CAVITY WALL TIE CORROSION

Cavity walls incorporating metal ties have been in use since the beginning of the nineteenth century .These ties hold the outer leaf of the wall to the inner leaf. The early metal ties produced mainly from wrought or cast iron performed satisfactorily but in more recent years mild steel ties have been used and despite galvanising or bitumastic treatments have been found to suffer from corrosion.

A number of cases of distress of cavity walls, attributable to wall tie failure, have occurred in the United Kingdom and it is now clear that the problem could eventually affect all the cavity walled structures built before 1981, some 10 million dwellings, and will not in future be confined only to cases of poorly made ties.

Wall tie failures reported to the Building Research Establishment (BRE) include inferior coatings of bitumen, insufficient zinc galvanising on mild steel, aggressive mortars (particularly black ash), exposure to marine climates and permeable mortars such as lime that permits rapid carbonation.

Once the steel of the wall tie is exposed to air and oxygen the all too familiar rusting cycle begins. The rate of corrosion will be governed by site conditions but in severely exposed locations the life of the twist tie could be reduced to ten years. It was estimated in the 1986 Survey of English Houses that around 12 million properties exist with cavity walls of which the number requiring some repairs to wall structures is approaching one million.

It must be stressed that wall tie failure, or cracking of the outer leaf of a domestic property is rarely an immediate threat to the safety of the occupants. The risk is greater to pedestrians and adjacent property and measures must be taken to restrict access to the risk areas if severe wall tie failure is diagnosed.

The installation of wall ties in an existing property may be needed to:

.• Stabilise a cavity wall damaged by corrosion of vertical steel twist ties.
• Stabilise a cavity wall rendered unsafe by the loss of wire ties by corrosion.
• Increase the number of walls ties in a cavity wall constructed with insufficient ties.
• Tie back existing cladding walls to concrete, steel or timber frame structures.
• Stabilise two parallel single-leaf walls with space between not exceeding 25mm tied together with solid mortar, where the two leaves are separating.
• Tie new walls or bulging walls back to existing cross walls.
• Tie walls either side of cuts made for insertion of openings or movement joints.

Wall Tanking

Unused areas of properties (basements, vaulted cellars, coal stores) can be converted into part of the living accommodation or additional offices/store rooms if part of a commercial property.

Once having carried out a survey we would supply a detailed report and specification for the method of tanking/dry lining that is appropriate.

Even if running water is visible to wall surfaces or standing water is present on floor surfaces a suitable tanking/dry lining system can be recommended.