The British Standard for wall ties – BS1243 was introduced in 1945 and was a ‘prescriptive’ standard – ie all ties had to conform to the approved design of the time, and the ‘butterfly’, ‘double triangle’ and ‘fish-tail’ were born. For cost saving, these were made from galvanised mild steel and it was only in the late 1990s that manufacturers applied for a relaxation in the ‘design’ element of the standard (DD140), so they could produce a competitively priced wall tie in stainless steel. At this time, in the worst cases of exposure and aggressive mortars, some mild steel ties were found to be corroding at an alarming rate of between 13-26 years.
Over 30 years ago, Professor Malcom Hollis calculated that over 12m cavity wall houses in the UK could be affected by wall tie corrosion to galvanised mild steel ties and, as a result, the ‘Preservation Industry’ has developed various neat and effective solutions to replace these ties.
Initially, these were ‘mechanical expanding anchors’ to tie both leaves of the wall back together again, or ‘resin ties’ where resin was used to bond both ends of a stainless steel tie to the masonry. Lately, drive ties, where an 8mm diameter helical anchor is forced into a 6mm diameter hole, are the norm.
Modern housing design incorporates lateral restraints (metal straps that connect the masonry to the floors and roof) to prevent lateral outwards movement of the masonry.
This, however, was not a requirement in older properties and while undertaking wall tie surveys we often see minor bulging of walls, where the building has moved over time. This can be controlled by the use of 12mm diameter stainless steel rods inserted from the outside and connecting to the first three parallel running joists, or in the case where joists bear into the affected wall – into joist noggins. The floor diaphragm thereafter ‘ties’ the wall back to the floor.
Lateral restraints are normally inserted at around 900mm horizontal centres, and once installed and neatly pointed, the holes are barely visible from a couple of metres.
If the movement of a wall causes cracks then the cause of these must obviously be investigated and a cure implemented. The resultant cracked masonry or mortar joint can be stabilised and strengthened by the use of stainless steel helical bars, set into the mortar joint with a cementitious grout.
If you would like to know more about these services, visit our Specialist Expertise Page or call us free on 0808 273 2138.