TECHNICAL FIELD OF THE INVENTION

This invention relates to the weatherproofing of flat rooves and similar flat substantially horizontal surfaces comprising boards, sheets slabs or the like supported on beams.

BACKGROUND

Flat rooves presently present a major leakage problem. Most frequently, boards are covered with waterproof layers of felt and bitumen, but this kind of finish has a limited life, can only be applied under dry conditions, and is not inherently fire proof.

Another important consideration relates to the problem of condensation on the underside of the roof, within the building itself. In order to prevent condensation, which can rapidly lead to decay of the roof timbers, it is desirable that the roof structure should be porous to allow air and moisture to pass through from inside the building. On the other hand this is inconsistent with the requirement for total waterproofing to prevent entry of water into the building.

GB 239 647, GB 581 446, US4 241 107 and US 4 349 398 disclose surface coatings comprising a cementitious material containing a single layer of reinforcing mesh.

An aim of the present invention may be viewed as being to provide a form ofweatherproofingwhich is patentably different from existingweatherproofing methods applied to flat surfaces.

SUMMARY OF THE INVENTION

The present invention proposes a method of weatherproofing a flat roof or similar structure, which comprises:

- securing at least three layers of metal mesh to the said structure;

- applying a skim of hydraulic cementitious material to the mesh; and

- thereafter keeping the skim wet for a continuous period of days sufficient fo r the skim to cure and form a water-impermeable layer.

The invention aiso provides a weatherproofing layer applied to a flat roof or similar structure, which comprises at least three superimposed layers of metal mesh secured to the underlying structure and embedded in a cured layer of hydraulic cementitious material.

It is important that the cured layer should contain a relatively high proportion of mesh to prevent cracking due to shrinkage, thermal movement of the layer or, in the case of a flat roof, due to someone walking over the roof. The number of layers of mesh secured to the roof structure is ideally between 3 and 12 inclusive. The mesh preferably forms from 15% to 50%, and most

preferably from 20% to 35%, of the total cured weight of the mesh layers and cementitious skim.

The mesh can be nailed down to the underlying structure through any existing felt.

The preferred form of mesh comprises two sets of substantially parallel metal strands secured together in a substantially perpendicular orientation. The pitch of the mesh strands is preferably between 3mm and 15mm.

In structures which are particularly prone to flexing the mesh is preferably secured to the underlying structure by fastening elements which each have a first portion for attachment to the mesh and a second portion for insertion into the underlying structure, with provision for relative movement between said portions. The first portion may comprise a simple wire tie which is preferably looped around a stem of the second portion such as to allow sliding movement along said stem.

The cementitious material preferably comprises hydraulic cement and sand. The ratio of sand to cement is preferably between 3: 1 and 1 : 1 to form a layer which is water-impermeable when fully cured. To ensure that the layer is permeable to air and moisture and minimise condensation problems the sand preferably comprises a washed non-porous sand such as silica sand, substantially consisting of sand particles in the range 90 microns to 750 microns inclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings:

Figure 1 is a vertical section through a flat roof which has been weatherproofed by the method of the invention, with some of the layers shown separately for convenience of illustration, and

Figure 2 is a sectional detail of a similar form of flat roof, including a fastening element.

DETAILED DESCRIPTION OF THE DRAWINGS

A flat roof comprising timber beams 1 and boards 2 is covered with an old layer 3 of felt and bitumen, which may have started to leak.

If the roof is leaking badly a layer of polythene sheet 4 may be applied over the old felt layer 3 after first cutting out any bubbles which may have formed in the existing felt layer. However, where there is a requirement to reduce condensation on the underside of the roof the sheet 4 can be omitted or microporous sheet can be used.

