Wall Tie Replacements - Cavity Wall Ties

History & Background

Cavity walls incorporating metal ties have been in use since the beginning of the 19th century, but it was the 1930’s building boom that produced large numbers of cavity construction dwellings. From 1945 cavity wall construction became the normal for houses and many other buildings. The early metal ties produced mainly from wrought or cast iron performed satisfactorily, but in more recent yearsmild steel ties have been used and despite galvanising or bitumastic treatments have been found to suffer from corrosion. A British Standard was produced in 1945. 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.

General

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 therefore measures must be taken to restrict access to the risk areas if severe wall tie failure is diagnosed.

Symptom

Horizontal cracking at regular intervals. Normally every five to seven courses and frequently evidenced by re-pointing, resulting in wide joints. The metal detector will confirm the presence of wall ties along these joints. Outward bulging of brickwork may indicate wall tie corrosion. The inner leaf of the wall being more firmly tied into the construction is less likely to move, with the rust lamination of the wall ties, than he outer leaf. This outer leaf may lift at the horizontal cracks, and/or be forced outwards. A pagoda roof effect may be seen at gable ends where the height expansion of a gable wall has caused the roof to lift. Misalignment of slates at the roof verge is an indicator. Lifting or sagging of lintels and cills can indicate wall tie corrosion.

Internal Defects

Windows reveal shows positive movement between the frame ad the plaster, or uneve depth of reveal where gaps have been filled prior to decoration. Cracked comers are visible at the point internal walls abut eternal walls. Watch for outward movement of the outer wall that has been camouflaged by decoration, but a spirit level will confirm the lean of the outside wall. Cracking may be visible at the wall ceiling joint. Stair strings and skirting boards may separate from the walls. A tell tale paint line on the floor boards may indicate the skirting board has moved with the wall and not necessarily separated. In extreme cases horizontal cracking may be visible, or decorations may show ridges at 450mm intervals.

Summary

It is observation and careful use of an endoscope that will complete the story. Removal of a brick will confirm the state of the tie and give the opportunity for a more thorough cavity inspection.

Inspection of the Wall Tie

Requiring a specialist Dampshield survey would involve a hole being drilled and/or to the side of at least six ties for an inspection using an endoscope to establish the type and condition. Viewing through the endoscope will not always indicate the extent of the corrosion that can oly be assessed by visual inspection.



Cavity Tie Replacement

Dampshield offer a range of wall designs from resin ties, auto torque mechanical ties. Replacement ties are available in various lengths to suit all cavity widths. The replacement ties incorporate a centre drip to prevent transmission of water across the cavity. On completion of work a wall tie guarantee is available.