Cavity Wall Ties
The introduction of the cavity wall in the 1920s helped overcome the three problems described previously. An early cavity wall basically comprises two 100 mm skins or leaves of brickwork separated by a gap of 50-70mm.
Two 100 mm skins of brickwork each acting independently are not very strong or stable and it is a requirement of the Building Regulations that the two leaves of the wall are tied together at regular intervals. This is done by the use of wall ties which can be made from galvanised steel and, more recently, plastic and stainless steel. Some of the very early ties were also made from iron; these are not acceptable in modern construction; they are too brittle and they rust very quickly. Current Building Regulations specify stainless steel ties although other ties will be acceptable as long as they have the approval of the British Agrement Board.
The cavity prevents water from reaching the internal skin and improves the thermal efficiency of the wall as the air in the cavity is a good insulator.
It is important that the tie is correctly positioned to ensure that any water in contact with the tie does not reach the inside skin. The frequency and position of the ties is set out in the Building Regulations but generally they occur 900mm apart horizontally and 450mm vertically (every sixth course of bricks). Extra ties are required around window and door openings.
Underground structures being brought into use as part of the living accommodation are likely to be subject to building regulation approval.
Waterproofing is a high risk area. Conversion works are costly and are entirely dependant upon the efficacy of the water proofing materials and installation work itself – if this fails the building works are at high risk and may require removal.
The waterproofing is nearly always covered up by new masonry walls or dry lining. Thus it is not readily accessible in order to establish ongoing efficacy. Thus the first an unsuspecting owner may know of failure of the waterproofing is sudden ingress of built up moisture or water from a defective seal in the materials used.
Surveys for this work should take into account soil type, strata and bed rock external and nearby drainage including taking into account that of any neighbouring property.
The latest British Standard ( revision of the standard BS8102:1990) – of November 2009 was brought out due to an uplift in demand for extension of living accommodation into basements and cellars. In 2010 Building Regulations require all such works to be Radon gas proof.