Wall Tie Spacing on a Cavity Wall with a Wide Leaf Gap
You are laying a 102.5 mm outer leaf of facing brick against a 100 mm inner blockwork leaf. The cavity was specified as 50 mm, but on site the inner leaf is set back further than drawn, and your string line plus a straightedge show the actual clear cavity is a uniform 75 mm at the base and 80 mm at the top of the first lift. The specification calls for wall ties at 900 mm horizontal centres and 450 mm vertical centres, with a maximum of 5 ties per square metre. Your tie stock is a type with a 200 mm overall length and a 25 mm safety end embedment. The bricklayer next to you says the ties will still reach and wants to carry on.
1. Using the measured cavity, decide whether the supplied 200 mm ties are suitable, and state the maximum cavity the tie can serve.
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Work from the embedment requirement. A tie must have a set minimum embedment in each leaf; here that is 25 mm at the safety end. The tie length must cover: outer leaf embedment + actual cavity + inner leaf embedment.
- Required length at 75 mm cavity = 25 + 75 + 25 = 125 mm minimum, so a 200 mm tie is long enough in pure length terms.
- But the controlling issue is not just length: the tie must also be installed with a slight fall to the outer leaf and must not be bedded so deep that the drip is buried. Check the drip position: the drip must sit in the clear cavity, not inside the mortar.
- At 80 mm cavity at the top of the lift, the tie still spans it, but the cavity exceeds the 50 mm design value by 30 mm, which is a 60% increase. That is a design change, not a bricklaying decision.
Conclusion: the ties physically reach, but a 75–80 mm cavity is outside the specified 50 mm and outside typical tie/insulation detailing. Stop, record the measurement, and refer to the designer. Do not simply carry on as the adjacent bricklayer suggests.
2. The supervisor tells you to continue and just 'push the ties in further'. Explain why this is unsafe and what you should do instead.
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Pushing ties in further removes the safety end embedment in the inner leaf and can bury the drip, so water crosses the cavity. It also changes the tie's effective embedment and its ability to transfer wind load, and it may compromise the insulation if insulation is later inserted.
The correct actions:
- Record the actual cavity at several heights, with a straightedge and tape, and photograph it.
- Stop laying the affected lift so the defect is not built over.
- Refer the deviation to the designer or site engineer for a revised detail - either a longer tie type, a revised cavity, or a re-set inner leaf.
- Only proceed once you have a written instruction, and keep the record with the site paperwork.
A bricklayer can refuse to build a detail that departs from the specification; the trade judgement is recognising that this is a design query, not a 'make it fit' problem.