A known ceiling board comprises a layer of chalk sandwiched between two sheets of fibreboard. A first disadvantage of the known ceiling board is that it is relatively heavy i. e. approximately 6.6 kg/m2. Transport thereof is therefore a significant cost factor and a substantial amount of timber beams and rafters have to be provided for supporting the weight of the board. Another disadvantage of the known ceiling board is that it is relatively brittle and between 15 to 50 % of ceiling boards are damaged during transportation and installation. Installation of the known ceiling boards is also labour intensive, thus increasing the overall cost thereof.

A further disadvantage is that the known ceiling board has to be nailed to the timber beams and can be attached to metal beams only with difficulty. The combined weight of the timber, ceiling board and nails makes the conventional ceiling board unsuitable for use with relatively low cost houses as the roof structures, walls and foundations of such houses are usually unable to support said combined weight.

If the nails used in securing the conventional ceiling board are not treated properly, they tend to rust after a certain period of time resulting in the discoloration of the ceiling board and also the weakening of the installation.

Attempts have been made in the past to produce a lightweight ceiling board from cardboard but these attempts have failed mainly due to a variety of factors, including inter alia that the cardboard was manufactured from recycled paper of insufficient weight. Another disadvantage of the known cardboard ceiling boards is that they tend to bulge when painted.

Furthermore, the known cardboard ceiling displayed a tendency toward excessive atmospheric moisture absorption in relatively high humidity conditions and this resulted in the distortion of the cardboard's surface area.

The cardboard ceiling was also prone to excessive movement due to expansion and contraction resulting from a difference in day and night temperatures over a period of time.

OBJECT OF THE INVENTION It is therefore an object of the present invention to provide a ceiling board, a building structure and a method of installation with which the aforesaid disadvantages can be overcome or at least minimised.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a ceiling board comprising a first corrugated sheet disposed between first and second flat sheets; the ceiling board being characterised in that at least the first flat sheet is of virgin paper.

The ceiling board is further characterised in that the first flat sheet has a weight, which is more than 225 g/m2 and less than 750 g/m2.

The weight of the first flat sheet may be more than 245 g/m2 and less than 255 g/m2, preferably 250 g/m2.

The ceiling board is further characterised in that a paintable sealing layer is applied to the surface of the first flat sheet opposite the corrugated sheet.

The sealing layer may cover between 50 to 90%, preferably 70% of the ceiling board.

The sealing layer may have a weight of between 5 to 20 g/m2, preferably 8 glum2

Further according to the invention the ceiling board includes a second corrugated sheet disposed between a third flat sheet and the surface of the second flat sheet opposite the first corrugated sheet.

The corrugated sheets and the second and third flat sheets may also be of virgin paper.

A sealing layer may similarly be applied to the outer surface of the third flat sheet opposite the second corrugated sheet.

The weight of each of the two corrugated sheets may be between 100 to 200 g/m2, preferably 160 g/m2.

The weight of each of the second and the third flat sheets may be in the region of between 200 to 600 g/m2, preferably 225 g/m2.

The total weight of the ceiling board may be less than 5 kg/m2, preferably less than 1,5 kg/m2.

According to a second aspect of the invention there is provided a building structure provided with the above ceiling board.

According to third aspect of the invention there is provided a method of installing a ceiling in a building structure, the method including the steps of: -providing a ceiling board having a weight of less than 5 kg/m2 ; -providing a support for the ceiling board; and -gluing the ceiling board to the support.

The ceiling board may be of the type according to the first aspect of the invention.

The support may comprise a plurality of spaced rafters.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of a non-limiting example with reference to the accompanying drawings wherein: figure 1 is a longitudinal sectional side view of a building structure provided with a ceiling board according to a preferred embodiment of the invention; figure 2 is an end view of the ceiling board in the direction of arrow R in figure 1; and figure 3 is part of a cross-sectional end view along lines A-A'in figure 1.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Referring to the drawings, a ceiling board according to a preferred embodiment of the invention, is generally designated by reference numeral 10.

The ceiling board 10 comprises first and second corrugated sheets 12 and 14 sandwiched between first, second and third flat sheets 16,18, and 20 respectively, all of virgin paper.

The first flat sheet 16 has a weight of 250 g/m2, the two corrugated sheets 12 and 14 a weight of 160 g/m2, and the second and third flat sheets 18 and 20 a weight of 225 g/m2. A sealing layer 22 such as Aquamol 877 having a weight of at least 8 g/m2 is applied to the exposed outer surfaces of the first and third flat sheets 16 and 20, to cover 70% thereof. The total weight of the ceiling board is therefore in the order of 1. 034 kg/m2.

Because the sealing layers 22 cover only 70% of the outer surfaces of the ceiling board, these surfaces are paintable with a large number of different paints. In order to improve the moisture resistance of the ceiling board 10 further, the different layers 12 to 20 are bonded to each other with a water- resistant starch based adhesive.

Referring particularly to figure 1, a building structure 24 having a side wall 26; a roof 28; a main beam 30; and a plurality of rafters 32, is provided with a ceiling

as follows. The ceiling board 10 is connected to the rafters 32, simply by applying an elongate layer 34 of silicon based adhesive (figure 3) to the outer exposed surface of the third flat layer 20 and the lower surface of the rafters 32, and pressing the ceiling board 10 against the rafters 32. The adhesive 34 is allowed to cure for approximately 6 hours and the outer exposed surface of the first flat layer 16 painted.

It will be appreciated that due to the light weight of the ceiling board 10, the transport and installation cost thereof is relatively lower than that of conventional ceilings, due to the use of relatively lighter vehicles and less personnel to handle the ceiling boards. The overall cost of a roofing structure incorporating a ceiling constructed with the ceiling board 10 will also be lower as much less intermediate timber supports are required between the main rafters 32.

The applicant has found that the ceiling board 10 provides effective insulation and is paintable relatively easily with a range of different types of paint without significant deformation. Because the ceiling board 10 is flexible to an extent, relatively less damage of the ceiling board 10 takes place during transportation and installation.

It will be appreciated further that variations in detail are possible with a ceiling board, a building structure and a method of installation according to the

invention, without departing from the scope of the appended claims. For example, only one corrugated sheet, which is sandwiched between two flat sheets, can be used, and in such case the weight of the flat sheets will be in the region of 400 g/m2 and the weight of the corrugated sheet in the region of 195 glum2.