Insulation can be incorporated into a flat roof construction either over or under the waterproof membrane of the roof. Where the insulation system is placed on top of the waterproof membrane this is usually referred to as an inverted roof system. Such an inverted roof system protects the waterproof membrane from thermal shock, the effects of sunlight and weathering, and damage by roof traffic. Another advantage of the inverted roof system is that the insulation of an existing flat roof can be upgraded without replacement of the existing waterproof membrane.

In a conventional inverted roof system the insulation is provided by foamed slabs which are placed on top of the waterproof membrane and to prevent the slabs being blown, or floating off, the existing waterproof membrane it is necessary somehow to anchor them in place. In general it is not possible to use mechanical fixings since such fixings penetrate the waterproof membrane causing leaks. Accordingly, at present, the insulation slabs are laid loosely on top of a waterproof membrane on a flat roof and ballasted with gravel or paving slabs having a weight of at least 80 kilograms per square meter.

There are two other conventional systems referred to as "lightweight" inverted systems. In these the insulation material has the form of rectangular slabs coated on one side with a cementitious screed. In one, the slabs have a tongue and groove formed on opposite long sides of the slabs and the insulation slabs are laid in a brick bond pattern with the tongues and grooves of adjacent slabs engaged with one another. In the other, all the edges of the slabs are rebated and the slabs are laid in a chevron pattern with adjacent edges overlapped. Such systems typically have a total weight of between 20 and 25 kilograms per square meter. Usually an additional restraint in the form of additional ballasting, mechanical fixing or adhesion to the membrane, or a combination of them around the periphery of the roof is also required. Although these lightweight inverted systems are much lighter than the normal conventional inverted system it is still some 5 to 15 kilograms per square meter heavier than an equivalent insulated roof system where the insulation is placed under the waterproof membrane.

The majority of roofs are of lightweight construction and cannot carry the loads imposed by the inverted roof systems currently on the market. At present such roofs are insulated by warm roof systems mainly using a foamed polyurethane as the insulating material. However such insulating material requires the use of chlorofluorocarbons during its manufacture and since these are environmentally damaging it is desirable to provide an alternative system.

According to a first aspect of this invention an inverted roof comprises a weatherproof membrane covered by rectangular, foamed polystyrene insulation boards arranged in a herringbone pattern, each of the insulation boards having a ratio of long to short side of 2:1 with tongues formed on an adjacent long and short side and corresponding grooves formed on the other two sides, and a permeable membrane having a weight of between 3 and 15 kilograms per square meter covering the insulation boards.

According to a second aspect of this invention an inverted roof system for insulating an existing flat roof comprises rectangular, foamed polystyrene insulation boards each having a ratio of long to short side of 2:1 with tongues formed on adjacent long and short sides and corresponding grooves formed on the other two sides, and a permeable membrane having a weight of between 3 and 15 kilograms per square meter.

Preferably the foamed polystyrene insulation boards are formed by extrusion into a vacuum with their edges subsequently being profiled. Preferably the tongues and grooves include a slight taper to facilitate their insertion and the arrangement of the insulation boards in a herringbone pattern. The density of the foamed polystyrene insulation boards may be 25 kilograms per cubic metre and they may have a compressive strength of the order of 220 Kpa. However it is preferred that they have a density of 35 kilograms per cubic meter and a compressive strength of the order of 350 Kpa. Typically the insulation board may come in a range of sizes depending upon the degree of insulation required and typically the boards come in thicknesses of 60, 75 and 100 mm. For boards of this thickness it is preferred that the length and width of the board are 1200 mm by 600 mm.

Preferably the permeable membrane has a weight of between 4 and 7 kilograms per square meter. The permeable membrane typically has the form of an elongate strip having a width generally similar to that of the length of each insulation board. In this case the strips of permeable membrane are preferably laid at 45° to the edges of the boards arranged in their herringbone pattern and each strip extends over a number of the boards. One suitable material for the permeable membrane is made from a rebonded rubber crumb material. This is particularly cheap since it is formed from waste materials yet its properties are ideally suited to the present invention. One example of this material is sold under the trade name of RYPOL to be used as an outdoor recreation surface, in running tracks, playing fields, and children's playgrounds.

