Unsupported 900mm opening in cavity wall
- Thread starter petend
- Start date
It depends largely on the strength of the mortar, and also the care which has been taken to make the opening.
Modern cement mortar can have good adhesion to the brick and may well hold up the triangle of brickwork across the opening.
Older, lime mortars are not so reliable in this respect.
It's like 'how long is a piece of string'?
Modern cement mortar can have good adhesion to the brick and may well hold up the triangle of brickwork across the opening.
Older, lime mortars are not so reliable in this respect.
It's like 'how long is a piece of string'?
- Joined
- 24 Oct 2025
- Messages
- 31
- Reaction score
- 12
- Country
It won’t stay up for long. Five courses over a 900mm opening is already risky without support. The blocks can crack or drop at any time, even while you’re working. Get a lintel or at least a strong prop in before going any further.
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