Background of the invention A large number of various roofings is used for the roofs of open sites. These roofings prevent either from the approach of various kinds of contaminants, animals, insect and others, or from the fall of persons into these sites, or they create a certain air-conditioning. Various roofs of farming lands are familiar from technical experience, such as glasshouses, foilhouses and others used for the roofs of water surfaces.

To provide such water surfaces as garden pools with roofs, we can use for instance mobile roofs. One of these prospective roofs is described for instance in the document FR 2 715 685. A disadvantage of this type of roof is constituted by the need of at least two persons to move the roof away from the pool and eventually to return it to its position over the pool. Another disadvantage is the need of a place for putting aside the roof. Next we also know designs when a sheet is moved above the water surface to protect this surface. A system like this is described for instance in the document FR 2 729 176. The disadvantage of this design is the need of the aside-putting place for the sheet and the need of a movable and down-laying system. Other designs are described for instance in the document FR 2 708 957, when the water surface is roofed by means of a structure placed over the water surface. The disadvantage of this design is the necessity of a structure and in some cases an insufficient protection of the access to the water surface. A telescopic system advantageous for the roofing of rather small water

surfaces is described for instance in the document FR 2 727 710. The disadvantage of this design is a rather complicated design of individual parts of the telescopic roof consisting of broken elements. Joints in the broken points must be sealed.

These joints are difficult to be made, a lot of joining material is needed and the whole design is bulky and aesthetically not very good. And what is more the sealing between individual parts inserted into one another is difficult to be done with respect to the breaks.

Summary of the invention The above-mentioned shortcomings are made up for, to a certain extent, by the roof of the open site, especially the pool consisting of rooflight sheets attached to the bearing structure according to this invention. It is based on the fact that it consists of at least two rooflight sheets bent to be of an arched shape. Each rooflight sheet is attached to at least two subarches made of sections. At its ends the arches are interconnected with a cross arm provided with elements enabling placing on parallel guides. The diameter of one of the rooflight sheets is smaller than the diameter of the other rooflight sheet.

It is convenient to form the arch on the inner side of the sheets with a rectangular section and on the outer side of the sheets with belt material. The rectangular section is interconnected with the belt material by means of connecting elements.

Another convenient design proposes the arch of a self-supporting section of the H-letter shape whose one bearing side consists of a rectangular section and the other one is flat.

One more convenient design proposes the arch of two sections of two sections of the U-letter shape, with a thick bottoms and they are interconnected by means of their bottoms to form one unit.

The cross arm can have a section of the H-letter shape whose one bearing side consists of a rectangular section opened on one side and its other side is flat. In this case, in one of its inner room there is a sheet and in the open rectangular section there are guide elements. The connection between the cross arm and the arches consists of screws fitting into holes running through the sections of the arches.

A safety pin whose other end is loosely placed in the guides can be connected to the cross arm.

It is advantageous to provide the inner surfaces of the subarches with sealing elements in the form of a rubber belt and/or a belt of hairs. In this case the inner surfaces of the subarches can be provided with a groove for inserting the sealing elements.

The parallel guides can be made of at least two guideways of a tube shape.

The cross arm can be of a section of the U-letter shape within whose inner room there is a sheet. The cross arms can be provided on the inner sides with sealing elements in the form of a rubber belt and/or a belt of hairs.

The rooflight sheets, in addition to the prevention of the pool from pollution, insect, leaves, needle-leaves and others have also the function of solar collectors.

The incoming solar energy warm the water in the pool and the rooflight sheets also prevent heat leakage into surrounding environment. If there is a roof over the pool, the pool can be used for a much longer period of time, basically from April to October. Another advantage of this design is constituted by the prevention of an unexpected fall into the pool. At least two rooflight sheets bent to be of the arched shape provide the above-mentioned advantages and cater for the aesthetic taste.

With respect to the fact that each rooflight sheet is attached to at least two subarches made of sections, sufficient rigidity and strength of the whole structure are guaranteed. This rigidity and strength are increased by means of the interconnection of the arches with the cross arm. The cross arm may be provided with guide elements enabling placing on parallel guides, both for stationary and

mobile placing on the guides. With respect to the fact that the diameter of one of the rooflight sheet is smaller than the diameter of the other one, it is possible to insert the sheets into one another on the principle of a telescope or it is possible to slide one sheet along the other one as long as these sheets are not placed from the cross arm at one end of the arch to the cross arm at the other end of the arch.

The rectangular section on the inner side of the sheets and the belt material on the outer side of the sheets form a simple and price obtainable structure that has the required properties. With respect to the fact that the rectangular section is interconnected with the belt material by means of connecting elements, the whole structure is sufficiently strong. The arch consisting of the rectangular section and the belt material is able to relieve a certain tension resulting from the outer influence such as for instance the change in temperature.

