As part of a pre-mortgage inspection the lender often requests that a wall tie inspection is undertaken by a member of the PCA to ensure there are no structural issues.
Wall ties are a means of connecting the inner and external leaf of a cavity wall together, providing support and preventing deflection due to the effects of the wind.
This property was located in south Manchester, a typical semi-detached constructed of a 275mm cavity wall construction, with a dash cement render to the front elevation. It was established that the property was indeed affected by, wall tie corrosion and required replacement wall ties. The report was accepted by the lender and the client returned the contract for the works to be undertaken, so the retention held against the property could be released.
Works were started as normal working from the rear of the property, back towards the front it was on the front elevation that problems were encountered. We were aware of the cracked brickwork to the arch over the front door. This had been confirmed as historical movement so a crack stitching specification had been included as part of the wall ties works.
On drilling the external brickwork to install the wall ties to the front elevation, the brickwork started to move, this creates a problem because a wall tie is only as strong as the cement, which bonds the bricks together, in this case the mortar was Black ash based, which is made from the waste slag of power stations and when this become damp, this results in a weak acid forming, which attacks any metal embedded within it, in this case the old wall ties corrode and the mortar had become weak.
Normally in this situation the recommended method of repair, is to take down the external leaf of brickwork and reconstruct off new OPC mortar, with new resin wall ties fixed into the inner leaf. In this case due to the ornate construction of the archway and dash render installed to the upper elevations of two properties and the Bow bay window construction, this was not really an option.
Luckily as members of the PCA we have a wealth of technical information and experience we can tap into, when unusual issues like this occur, following consultation with the structural engineers and conformation from the lenders, a course of action for the best remedial system was established.
It was decided that the method of dropping and rebuilding the external elevation was, both too expensive and would also result in a loss of character to the property.
The method of stabilisation decided on, was to deep grind all the mortar beds of the lower half of the front elevation to a minimum depth of 30mm, the joints were washed clean and then repointed using a power pointing system installing an SBR strengthened cement, which would ensure the joints were filled to their full depth and any voids within the frogs of the brickwork would be filled and bond to the brickwork, so stabilising the outer wall construction. The engineers deemed that the installed dash render, which was in a good condition would provide adequate support to the wall structure, from the effects of the wind suction, due to the location of the property, and sheltered location.
To provide further support. along the largest expanse of brickwork and also to correct the cracking defect, then Retro-installation of bed joint reinforcement in masonry to BRE GBG 62 would be undertaken using Thorhelical crack stitching systems.
Click the links below to download this information in a PDF format.
How to fix a wobbaly wall
Crack Stitching a Cavity Wall