At least three layers of metal mesh 5 are then applied over any polythene sheet 4 and secured to the underlying boards 2 by galvanised nails 7, rust¬ proof staples or the like. Up to twelve layers may be used, depending upon the application. The mesh may be of the kind known as expanded metal mesh, formed from a metal sheet with parallel siits which is pulled to open

up the slits. The layers of mesh are preferably laid with the direction of the slits disposed perpendicularly in alternating sheets. The preferred kind of mesh is formed of two sets of parallel wires which are secured one on top of the other with the wires mutually perpendicular and joined by welding or the like. Such mesh is inexpensive and has maximum strength with a high resistance to distortion combined with a minimum thickness. The preferred mesh has square apertures with a wire pitch of about 6mm or 12mm.

When the old felt has been covered with the required number of layers of mesh, a wet cementitious skim 8 is trowelled into the mesh until the mesh is completely covered. Although a standard mix of sand and cement would achieve a hard roof which is waterproof to a certain extent, the mix is important to achieve a waterproof covering which is sufficiently porous to prevent internal condensation. The preferred sand is a highly purified and washed (sharp) silica sand with a grain size in the range 90 microns to 750 microns inclusive. Other non-porous washed sands of suitable grade (particle size) may also be suitable. The size and purity has been determined to be important as it appears that the grains fit together in such a way as to form a micro-sieve which is permeable to air and water vapour whilst, at the same time, being impermeable to any water lying on the roof The sand and hydraulic cement are mixed in a proportion by volume of about 2' 1 , together with a waterproofing additive which further aids water repulsion.

The overall thickness of the combined mesh and cementitious layers will generally be between 6mm and 25mm inclusive. The mesh will generally form between 20% and 35% of the total cured weight of the mesh layers and cementitious skim.

Curing is crucial to the permeability, strength frost resistance, abrasiveness and overall quality of the cured product. Normally, the layer should not be allowed to dry out for at least twenty eight days. Under dry conditions a polythene sheet may be temporarily applied over the layers to retain moisture until the cement has fully cured. Alternatively, a known moisture sealant may be permanently painted onto the skim after the initial hardening has taken place. Under damp air conditions the material can be allowed to cure naturally without any such measures.

Further curing may take place over a period of weeks or months, but even after the initial curing period the treatment provides a permanent tough, waterproof coat which is able to flex without cracking, even under the weight of a man or a vehicle. The treatment can therefore be used on rooves and similar structures to extend its range of uses, e.g. by providing walkways or even car parks. The minimum life of the treatment will generally be measured in decades so that replacement is rarely necessary.

It is important that the metal content is relatively high. A mesh content of at least 22 Ibs/cu ft, and preferably 28 to 50 Ibs/cu ft is required as compared with 8 to 10 Ibs/cu ft for normal reinforced concrete. The total weight of the cured layers of mesh and cementitious skim is about 147 Ibs/cu ft.

Closely controlled structural tests have shown that the cured material has a coefficient of thermal expansion of 6.5 x 10 ^'6 per °C in the range of 6 to 26 ^C C. Thus, the movements induced by temperature changes are extremely low.

The treatment can of course be applied directly to the boards 2 in the case of a new roof or in a case where the old felt layer is so badly deteriorated that

it is best removed.

Many industrial rooves incorporate a layer 9 of fibrous heat-insulation material, as shown in Fig. 2. These may be treated with layers of mesh and a sand and cement skim as described above but using special fastening elements to secure the mesh to the underlying boards 2. A simple form of fastening e'ement comprises a screw or nail 10 having a shaft 1 1 and a head 12. A length of corrosion-proof wire 13 is loosely wrapped twice around the shaft 1 1 so that it is free to slide along the shaft. Holes 14 are drilled at intervals in the insulating layer 9, through which the screw or nail 10 is inserted into the underlying layer 2. The layers of mesh 5 are laid on the insulation 9 with the wire tails projecting through the mesh, which are then twisted together to secure the mesh In the completed roof, the layers of mesh and skim are able to flex and move under the weight of a man due to the sliding of the wires 13 along the elements 10. The same movement also allows for flexing of the layers caused by wind pressures, which may otherwise result in structural damage

In some rooves a layer of concrete or other material supported on reinforced concrete or steel beams may be present in place of the boards 2

Whilst the above description lays emphasis on those areas which, in combination, are beheved to be new, protection is claimed for any inventive combination of the features disclosed herein