Bay window movement: causes, signs and repairs
Bay windows crack and move more than the rest of a house because they usually sit on shallower foundations, carry less weight and catch the worst of the ground's seasonal movement. Most cases are repairable, but the right fix depends on whether the cause is below ground or above it.
Why do bay windows move?
On many Victorian and Edwardian houses, the bay was built as a lightly loaded projection on its own shallow footing, often not tied in well to the main front wall. On London Clay, that combination makes it the part of the house most likely to move. The clay shrinks in dry summers and swells in wet winters, and a light, shallow bay rises and falls more than the heavier main walls beside it.
Bays also tend to sit close to front garden trees, street trees, gullies and rainwater pipes, all of which change the moisture in the ground. Above ground, rotten timbers, failing lintels and poorly supported replacement windows can cause cracking that looks similar but has nothing to do with the foundations.
Signs to look for
Bay movement usually shows first at the junction between the bay and the main house, inside and out. Note where the cracks are and whether they change between summer and winter, as that helps the engineer separate ground movement from other causes.
- A vertical or tapering crack where the bay meets the main front wall
- Cracks above the bay windows, or a sagging line in the brickwork over the openings
- Gaps opening between the bay ceiling and the main ceiling inside
- Sash windows in the bay that stick or have dropped in their frames
- The bay roof or its lead flashing pulling away from the front wall
- Cracked or tilting front steps and paths beside the bay
Causes below ground
Clay shrinkage driven by tree roots is the most common ground cause in London. A tree in the front garden or on the pavement can draw moisture from beneath a shallow bay footing, and the cracks typically open in late summer and partly close again over winter.
Leaking drains are the other usual suspect. The gully and rainwater pipe are often right beside the bay, and a cracked pipe can wash fine material out of the ground or soften the clay under the footing. Paving over a front garden can also change how water reaches the ground. Why these effects are so strong on clay is covered in our London Clay and trees guide.
Timing gives useful clues. Ground movement from trees tends to follow the seasons, with cracks widening after a dry summer. Movement from a leaking drain is less seasonal and can continue steadily all year. Heave, where clay swells after a large tree near the bay has been felled, can lift the bay over several years and is easily mistaken for something else.
Causes above ground
Not all bay cracking is subsidence. Many bays are spanned by a timber bressummer or lintel over the windows, hidden behind brickwork or render. If water has got in for decades, that timber can rot and sag, and the brickwork above cracks and drops. Some bays have brick arches or iron lintels that have failed in a similar way.
Replacement windows are another common cause. In some timber bays, the original window frames and mullions carry part of the load from above. When they are replaced with frames that are not designed to carry load, and no new lintel is installed, the structure above can settle onto the new windows. The bay roof itself can also spread or rot, pushing the walls outward.
How an engineer investigates
The engineer starts by mapping the cracks, checking levels and verticality, and looking at the bay roof, lintels and windows. That often shows whether the problem is in the ground, in the structure above, or both. Where the ground is suspected, the next steps are usually a drain survey and a trial pit beside the bay to see the footing and the soil.
If it is not clear whether the bay is still moving, the engineer may recommend crack monitoring over several months, ideally covering a summer. Our crack monitoring baseline report sets up the reference readings, and later visits show whether the cracks are opening, closing or stable. That record is valuable if you need to make an insurance claim.
Repair options
The repair follows the cause. Fixing the cause first and the cracks second avoids repairs that simply reopen.
Partial underpinning is common for bays because the bay is often the only part of the house on a shallow footing. Done well, it brings the bay down to the same level of support as the main walls. Our guide on what underpinning involves covers the methods and the risks at the junction with the main house.
- Repair leaking drains and gullies, then monitor before structural work
- Manage or remove trees where it is safe and permitted, taking advice on heave
- Replace a rotten bressummer or failed lintel with a new steel or concrete lintel
- Tie the bay back to the main front wall with stainless steel helical bars
- Stitch cracked brickwork with helical bars set into the mortar joints
- Underpin the bay where ground movement continues after the cause is dealt with
Replacing bay windows safely
If you are planning new windows in a bay, find out what the existing frames are doing before they come out. A competent window installer will prop the structure above during the work, but they cannot know whether the new frames need a lintel above them unless someone has checked. A short engineer's visit can confirm the load path and specify a lintel if needed.
The same applies to removing a bay entirely or opening it up to a larger window. That is a structural alteration and needs Building Control approval. If the bay has already moved, a report before the windows go in gives you a clear record of its condition, and ours carry the sign-off of a chartered engineer with MICE membership (Member of the Institution of Civil Engineers).
Frequently asked questions
Is a crack between my bay window and the house serious?
It is common and often not serious, but it is worth understanding. A fine crack that opens slightly in summer and closes in winter usually reflects seasonal clay movement. A crack that keeps widening, is wider than a few millimetres, or comes with sticking windows and a leaning bay should be inspected. An engineer can tell you whether it needs monitoring, a simple repair or more.
Will my insurer cover bay window subsidence?
Many buildings policies cover subsidence, and bay movement caused by tree roots or leaking drains is a common type of claim in London. Policies usually have a higher excess for subsidence, and insurers will want evidence of the cause. Rot in a timber lintel or problems from replacement windows are generally treated as maintenance rather than insured damage. Check your policy and speak to the insurer early.
Does a moving bay always need underpinning?
No. If the cause is a leaking drain, repairing it may be enough once the ground settles. If a tree is responsible, managing it can stabilise the bay, although removing large trees on clay needs care because of heave. Underpinning is generally kept for bays that keep moving after the cause has been dealt with, or where the footing is plainly inadequate.
Can I remove my bay window altogether?
Sometimes, but it is a structural alteration. The front wall above may need a new beam, the opening has to be rebuilt, and in many streets the council will need to grant planning permission because the front elevation changes. In conservation areas, removing an original bay is often resisted. The engineer can design the structural side, but check planning before committing to the work.
Who should I call first: a builder or an engineer?
If the bay is cracking or moving, call an engineer first. A builder can repair the symptoms, but without knowing the cause the cracks may come back. An engineer's inspection identifies whether the problem is in the ground, the lintel or the windows, and sets out what the builder should do. A Specific Structural Inspection report is designed for exactly this kind of single defined concern.