Bay windows are one of the most common places for subsidence cracking in older Liverpool homes, because many Victorian, Edwardian and inter-war bays were built on shallower footings than the main walls. A crack at the junction between the bay and the house, or a bay that seems to lean away, does not automatically mean underpinning. A chartered structural engineer first works out why the bay has moved; the repair is then chosen to suit that cause, and may range from drain repairs and crack stitching to partial underpinning of the bay alone.
Why do bay windows move more than the rest of the house?
A bay is a small, lightly loaded projection bolted on to a much heavier wall. Builders often gave it a shallower footing than the main house, and sometimes much less. That shallow footing sits in the zone where clay soils dry and swell with the seasons, where roots from front-garden trees and hedges reach, and where water from a cracked gully, downpipe or front drain run soaks in. The main wall, deeper and heavier, may barely notice the same conditions.
There is also the structure above the foundation. Many bays rely on a timber or steel lintel, or a bressummer beam, over the window openings. If that has rotted or corroded, the brickwork above can sag and crack in a way that looks very similar to subsidence. Telling the two apart is part of the engineer’s job, and it changes the repair completely. Where the problem is the lintel rather than the ground, our lintel replacement work is the relevant service.
What signs point to bay window subsidence?
- a crack at one or both junctions where the bay meets the front wall, often wider at the top;
- diagonal cracking below or beside the bay windowsills;
- the bay roof or lead flashing pulling away from the main wall;
- internal cracks in the bay corners, or a gap opening between skirting and floor in the bay;
- sash or casement windows in the bay starting to stick or rattle;
- a crack that runs down through the damp-proof course into the brickwork below ground.
Several of these are the classic signs RICS lists for subsidence, and cracks that run below the damp-proof course are often linked to foundation movement. But none of them is a diagnosis on its own. RICS says cracks that appear suddenly and are wider than 3 mm are the main cause for concern, and BRE guidance stresses that whether a crack is getting wider over time matters as much as its width today.
If you suspect subsidence, contact your buildings insurer before arranging investigation or repairs. On a claim, the insurer and its loss adjuster decide on investigation, monitoring and the repair method, including whether underpinning is needed. Please don’t fill and paint over bay cracks in the meantime: the record of how they change is useful evidence.
How is the cause of bay window movement found?
The engineer looks at the pattern of cracking inside and out, the age and construction of the bay, nearby trees and shrubs, and the drainage at the front of the house. Investigation might include a trial pit beside the bay to see how deep the footing actually is and what it sits on, a CCTV survey of the front drain and gullies, and soil tests if clay and vegetation are suspected. If it is not clear whether movement is still happening, crack and movement monitoring over several months, often around a year on clay, separates seasonal opening and closing from steady progression.
What are the repair options for a subsiding bay?
| Cause found | Typical direction of repair (engineer decides) |
|---|---|
| Leaking front drain or gully | Repair or reline the drain, allow the ground to recover, then repair cracks |
| Tree or shrub roots on clay | Vegetation management on arboricultural and engineering advice, monitoring, then crack repair; underpinning only if the cause cannot be controlled |
| Very shallow or poor bay footing | Partial underpinning of the bay, by mass concrete, mini-piles or occasionally resin |
| Failed lintel or bressummer | Lintel replacement with temporary support and rebuilding of affected brickwork |
| Historic movement, now stable | Crack repair, for example helical bar stitching, and redecoration |
Once the cause has been dealt with, structural crack repair restores the masonry, usually with stainless steel helical bars bonded across the cracks and repointing to match. Our wider subsidence repairs page explains how cause removal and repair fit together for the whole house.
What is partial underpinning of a bay, and what is the catch?
Partial underpinning means deepening or supporting only the bay foundation rather than the whole front wall. It can be a sensible, proportionate answer where the bay footing is the weak point. The catch is the “hard spot”: once the bay is on a deeper, stiffer foundation, it can behave differently from the shallower main wall beside it, and movement can concentrate at the junction. The engineer has to design for that, sometimes by extending the underpinning a short distance along the main wall, sometimes by detailing a controlled joint. It is one more reason underpinning a bay is never a standard kit.
If the bay is attached to next door’s, as on many semi-detached and terraced fronts, the Party Wall etc. Act 1996 usually needs to be considered before digging near the shared foundations.
Which Liverpool homes have this problem?
Bay windows are a feature of huge numbers of Merseyside houses. Victorian and Edwardian terraces and villas in Aigburth and Wavertree, and inter-war semis in Allerton and many other suburbs, commonly have bays on the front elevation, often near street trees or front-garden planting. That is why bay cracking is one of the most frequent reasons people get in touch with us about subsidence repair in Liverpool.
What affects the cost of a bay window repair?
Because the repair depends on the cause, the cost does too. Drain repairs and crack stitching are very different jobs from partial underpinning. Factors include the investigation and any monitoring needed, the depth to suitable ground, the method the engineer chooses, whether a lintel also needs replacing, Building Control and party wall involvement, access at the front of the house, and making good the window reveals, plaster and decoration. We give a written quote for the engineer’s scheme once the design is known.
Crack between your bay and the house?
Call 07777 659786 or use the form. Tell us where the cracks are, roughly how wide, whether they are new or getting bigger, and whether there are trees or drains near the bay. We will call back, talk it through and arrange a visit, with an engineer’s inspection where the cause is not yet clear. More on our underpinning and subsidence specialists in Merseyside is on the homepage.
Frequently asked questions
Is a crack between my bay window and the wall subsidence?
It might be, but it can also come from a failed lintel, old settlement that has long stopped, or thermal movement. A crack that is new, growing, wider at the top or running below the damp-proof course is worth having inspected by a structural engineer. Our subsidence and cracks guide explains what to record.
Does a subsiding bay always need underpinning?
No. Many bay problems are solved by repairing a leaking drain or managing nearby vegetation, letting the ground recover and then repairing the cracks. Partial underpinning is considered when the bay footing itself is inadequate or the cause cannot be removed. The engineer, or the insurer on a claim, decides.
Will my insurance cover bay window subsidence?
Buildings insurance usually covers subsidence damage to the home, according to the ABI, but every policy is different and a subsidence excess normally applies. Tell your insurer as soon as you suspect it. The insurer decides whether a claim is accepted and how the repair is handled.
Can a bay window be rebuilt instead of underpinned?
Sometimes, where the bay brickwork is badly distorted, the engineer may specify taking down and rebuilding it on a new or underpinned foundation. Whether that is needed depends on the condition of the masonry, the cause of movement and the foundation design. It is an engineering decision, not a cosmetic choice.
Should I sell a house with a cracked bay window?
Many houses with bay cracking sell, but buyers’ surveyors will spot it. Getting a structural engineer’s report, and evidence of any repair and sign-off, gives buyers and lenders something firm to rely on. Never hide past movement from an insurer, buyer or lender.