The arch consisting of the self-supporting section of the H-letter shape, whose one bearing side consists of a rectangular section and the other one is flat, enables easy assembly with a minimum need of further connecting elements. The said self- supporting section is rigid enough and has suitable mechanical properties. The advantage of this section is also constituted by the fact that the operation of bending further belt material is saved.

The arch consisting of two sections in the shape of the U-letter with thick bottoms that are connected through their bottoms to form one unit, is sufficiently rigid and enables to flush certain roughness between neighboring sheets. The advantage of this design is the possibility of placing sheets within a smaller distance of one another which means that in case of a longer pool the last arches have diameters large enough to cover longer surfaces.

If the cross arm is of the H-letter shape, whose one bearing side consists of a rectangular shape opened on one side and the other side is flat, then there is a sheet in one of its inner rooms. The advantage of this design is that the sheet can freely move and thus balance possible dilatation. The guide elements placed in the open rectangular section are covered and the attachment of their axes is not visible on

the outer side and therefore this arrangement is of an aesthetic impression, there is no danger of catching on protruding connecting elements and these connecting elements are protected against mechanical damage. The attachment of the cross arm to the arches is implemented by means of screws that fit into longitudinal holes in the sections of the arches which means that these screws do not protrude from the surface and do not disturb the smooth impression of the structure. What is saved is also the operation of marking and predrilling holes in each section.

The safety pin, fitting into the hollow created by opening a tube, secures the roof against moving during unexpected gusts of wind and similar events.

The sealing elements in the form of the rubber belt and/or the belt of hairs prevent from the penetration of impurities into the pool and heat leakage from the inner room. In case the inner surfaces of the subarches are provided with grooves for inserting the sealing elements, the assembly of the roof is made easier.

The parallel guides consisting of at least two guideways in the form of a tube enable the sliding of sheets and at the same time prevent from the sliding of the sheets in an undesirable direction or the undesirable lifting of the sheets, for instance owing to violent winds. Guideways can also have different shapes, for instance the double-tee, but they can also be constituted by mere grooves in which guide elements in the form of wheels, especially rubberized wheels, move.

The provision of the cross arm on the inner surface with sealing elements in the form of a rubber belt and/or a belt of hairs fulfills the same function as the seals on the inner side of the arch, but in this case the sheets do not slide along the guides, however, they slide against one another.

Brief description of the drawings The said invention will be explained in greater detail by examples of concrete models by means of enclosed drawings, where Fig. 1 shows an example of a

scheme of the roof from the front view. Fig. 2 shows the right view of the roof.

Fig. 3 shows the scheme of the front view of another possible model of the roof.

Fig. 4 shows a detail of the cross-section of the arch. Fig. 5 shows the detail of the cross-section of another arch and Fig. 6 shows the detail of the cross-section of one more arch. Fig. 7 shows the cross-section of one cross arm and Fig. 8 shows the cross-section of another cross arm Detailed description of the preferred embodiments The model roof of the pool consists of two rooflight sheets 1 bent into an arched shape. Both the rooflight sheets 1 are attached to two subarches 2 made of section material. At their ends the arches 2 are connected with cross arms 3 provided with guide elements 4. The diameter of one of the rooflight sheets 1 is smaller than the diameter of the other rooflight sheet 1.

The arch 2 is in one case created on the inner side of the sheets 1 with the rectangular section 21 and on the outer side of the sheets 1 with the belt material 22. The rectangular section 21 is connected with the belt material 22 by means of rivets.

In another example the arch 2 consists of the self-supporting section 23 of the H-letter shape whose one bearing side is created by a rectangular section and the other one is flat.

In one more example the arch 2 consists of two sections 24 of the U-letter shape with thick bottoms 25 to form one unit. The connection is implemented by means of screws.

The cross arm 3 has the form of the H letter, and at the same time in one of its inner rooms there is placed the sheet 1 and in its other room there are guide elements 4 in the form of wheels that move along parallel guides 5 in the form of the tube 51.

In another design the cross arm 3 is of the H-letter shape whose one bearing side consists of a rectangular section opened on one side and the other side is flat.

In one of its inner rooms there is the sheet 1 and in the open rectangular section there are guide elements 4. The connection between the cross arm 3 and the arches 2 consists of screws fitting into longitudinal holes in the sections of the arms 2. The safety pin 31, whose second end is placed loosely in the open room of the tube 51 of the guide 5, is attached to the cross arm 3 on the side.

The inner surfaces of the subarches 2 are provided with grooves 27 in which there are inserted sealing elements 26 in the form of a rubber belt.

Another example of the design shows cross arms 3 of the U-letter section in whose inner room there is the sheet 1. Cross arms 3 are provided, on the inner surfaces, with sealing elements 32 in the form of a rubber belt.

Industrial applicability The roofs of open sites according to this technical design will be mostly used for garden pools.