Since the 1930’s, cavity walls have become the standard form of construction for housing and buildings of a similar scale. The purpose of cavity construction is to improve the weather resistance of the traditional nine-inch masonry wall by splitting it into two leaves of 4 inch brick, linked with metal ties. The floor- and roof-loads are carried on the inner leaf, the outer leaf is simply a weatherproof skin while the ties contribute to the stability by improving what engineers refer to as the ‘slenderness ratio’.
Structurally, the cavity wall is a load sharing system with ties laid out in a uniform spacing pattern: a diamond formation with ties at 900mm centers in courses set 450mm apart. At door and window openings the spacing is reduced to 300mm vertically. This spacing is most important where cavity walls are most vulnerable: in small areas of brickwork between openings and in large un-reinforced areas in exposed positions such as gables, where, in the absence of the lateral restraints nowadays required in housing, the walling derives no restraint from the junction with the roof. The main danger in these positions is from wind suction which, on the leeward side of a building, can exert considerable force which the brickwork, strong in compression but weak in tension, cannot withstand without effective ties at the correct spacing.
Although ties in bronze, cast iron and slate are sometimes seen, the commonest material for tie manufacture is mild steel and the commonest manufactured forms are the fishtail or vertical twist ties (either galvanized or bitumen coated) and the galvanized butterfly wire tie. All mild-steel ties are susceptible to corrosion over time, as the Building Research Establishment has pointed out in its publications, particularly Digests 329 and 401. Because the service life of the masonry is much longer than that of the mild steel it follows that at some point in its life the wall may need replacement of the ties.
The process of corrosion of mild steel causes expansion of the ties because the metallic oxide occupies a greater volume than the pure metal did. This expansion has the effect of forcing apart the bricks above and below the bed joints in which the ties are laid, producing a pattern of lateral cracks coinciding with the tie positions approximately every six courses. In fair-faced brickwork this will frequently have been re-pointed in the past, resulting in thicker bed joints at regular intervals. There may be some associated diagonal cracking. Because of their lightweight the corrosion of butterfly wire ties produces almost no detectable external symptoms. The tie can corrode right through without producing any evidence on the outside of the building of what is happening. But fishtail and vertical twist ties with their greater bulk of metal, do produce sufficient volume expansion in corrosion to cause a visible splitting of the mortar bed in which they are fixed.
Many cavity wall properties in the UK have received retrofit Cavity Wall Insulation as part of various Government initiatives. Whilst this doesn’t impose any immediate danger, prolonged wet cavity wall insulation has been demonstrated to activate and exasperate the corrosion process to some wall ties. In the case of insured perils such as flood, fire and escape of water, cases have been seen by CES Northwest where this has had a detrimental effect on the wall ties where cavity wall insulation has been left in-situ for many months following the peril exposing the insulation to large amounts of moisture.
The remedy for wall tie corrosion is to identify the positions of existing ties by way of a hand-held metal detector; install replacement remedial wall ties (Helical) in positions staggered from the originals at the appropriate tie density of 2.5 per sq. meter and treat the old ties to prevent any further damage from expansive corrosion; this is called isolation. Installing replacement ties alone will restore structural stability but will still leave the masonry prone to damage by expansive corrosion. Except in the case of some thin butterfly wire ties, remedial treatment of the existing ties is essential for a complete repair.
In the course of remedial work it is usually possible to bring older buildings up to modern standards with the use of lateral restraints to secure floors and roofs to the masonry. Repointing of the masonry will also enhance its stability and should be considered as part of the work.
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Cavity Extraction Specialists (NW) Ltd 779137 is an Appointed Representative of Improveasy Ltd., a company registered in England and Wales (Co. Reg. number 7807352). Registered Office: Manchester Business Park, 3000 Aviator Way, Manchester M22 5TG. The firm is authorised and regulated by the Financial Conduct Authority (FRN 708623). The firm is authorised as a Lender (limited only to green deal finance plans). It also acts as a Broker, not a Lender, and offers credit facilities from a panel/one lender. The credit is subject to application and status.