel extending ridge formations.

According to a third aspect of the invention there is provided a building having the above roof structure.

According to a fourth aspect of the invention there is provided a method of constructing a roof structure for a building having a non-rectangular annular circumferential profile, the method including the steps of: erecting perimeter walls of the building ;

constructing a roof structure according to the first aspect of the invention in a position remote from the walls ; and -lifting the roofs structure onto the walls.

The method may include the further step of securing the roof structure to the walls.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further by way of non-limiting examples only with reference to the accompanying drawings wherein: figure 1 is a side view of a roof structure according a first embodiment of the invention; figure 2 is a plan view of the roof structure of figure 1; figure 3 is a cross-sectional side view along line A-A'in figure 2; figure 4 is a perspective view of two adjacent roof sheets of the roof structure of figure 1; figure 5 is a plan view of the roof sheets of figure 4; figure 6 is a cross-sectional end view along line B-B1 in figure 5;

figures 7 and 8 illustrate a fastening means for fastening a roof structure having a roof sheet according to a second embodiment of the invention on top of a wall of a building ; figure 9 is a view similar to figure 3, illustrating an attachment means for lifting the roof structure; figure 10 depicts a comparison between the side meeting edges respectively of the roof sheets of figure 6 and figure 7; figure 11 is a plan view of two roof sheets according to the second embodiment shown in figures 7 and 8; figure 12 is a perspective view of the two roof sheets of figure 11; and figure 13 is a perspective view of two roof sheets according to a third embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Referring to figures 1 and 2, a roof structure according to a first embodiment of the invention, is generally designated by reference numeral 10.

The roof structure 10 comprises a plurality of roof sheets 12 (shown in more detail in figures 4 to 6). The roof structure 10 is particularly suitable for use with an octagonal house 14 (figure 1). The roof structure 10 can, however, further be adapted for use with any building having a non-rectangular annular circumferential profile.

Referring to figures 4 to 6, the roof sheets 12 each comprises an elongate triangular panel comprising an upper end 12.1; a base 12.2; and two sides 12.3 and 12.4. Each panel 12 further comprises a stiffening formation in the form of an elongate triangular rib formation 12.5 extending partially along the length of the panel 12.

Referring particularly to figure 6, one side 12.3 of each panel 12 is provided with a connecting formation in the form of an elongate ridge formation 12.6 for mating with an elongate groove formation 12.7 provided on an opposite side 12.4 of an adjacent panel. Alternatively, as shown in figure 10, both edges could be in the form of ridge formations 13, and these are connected to each other by an elongate channel member 15 which mates with the two ridge formations.

Referring to figure 3, the roof structure 10 further comprises a connecting formation 16 for connecting the upper ends 12.1 of the roof sheets 12 together.

The connecting means 16 comprises two spaced conical members 16.1 and 16.2, the lower one 16.2 having a hollow polyhedral configuration, for receiving the upper ends 12.1 between them so that the upper ends are sandwiched between them. The connecting means 16 further includes a fastening means in the form of a bolt 16.3 for fastening the two conical members 16. 1 and 16. 2 to each other.

In use, the walls 14.1 of the building 14 are constructed up to roof level and the base 12.2 of the roof sheets 12 disposed on top of the wall 14.1, with the upper ends 12.1 of each roof sheet resting on the inner conical member 16.1, and with the sides 16.3 and 16.4 of adjacent roof sheets 12 intermeshing as indicated in figure 6.

The applicant foresees that a temporary support (not shown) for the inner conical member 16.1 can be provided, and which support is optionally later removed. The upper conical member is disposed on top of the upper ends 12.1 and the bolt 16.3 fastened to sandwich the upper ends 12.1 of the roof sheet between the two conical members 16.1 and 16.2. The temporary support member can now optionally be removed.

The roof structure 10 is now ready for use and no further supports are necessary to support the roof in position. It will be appreciated that the roof structure 10 can be erected in a relatively short time and that a substantial amount of money is saved by not using any rafters or beams. Further saving is achieved by not using a permanent central support in supporting the apex of the roof.

An alternative method of erecting the roof structure 10, includes the steps of: erecting perimeter walls of the building ; constructing the roof structure 10 on a jig (not shown) located adjacent or in a position remote from the walls ; and -lifting the roofs structure 10 onto the walls.

This alternative method presents the advantage that the perimeter walls and the roof structure can be constructed simultaneously, and not sequentially, and a substantial amount of time is saved.

Referring to figure 9, the roof structure 10 is provided with an attachment means 30, to which any suitable lifting means such as the hook of a crane can be connected, for lifting the roof structure onto the walls.

Referring to figures 7 and 8, the method includes the further step of securing the roof structure to the walls. This could of course be done in many ways. One

method which is envisaged is to provide an elongate metal strap 32 having a bent back end 32.1, which is trapped between the channel formation 15 and the side meeting edges 13 of adjacent roof sheets. The lower opposite end of the strap 32 is then secured with bolts 34 to the perimeter walls, once the roof structure has been placed in position.

Preferably the method includes the further step of preparing the upper edge of the walls by applying a layer of settable material 36 thereto and working the layer off to the appropriate angle for flushly abutting the inside of the roof sheets.

Referring to figure 11 to 13, a roof sheet according to alternative embodiments of the invention are designated by reference numeral 12B and 12C respectively. The embodiment illustrated in figures 11 and 12 includes parallel extending elongate ridge formations or pleats 12.5B, extending from the base for approximately one third of the length of the sheets.

The embodiment illustrated in figure 13 is provided with and elongate stiffening formation in the form of upstanding edges 13.1 extending along the sides of the sheets 12C and no intermediate ridge formations are provided.

It will be appreciated that variations in detail are possible with a roof structure; roof sheeting for a roof structure; a method for constructing a roof structure; and a building provided with such roof structure according to the invention without departing from the scope of the appended claims.