Avoid backfalling wall ties

This image shows an external cavity with the insulation removed because of damp penetration through the wall. If you look closely, the wall ties fall backwards to the internal leaf - allowing water to track across the cavity. This is because the mortar bed to the brick skin is too thick, resulting in the wall height of the three brick lifts being slightly greater than that of the single block.

This can sometimes be overcome by bending the ties upwards so that they sit in the top of the block mortar bed, but in this case the blockwork was built first - meaning the backfall worsened as the wall got higher. The cavity is also slightly too wide and the glass fibre insulation slabs leant towards the outside brick leaf, allowing water to track through the slabs onto the internal skin and leaving damp patches. The building is on an exposed site and so driving rain is a greater problem and damp staining was happening even before first fix.

Getting cavity wall ties right

• Incorrect use of cavity wall ties is one of the common problems found on site by our building control surveyors. Our guidance on how to use cavity wall ties correctly gives more information on getting the job right. Read it here.
• For ‘full fill’ cavity insulation it’s important for the insulation to completely fill the cavity with no air space between the insulation and the inner leaf. Gaps allow air to circulate and reduce the effectiveness of the insulation.

Further reading

Also read How to use cavity wall ties correctly

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.