Repair or Replace: What Is Best for an Ageing Flat Roof?

The Small Flat Roofs on a Bebington House

The inter-war semis that dominate the area come with a set of small flat roofs that people forget they have.

The bay window roof. A small flat or shallow-pitched roof over the front bay, with its own upstand against the wall, its own lead or felt, and its own tiny drainage arrangement. Almost nobody looks at it.

The porch canopy.

The garage roof, either attached or at the end of the drive.

The rear extension added at some point since.

Each is a few square metres. Together they are a significant part of the roof area of the house, and they are the parts that leak.

Why the Usual Arithmetic Does Not Apply

On a large flat roof, materials and labour on the covering are the bulk of the cost, and a repair is genuinely much cheaper than a replacement.

On a bay roof of two or three square metres, that is not true. The covering material costs very little. What costs money is somebody getting there safely, working at height above a public-facing elevation, and setting up and clearing away.

So the gap between "patch the bay roof" and "recover the bay roof" is much narrower than the gap between the areas involved suggests - and a patch that buys two years is poor value when a full recover buys twenty for not much more.

That is the single most useful thing to understand about small flat roofs, and it is the opposite of the advice that applies to big ones.

The Bay Roof Specifically

It deserves its own paragraph because of what it damages.

A leaking bay roof does not just stain a ceiling. Water gets into the top of the bay structure - which on these houses is often timber-framed, sometimes with rendered or tile-hung panels - and into the head of the window frame below.

By the time it shows inside as a stain on the bay ceiling, it has usually been in the structure for a while. Left longer, the repair stops being roofing and starts being joinery and rendering.

The lead or covering on these is frequently original, which means it is around ninety years old. Ninety-year-old lead may well be fine - lead does not perish - but ninety-year-old felt is not, and ninety-year-old detailing at the upstand against the wall has usually had a few applications of mastic in the meantime.

It is a small job when caught early and it is a genuinely disproportionate one when not.

Do Them Together

The corollary of access dominating the cost: do the small flat roofs at the same time as each other, or at the same time as something else.

A bay roof and a porch canopy done in one visit costs meaningfully less than two visits. A garage roof done while a scaffold is up for a re-roof, gutters or render is cheaper again.

And if the fascias, soffits and guttering are due, the flat roof edges that discharge into them should be done at the same time - the drip edge and the gutter are the same detail and doing one without the other is a missed opportunity.

It is worth asking us what else on the building is at a similar stage. On a house of this age the answer is frequently "the hips and the bay roof", and those two together is one visit rather than two.

When Repair Is Still Right

None of the above means never repair.

A specific, identifiable fault on a covering with life in it - a lifted trim, a failed upstand flashing, a blocked outlet - is a repair whatever the size of the roof.

Original lead on a bay roof that has simply come loose at the upstand is a repair, and a good one: the lead itself is probably fine and re-dressing it properly is far better than replacing it with felt.

And if the roof is going to be altered - an extension planned, a bay rebuilt, a garage converted - a repair that holds until then is the right call rather than the lazy one.

What to avoid is the third patch. If a small flat roof has been patched twice, the access has now been paid for twice and the covering is still old.