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Cracked Or Failing Lintel Above A Window
The beam over a window or door has cracked, sagged or dropped, and the brickwork above it has moved.
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What's happening?
A lintel carries the weight of everything above an opening and spreads it into the masonry either side. When it starts to fail, the load has to find another route, and the brickwork above tries to arch across the gap — which is what produces the classic signs: a sag in the courses over the window, a horizontal crack along the underside, diagonal cracks running up and out from both top corners, and sometimes a window frame that suddenly binds because the head is pressing on it. Older houses may have timber lintels that have decayed, shallow brick arches that have slipped, or steel that has rusted and expanded, splitting the surrounding brickwork as it goes.
What usually causes it?
- Rusting steel lintel or catnic expanding as it corrodes, forcing the brickwork above apart
- Timber lintel in an older wall decayed by long-term water ingress from a failed sill, gutter or pointing
- Brick or stone arch that has dropped because the mortar has perished or an abutment has moved
- Lintel with too little end bearing, so it is not sitting far enough onto the masonry either side
- Opening widened at some point without the lintel being upgraded for the longer span
- Overloading from above — a new roof covering, an extra storey, or a beam landing on the wall over the opening
- No lintel at all over an opening formed in the past, with the brickwork left to arch unaided
Is it dangerous?
Yes, and it is the most consequential common wall fault in a house. A lintel is holding masonry over a hole you walk under, and the failure mode is brickwork coming down. Approved Document A treats the support over openings as structural. Do not stand or work under a visibly sagging lintel, do not remove or open up anything below it, and keep the window shut so the frame stays as unintended support until a builder has looked. A rusting steel lintel can also be pushing a whole panel of brickwork out, which is a falling-masonry risk to the pavement or garden below.
Can I fix it myself?
No. There is nothing safe to do beyond gathering evidence and reducing what is wetting it. Photograph the sag and the cracks with a dated ruler, clear the gutter and downpipe above the opening, and stop water tracking onto the head of the window. Do not fill the crack, do not repoint over it, and do not take out the window — the frame may be carrying load it was never meant to, and removing it is how a bad lintel becomes a collapse.
When should you call a builder?
Call a builder promptly, and treat visible sagging in the courses above an opening, brickwork you can move by hand, or fresh cracks that are still growing as urgent. If masonry is loose above a public footway, deal with the falling risk first. Expect a structural engineer to be involved: replacing a lintel means propping the wall above it, and the propping design and the new lintel size are engineering decisions.
What will the builder actually do?
- Open up locally to identify what is actually over the opening — steel, timber, concrete, arch or nothing
- Assess the end bearings, the span and the load path, with a structural engineer where the replacement needs designing
- Needle and prop the masonry above the opening so the wall is supported before anything is removed
- Cut out and replace the failed lintel with the correct type, span and bearing, or repair a dropped arch
- Stitch and rebuild the cracked brickwork above and reinstate wall ties or cavity trays disturbed by the work
- Fix the water source — sill, flashing, pointing or gutter — so a timber or steel lintel is not wetted again
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