WALL TIE INSTALLERS FEDERATION


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Wall Tie Replacement

Why a Cavity Wall?

Cavity walls incorporating metal ties have been in use since the 19th century. They are common in dwellings built in the early part of the 20th century and large numbers of such houses were built during the building boom of the 1930's. Since 1950 the cavity wall has 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'.

How do you recognise a cavity wall?

By looking at the pattern or "bond" of the bricks. If all the bricks are laid with the long edges ('stretchers') facing you, the wall is a cavity wall. If alternate bricks are laid with the short edge ('header') facing you, the wall is probably a solid wall; 'headers' are used in this way to increase the strength and stability of the masonry.

Of course, if the wall is rendered or plastered and the bond cannot be seen then the easiest way to test its construction is to drill a small hole through the outer face to a depth of some 5 inches. If no cavity is found in this way the wall is probably solid. This will be confirmed by checking the wall thickness at door openings or window reveals. A cavity wall measures 10 or 11 inches (depending on the cavity width) a solid wall 9 or 13 inches (depending on the number of bricks in depth.

How It Works

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 centres 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 chief 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.

The Wall Ties

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 galvanised or bitumen coated) and the galvanised 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.

Tie Corrosion

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 horizontal 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 light weight 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.

Remedial Treatment

The remedy for wall tie corrosion is straightforward: identify the positions of existing ties; install replacement ties 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. 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 butterfly wire ties, remedial treatment of the existing ties is essential for a complete repair.

Ancillary Work

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. Proprietary systems are available through most registered installers as are systems for bed joint reinforcement outlined in Building Research Establishment Good Building Guides 29 and 62.

Repointing of the masonry too will enhance its stability and should be considered as part of the work.