Repair or replace? How to tell which conversation you are having

Age is a weak signal

Roofs are not replaced on a schedule. A slate roof can be a century old and have decades left, because the slate is barely worn and only the fixings have aged. A twenty-year-old roof laid badly can be finished. What matters is condition, and specifically the condition of four things.

One: how much of the covering has failed

A handful of slipped or cracked units in twenty years is maintenance. A dozen in two winters, with more visible from the ground, is a process rather than an event — and each one that slips exposes the one below it. That accelerating pattern is the clearest single indicator that repair has stopped being economic.

Two: the fixings

On slate roofs the usual failure is nail sickness: the nails corrode and slates begin to release. It happens across the whole roof at roughly the same rate, because they were all fitted at the same time. Once it starts you are paying repeatedly for the same problem to keep occurring somewhere new.

Three: battens and underlay

Many older London roofs have no underlay at all — the covering is the only barrier. Where there is felt, it perishes, and the tell is black rubbery fragments in the loft. Battens that are soft or split will not hold a new fixing. Neither can be replaced without stripping the covering off, which is what turns a repair into a re-roof.

Four: what the timber has been doing

Rafters and wall plates that have been getting wet for years are a different order of problem. That is the thing worth finding before it needs structural work rather than roofing work.

The arithmetic

Repairs are cheap individually and expensive in aggregate. Three repairs at a few hundred pounds each, plus access each time, plus the internal redecoration each time, adds up quickly against the cost of doing the job once. Access is the part people forget: it is often the largest line on a repair and it is paid again every visit.

The other half of the arithmetic is time. If you are selling in two years, a competent repair may be exactly right. If you are staying for twenty, spending on repairs to a roof that is going to need replacing anyway is spending twice.

What to ask for

Ask for the assessment in writing with photographs, and ask specifically: what proportion of the covering is at or past the end of its life, what condition are the battens and underlay in, and what would you expect to be repairing again within five years. A roofer who will answer those three questions in writing is telling you something useful. One who says only "it needs doing" is not.

What a roof inspection covers.