Background Art In the nineties, the applicant has been marketing a roof cladding system under the trademark LindabToplinee which includes roof cladding elements of sheet metal of the type mentioned above. The LindabTopline system is described in two Danish-language pamphlets"Gi'dit hus en ny profil..."and"Vejledning-projektering og mon- tage"issued by Lindab Profil A/S in 1994, as well as in the German-language pamphlet"Lindab Dachpfannen"issued by Lindab VM GmbH. These known roof cladding elements are profiled in their longitudinal direction and thus have a wave-shaped cross-section. Further, the elements are stepped in their transverse direction for forming tile- like formations similar to traditional roof tiles of clay. The roof cladding elements are produced in roll- forming machines, for instance of the type described in a pamphlet"Integrated Sheet Metal Technology"issued by the Finnish company Samesor Oy.

Normally, the elements are delivered in lengths of several metres, so-called full length plates which require two people in mounting on the roof. Lately, how- ever, there has been an increasing demand for shorter roof cladding elements of this type, so-called panels having a length of about 1-2 m which can be mounted by a single person. Such short roof cladding panels have several advantages: they can be stored in standard lengths, they can easily be placed on a small trailer for transport and they can easily be mounted on the roof by one person.

Short roof cladding panels of this principal type are disclosed in the Danish Utility Model Publication DK-U-96 00008 corresponding to the German Utility Model Publication DE-U-297 00 208. The rectangular panels shown in these publications have longitudinal waves or undula- tions and are stepped in the transverse direction for forming tile-like formations. Further, the upper and lower edge portions of the panel are cut generally wave- shaped.

However, the known roof cladding elements (plates and panels) sometimes suffer from some common disadvan- tages. When elements are used which do not extend over the full length of the roof, specific overlap problems occur. These problems are particularly noticeable when the roof cladding is built up from several short panels, in which case a plurality of four-corner joints are pre- sent on the roof. Since the panels are mounted in an overlapping manner, these four-corner joints include the corner portions of four adjacent panels. Due to the four- corner joints having four layers of sheet metal, there is a build-up in height throughout the roof cladding which makes it hard to mount the panels in a straight horizon- tal direction. Further, of course, the four-corner joints having four layers create the same problem in the longi- tudinal direction of the roof cladding.

Another disadvantage is that the person who mounts the roof cladding elements does not know where to put the fastening screws on the elements for attachment to the underlying supporting structure of the roof. The support- ing structure is simply hidden by the panels.

Summary of the Invention An object of the present invention is to provide an improved roof cladding element, by means of which the above-mentioned disadvantages can be remedied. A particular object of the invention is to simplify and improve the mounting of the roof cladding elements.

These and other objects, which will appear from the following description, have now been achieved by a roof cladding element having the features defined in appended claim 1, preferred embodiments thereof being set forth in the subclaims 2-12. The objects of the invention are also achieved by a roof cladding system and the use of roof cladding elements in accordance with appended claims 13 and 14, respectively.

The roof cladding element of the invention is char- acterised in that two diagonally opposite corners there- of are obliquely cut. An upper cut corner portion of one roof cladding element is adapted to be matchingly assembled with a lower cut corner portion of another cladding element roof in a four-corner joint including also non-cut corner portions of a third and a fourth roof cladding element.

The invention confers several advantages. Since the four-corner joints only have two layers of material along the connection where the cut corner portions are put together, the former overlap inconvenience is avoided.

In the area immediately adjoining said connection between the cut corner portions, there are only three layers of material. Further, the mounting of the roof cladding is simplified.

Brief Description of the Drawings The invention will be described more in detail in the following with reference to the accompanying drawings which show non-limiting embodiments of the invention.

Fig. 1 is a perspective view of a short-tile roof cladding panel in accordance with the invention.

Fig. 2 is an end view of the panel of Fig. 1.

Fig. 3 is a perspective view of a long-tile roof cladding panel.

Fig. 4 is a plan view of four roof cladding panels as shown in Fig. 1 before assembling.

Fig. 5 is a perspective view of the four roof clad- ding panels shown in Fig. 4, portions of the four panels being cut away.

Fig. 6 is a perspective view of three of the four panels in assembling.

Fig. 7 is a perspective view of the four panels when assembled.

Fig. 8 is a perspective view of a short-tile roof cladding plate in accordance with the invention.

Fig. 9 is a long-tile roof cladding plate in accor- dance with the invention.

Figs 10 and 11 are side views, partly in section, showing a screw for fastening a roof cladding element of the invention to a supporting member.

Description of Preferred Embodiments Figs 1-2 show a rectangular roof cladding element 1 according to an embodiment of the invention. The roof cladding element 1 is of the short type also referred to as a roof cladding panel which has a length of about 1-2 m. The panel 1 is profiled in its longitudinal direc- tion and has a wave-shaped cross-section (see Fig. 2).

The lower edge 2 of the panel 1 is cut wave-shaped where- as the upper edge 3 is cut straight (cf. Fig. 4). The sides 4,5 of the panel are straight and adapted to be overlappingly connected to adjacent panels.

The panel 1 is stepped in its transverse direction, and the two steps are designated 6 in Fig. 1. The steps 6 define short tile-like formations. The lower edge 2 has the same wave-shape as the two steps 6. Further, the lower edge 2 has a downwardly directed flange 7 which will be described later on.

Two diagonally opposite corner portions 8,9 of the panel 1 are obliquely cut as will be described in more detail below. Further, the panel 1 has fixed marks or nests 10 for fastening means, such as screws, which also will be described below. Preferably, these nests

are formed as indentations in the upper surface of the panel 1.

