External wall issues

Defective wall ties (1850-1920)

Cast iron and wrought iron wall ties in cavity walls were sometimes protected by bitumen and were designed to be quite thick. They tend to snap rather than corrode through, and a significant number need to fail before serious collapse is imminent. As they rust they expand, and horizontal cracks appear in the mortar courses. Aggregates such as black sand, fuel ash and iron filings could be added so that a dark mortar was produced. Some of these aggregates are aggressive to iron, which could lead to the decay of iron ties. This practice was mainly found in industrial towns. Houses in exposed coastal areas are also at risk from the phenomenon. Snapped header walls are not as problematic as their Georgian counterparts, but there is a link to problems with the use of iron wall ties that may rust within the wall.

Wall ties: 1970s

Galvanised wire 'butterfly' wall ties in the 1970s were known to have too thin a coat of zinc. Houses built in the early 1970s are particularly prone to wall tie failure. This includes timber-framed houses with their brick faces tied to the timber frame by galvanised ties. The possibility of failure should not be over-emphasised though, as in sheltered locations the wall ties can perform well within expected lifespans, and there would be no reason to believe that the wall ties should fail prematurely if inserted in walls in sheltered areas.

There is no known case of complete collapse in low-rise domestic properties. The loss of the zinc coating does not necessarily mean the tie will fail. Buildings constructed using black ash mortar and exposed to a harsh marine environment are particularly vulnerable. However, most local builders and building control departments are well aware of the danger of the combinations.

High exposed gable walls have been known to completely collapse. As many gables are affected by direct positive wind pressure as are to negative pressure caused by the vortex created on the leeward side of the building. Look for cracking at about 450 mm centres and isolate bowing. Wall movements can be best seen between stair strings and plaster finishes. A white 'rust' on twisted wire wall ties is the corrosion of the zinc and indicates that the coating is beginning to struggle. Red rust deposits are a sign of advanced decay and the wall tie will fail sooner or later. In 1981, BS 1243 was amended to increase the thickness of the zinc coating.

The principal remedy for wall tie failure is to remove the old ones and either stitch in or drill in new ones and fix them with resin or expanding anchors. Products such as Helix spiral replacement wall ties and Triton wall ties are typical examples. The type of remedy and tie used must be matched to the type of failure that has occurred. If the wall has failed due to expansion of the tie within the brickwork, or if both leaves are cracked, a structural engineer may have to be consulted to assess the best remedial action. Some wall tie manufactures only make one type of tie and may not advise on other types of remedial action.

The range of remedies is published in BRE Digest 329.