Cavity wall tie replacement
Cavity wall tie replacement information
It must be stressed that wall tie failure or cracking of the outer leaf of a domestic property is rarely an immediate threat to safety of the occupants. This risk is greater to pedestrians and adjacent property and measures must be taken to restrict access to the risk areas if serve wall tie failure is diagnosed.
Wind suction in gale conditions is the normal cause of collapse of the outer leaf walls with the gable top being the most vulnerable area. Attention should be given to narrow unreturned walls such as those between openings and to the very large areas of walling such as gables. With the increase in a wall height by rust lamination of wall ties it is possible that the load bearing characteristics of a structure may change. The outer leaf of a wall may become load bearing with the resulting distortion of a roof structure.
The installation of cavity wall ties in an existing property may be needed to:-
1. Stabilise a cavity wall damaged by corrosion of vertical steel twist ties
2. Stabilise a cavity wall rendered unsafe by the loss of wire wall ties by corrosion.
3. Increase the number of wall ties in a cavity walls constructed with insufficient ties.
4. Tie back existing cladding walls to concrete, steel or timber frame structures.
5. Stabilise two parallel single leaf walls with space between not exceeding 25mm, tied together with solid mortar, where leaves are separating.
6. Tie new walls or bulging walls back to existing cross walls.
7. Tie wall s either side of cuts made for insertion of openings or movement joins.
Cavity wall tie replacement types
1. Cast and wrought iron wall ties are usually of heavy section and cannot easily be cropped or bent back and must be removed.
2. Stone and slate wall ties have poor mechanical grip and are likely to snap with movement. Slate and ceramic wall ties do not corrode or laminate and there fore need not be disturbed.
3. Brick and terracotta wall ties act as a cold bridge and transmit moisture and are likely to perish.
4. Metal ties have been made from cast iron, wrought iron, mild steel, copper and more recently stainless steel. Although the design is more practical than their predecessors most have the inherent enemy rust despite being galvanised or bitumastic coated.
Mild steel fish tail wall ties require to be isolated or removed from the mortar joint.
Butterfly wall ties are either heavy or light weight and only light weight wall ties in 12mm or thicker mortar joints can be left undisturbed. Heavy duty wall ties and all wall ties in mortar joints less than 12mm must be isolated or removed.
Copper wall ties are not found frequently; they are durable and have a long life.
Cavity wall tie replacement. What are signs of wall tie failure?
External wall tie defects
- Horizontal cracking at regular intervals, normally every five to seven courses and frequently this will be 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 the 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 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 sills can indicate wall tie corrosion.
Internal wall tie defects
- Window reveals show positive movement between the frames and the plaster work or uneven depth of reveal where gaps have been filled prior to decoration.
- Cracked corners are visible at the point internal walls abut external 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 he wall ceiling joint.
- Stair strings and skirting boards may separate from the walls. A tell-tale paint line on the floor boards may indicate 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.
Wall Tie Corrosion, indications and reasons for cavity wall tie failure
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