Bay window subsidence repairs

Victorian and Edwardian builders often gave the main walls deep footings but stood the bay on a shallow pad, sometimes little more than a few courses of brick. That shallow base sits in ground that dries, wets and is reached by roots and drain leaks. The bay then moves on its own, and a crack opens where it joins the house.

Not every cracked bay needs underpinning, and it is worth knowing that before spending money. Many junction cracks are old, stable and fixed with stainless ties and repointing. Where the bay is still dropping, its foundation is strengthened by traditional underpinning or mini-piles. An independent chartered structural engineer can confirm which applies, and we can arrange one. We then repair the bay and the plaster inside.

What are the signs you need bay window subsidence repairs?

• A vertical crack where the bay meets the front wall
• Bay roof or lead flashing pulling away from the main wall
• Bay window sash or casement sticking or binding
• A step or slope in the bay floor towards the window
• Cracks above the bay window head or below the sill
• Gaps opening around the internal bay window board

What causes it?

Shallow bay footings

Bays were frequently treated as an add-on, built on a narrower, shallower foundation than the house. In shrinkable clay that depth lies within the layer that changes with the seasons, so the bay moves while the main walls stay put. It is the most common reason bays crack.

Front garden trees and hedges

Street trees, front hedges and shrubs often grow within a few metres of the bay. Their roots draw moisture from clay beneath the shallow footing, causing it to shrink in dry summers. Take advice before removing anything, because felling on clay can lead to heave.

Drains and gullies near the bay

Rainwater downpipes, gullies and the main drain run often sit near the front corner of a house. A leaking joint or blocked gully can soften the ground beside the bay or wash fines away, letting the footing drop. A CCTV drain survey helps rule this in or out.

Lack of tie to the main wall

Many bays were simply butted against the front wall without bonding or ties. Even slight differential movement then opens a clean vertical joint. Sometimes the only real problem is this missing tie, with the bay foundation itself sound and stable.

How is it repaired?

How much will bay window subsidence repairs cost?

The biggest factor is whether the bay needs only a junction tie-in or a new foundation, and if so how deep firm ground lies. Two-storey bays, scaffolding, the window and internal finishes also affect the cost.

See regional prices on the prices page or work out a guide price for your home with the guide price tool.

What happens, step by step?

1. Photos and first view

   Send photos of the outside junction, the inside corners and the window, with a scale. We give a free initial assessment and tell you whether the crack looks old and stable or whether an engineer should inspect first.

2. Engineer and checks

   Where the cause is unclear, an independent chartered structural engineer inspects, sometimes with a trial pit at the bay and a drain survey. We can arrange the engineer. Monitoring may be suggested if the movement could be seasonal.

3. Agree the repair

   The engineer decides between tie-in repairs and underpinning. We confirm a fixed price in writing once the repair is specified, including scaffolding if needed, the bay window, roof flashing and internal making good.

4. Carry out the work

   Ties are fitted across the junction, or the bay is underpinned in sequence with building control inspecting the dig. The bay window and flashings are checked and eased or resealed where the movement has disturbed them.

5. Make good inside

   Plaster around the bay and along the junction is renewed, the window board is refixed and the room is ready for decoration. Building control paperwork, where it applies, is passed to you for your records.

How long will bay window subsidence repairs take?

A bay junction repair usually takes 1–2 days, while underpinning a bay is typically about a week on site.

Do you need building control or a party wall notice?

What if you have an insurance claim open?

A dropping bay is a frequent subsidence claim, and many buildings policies cover it subject to terms and the subsidence excess. Tell your insurer before permanent repairs start. They may ask for a drain survey, vegetation checks or monitoring before deciding. If the bay is repaired as part of a claim, we price to the loss adjuster’s or engineer’s schedule, and anything you add, such as replacement windows, is priced separately.

Bay window subsidence repairs: frequently asked questions