Fig. 3 shows a slightly modified panel 11 which cor- responds to the roof cladding panel 1 but which is of the long-tile type.

A long version of the roof cladding panel 1 is shown in Fig. 8. This short-tile plate 12 has several tile formations and can have a length of for instance about 4-6 m. A long-tile plate 13 is shown in Fig. 9 corresponding to the long-tile panel 11 shown in Fig. 3.

It should be noted that the present invention is not limited to roof cladding elements of particular lengths, since the four-corner joint problem occurs every time there is at least one transverse joint between two ele- ments of the roof cladding.

Fig. 4 shows from above four short-tile panels of the type shown in Fig. 1 before assembling. In order to simplify the understanding of the assembling of these four panels, they are designated la, lb, lc and ld in Fig. 4. The same references are used in Figs 5-7.

The four panels la, lb, lc, 1d are assembled in the following manner. First, the first panel la is attached to the roof-supporting structure. Then, the second panel lb is placed in an overlapping manner on the first panel la, the lower flange 7b being engaged with the step 6a of the first panel la. Thus, the lower edge portion of the second panel lb overlaps the upper edge portion of the first panel la. The second panel 1b is fastened to the roof-supporting structure.

In the following step, the third panel lc is con- nected to the first and second panels la, lb as can be seen in Fig. 6. The adjacent side edge portions of the first panel la and the third panel lc are overlapped.

The obliquely cut lower corner portion 8b of the second panel 1b snugly fits to the profile of the third panel lc. The cut corner portion 9c of the third panel lc is put matchingly together with the cut corner portion 8b of

the second panel 1b edge-to-edge. The third panel lc is attached to the roof-supporting structure.

In order to finish the four-corner joint, the fourth panel 1d is connected to the three panels la, lb, lc in such way that the non-cut corner portion 8d of the fourth panel Id is placed over the cut corner portions 8b and 9c of the first and third panels lb, lc, as is shown in Fig. 7. The flange 7d of the fourth panel ld is engaged with the step 6c of the third panel lc. The interconnect- ed side edge portions of the second and the fourth panels lb and 1d are overlapped. Seen from above, the two non- cut corner portions 8d, 9a of the first and the fourth panels la, ld are overlapped as well with the edge con- nection of the cut corner portions 8b, 9c therebetween.

The fourth panel ld is fastened to the roof-supporting structure.

Thus, it is the adjacent corner portions 8b, 9c of two diagonally mounted panels lb, lc that are obliquely cut. This principle is common for all four-corner joints all over the roof cladding which is built up by several panels of the type described.

Since the lower edge of the panel 1 is wave-shaped, the same panel 1 can be used close to the gutter or any- where in the roof cladding.

As can be clearly seen from Figs 6-7, the four-cor- ner joint connection is such that the wave-shaped steps of the panels are continuous in the horizontal direction.

It is also understood that there are only two layers of sheet metal along the connection where the edges of the cut corner portions 8b, 9c are put together, which eli- minates the four-layer overlap problem described by way of introduction. These two layers are the non-cut corner portions 9a, 8d of the first and the fourth panels la, ld. In the area surrounding said edge connection 8b, 9c, there are of course three layers of sheet metal.

Preferably, the edge-to-edge connection between the two cut corner portions 8b, 9c is rather tight. In par-

ticular, it is preferred that the edges 8b, 9c are in abutment with each other in the area where the flanges 7b and 7d of the second and third panels lb, lc meet, where- as there may be a small play (a few millimetres) in the rest of the edge connection, see Fig. 6. This small play facilitates the connection of the panels in mounting.

It should be noted that the panels and plates 1,11, 12,13 may be of other designs within the scope of the invention. For instance, it is preferred that the upper edge portion 3 of the roof cladding element is substan- tially straight, but alternatively the upper edge portion may be wave-shaped in accordance with the steps 6 and the lower edge portion 2. Further, it is preferred that the profiling and tile formations of the roof cladding ele- ment may vary in accordance with aesthetical desires and requirements.

A typical short-tile roof cladding panel 1 in accor- dance with the invention has a length of 1220 mm and a width of 1095 mm.

Figs 10-11 show a self-drilling screw with a head 14 and a threaded shank 15. The screw head 14 has a recess 16 for a screwdriver (not shown), and an annular rubber sealing 17 is mounted on the shank 15. When mounting the roof cladding element 1 on a roof-supporting member 18, the tip of the screw shank 15 is placed in one of the screw nests in the shape of an indentation 10 in the upper surface of the element 1, whereupon the screw 14, 15 is drillingly driven into the supporting member 18, causing a special deformation of the indentation 10. A very tight and secure fastening of the roof cladding ele- ment 1 is accomplished.

The screw nests or indentations 10 are arranged on the elements 1 in accordance with a predetermined pat- tern, which corresponds to the arrangement of the under- lying roof-supporting members 18. Thanks to these fixed indentations 10, the roof-mounting personnel just have to place the fastening screws 14,15 in the indentations 10

and activate the screwdriver which preferably is of the automatic type.

Since the underlying roof-supporting members 18 are arranged in accordance with standard lengths of the roof cladding plates and panels having fixed screw nests 10, it does not matter that the roof-supporting structure is hidden to the person mounting a roof cladding since there is a roof supporting member under each screw nest 10. This makes the assembling of the roof cladding much easier and quicker.

It is appreciated that the inventive concept by no means is restricted to the embodiments described, and several modifications are feasible within the general scope of the invention defined in the appended claims.

The specific appearance of the roof cladding element is not crucial as long as the element has the main features of the invention, especially in relation to the solution of the four-corner joint problem. Although the preferred material of the plates and panels is sheet metal, other materials can be used as well.