Bay Window Flat Roof Repairs: What You Need to Know

The small flat roof over a bay window is one of the most leak-prone areas of any older UK house. Their small size and complex detailing — with multiple joints, flashings, and transitions — mean they require careful attention.

Why Bay Window Roofs Fail

• Original felt was often poorly installed on small, complex shapes
• The junction with the house wall above is a common failure point
• Lead flashings over these roofs often haven't been maintained for decades
• Small size means water doesn't drain well — even minor ponding is a high percentage of the roof area
• Foot traffic from cleaning windows accelerates wear

The Best Solution

We almost always recommend GRP fibreglass for bay window flat roof replacements. The seamless nature of GRP means there are no joints to fail on a small, complex shape. It can be moulded around the bay's facets and angle changes, and it's easy to detail at the junction with the wall above.

The Junction with the House Wall

The most critical detail is the junction between the flat roof and the brickwork above. The membrane must be turned up and terminated behind a lead or aluminium flashing dressed into a mortar joint. This flashing must be sealed to prevent water running behind it.

Cost

Bay window flat roof replacement in GRP is typically a half-day job. Costs range from £350–£700 depending on the size of the bay and complexity of detailing. It's usually a straightforward repair that provides lasting results.