The permeable membrane also acts as a filter to prevent leaves and other debris from passing through and blocking the drainage channels and outlets of the roof. since the membrane is permeable it is also free-draining and so does not hold water to encourage the germination and rooting of seeds as can happen with conventional systems including ballast or a cementitious screed.

The permeable membrane may be bonded to the upper surface of the insulation boards. It may be bonded at spaced locations but preferably the edges of the permeable membrane are bonded to the boards that it covers. The membrane may be bonded using a modified bitumen adhesive or a moisture cured polyurethane. Preferably at the edges of the roof the permeable membrane is carried over the periphery of the insulation boards and secured to the underlying structure. Again to reinforce the edge it is possible for the insulation boards to be adhered to the underlying waterproof membrane but care must be taken to ensure that adequate gaps are left between spaced portions of adhesive to allow for water drainage between the insulation boards and the waterproof membrane.

By interlocking the insulation boards in a herringbone pattern when any attempt is made to lift one insulation board, for example as a result of wind uplift, this uplift is resisted by the surrounding boards resulting in the uplift being resisted by at least eight times the imposed weight of board. This is in direct contrast to the arrangement of the conventional "lightweight" inverted roof system in which an uplift of only three times the imposed weight of board can be resisted. Calculations to support this are set out in full in a Building Research Establishment Digest No. 295 dated March 1985 and entitled "Stability Under Windload of Loose-Laid External Roof Insulation Boards".

A particular example of a roof system in accordance with this invention will now be described with reference to the accompanying drawings, in which:-

• Figure 1 is a plan of a partly finished roof; and,
• Figure 2 is an exploded section drawn to a very much enlarged scale showing the edge details of two adjacent insulation boards.

Figure 1 shows a rectangular insulation board 30 having a ratio of long to short side of 2:1 with tongues formed on an adjacent long and short side and corresponding grooves formed on the other two sides of the board 30. Figure 2 is an exploded cross-section through the joint formed between an adjacent pair of insulation boards and shows the tongue 31 and groove 32. The tongues 31 and grooves 32 have an extent of 24 and 25 mm respectively and include a 2 degree taper to facilitate insertion of the tongues 31 into the grooves 32. The boards 30 typically have dimensions of 1200 mm x 600 mm.

Typically to install a roof in accordance with this invention a start is made in one corner by placing a half board 1 in the corner and then fitting a second board 2 beside it. A third board 3 is then used to lock together the boards 1 and 2 and a fourth board 4 interfitted with the third board 3. Fifth, sixth, seventh and eighth boards are then joined on as shown in Figure 1 so that, for example, considering board 6, board 6 forms a tongued and grooved connection with half the side of board 5 and board 2, the end of board 3 and half the side of board 7. Additional boards are laid in the order shown in Figure 1 so that when the laying of the insulation boards has been completed block 6, for example, is interlocked with blocks 2, 3, 5, 7, 12 and 13.

As soon as a few of the insulation boards 30 have been laid a roll of permeable membrane typically having a width of one meter and a length of 6 or 10 metres is cut in half at a 45° angle. A resulting strip of permeable membrane 35 is then unrolled on top of the insulation boards 30 to extend generally at 45° to the edges of the insulation boards 30. The other half of the roll is laid beside it and butt jointed against it as shown by the reference numeral 36. Further strips of permeable membrane are butt jointed to the sides and extend over the remainder of the surface. Typically the permeable membrane 35 is formed by a rebonded rubber crumb material having a thickness of 6 or 9 mm and sold under the trade name RYPOL. Once away from the edge then whole strips of material are used which are butt jointed to the ends and sides of adjacent strips. Typically the permeable membrane 35 is strip bonded to the insulation boards 30 by applying a continuous strip of adhesive of width 25mm to each edge. The adhesive may be a modified bitumen type of adhesive, one such example being known under the trade name of TIXIFELT, or a moisture cured polyurethane.