Repair or replace: how to tell if a roof is finished

A slate roof laid in 1890 can be sound. A concrete tile roof laid in 1974 can be finished. Age only gives you a probability: Welsh slate outlasts a century, clay tiles sixty to a hundred years, concrete interlocking tiles forty to sixty, torch-on felt fifteen to twenty-five.

What actually fails first is almost never the tile. It is the nails, the battens, the mortar and the lead. So the question worth answering is not how old the roof is. It is how much of it has stopped working, and whether the parts you cannot see from the ground have gone with it.

Rule of thumb, and it is only that: if the failures are spread across more than about a third of a single slope, stripping and re-laying that slope costs less over ten years than repairing it again and again. Under a third, with sound battens behind, repair it.

The rule works because roof repairs do not scale the way people expect. Most of the cost of a small repair is getting someone safely to the spot — the ladder, the tower, the scaffold, sometimes a parking suspension on a narrow street. Doubling the number of slates barely moves the price. Coming back next winter means paying for the access again.

Note the wording: a slope, not a roof. Terraces and semis often have one slope taking every south-westerly gale while the other side sits sheltered and untouched. Re-roofing one slope and repairing the other is a normal job. It is worth asking for before you accept a price for the whole thing.

Every nail on a slope went in the same week, out of the same keg, and has had the same winters since. So when the fixings start letting go they do it together, and the slates that arrive in the garden are perfectly sound — pick one off the path and it is still hard and still rings when you tap it.

That changes the arithmetic rather than the diagnosis. Re-hanging one slate drives a fresh nail into a batten of the same vintage, so each visit buys a season and books the next. Enough of them and you have paid for a stripped slope and been given a patchwork instead. The question to put to whoever is quoting is not what is wrong with this slate; it is how many more of these they expect you to be buying.

Concrete tile roofs fail differently. Concrete absorbs water, so after forty-odd years the tiles are heavier than the day they were laid, more porous, and gritty where the cement face has weathered off. A slope that dips between the rafters is usually battens giving way under that extra weight rather than the rafters themselves.

Lift a slate and you should see a batten with clean timber around the nail, and behind it a membrane that is still flexible. That is a roof worth repairing.

Old bitumen sarking felt — broadly anything laid before the late 1990s — goes brittle. It cracks along the batten lines, tears at the nail holes, then sags between rafters and holds a puddle instead of shedding it towards the gutter. Torn felt rarely leaks by itself, but it removes the second line of defence, so every future slipped slate becomes a wet ceiling rather than a damp batten.

Battens fail at the nail. Water tracks down the shank, the timber softens around the hole, and the nail pulls through. Where a batten crumbles as a slate nail is drawn, nothing can be re-hung on it reliably, and no amount of care makes that repair last. This is also why nobody can give you an honest answer over the phone or from the pavement. A batten cannot be seen from the ground.

A torch and ten minutes on a dry day tells you most of it. Stand on the joists rather than the plasterboard, and do not go up at all if the hatch, the ladder or the boarding is poor.

Scaffold is what decides it. It is paid for every time someone needs to be up there rather than once. Repeated repair visits, each with its own access, add up to the price of a stripped and re-laid slope — without the decades that a new slope comes with.

So the crossover is not really about the price of a repair. It is about how many repairs you are honestly buying. One repair that fixes a known defect on a sound slope is excellent value. A fourth repair on a slope with nail fatigue is not, and anyone quoting for it without saying so is taking your money.

One more thing when you compare quotes: check whether each figure includes VAT. It is the commonest reason two prices look further apart than they are.

Most roofs are not finished, and a quote that opens with a new roof before anyone has lifted a slate deserves a second opinion. These are the cases where repair is plainly correct.

Stripping a slope is not simply new slates. It is new treated battens at the correct gauge, a breathable membrane in place of the old bitumen felt, fresh fixings throughout, and usually mechanically fixed dry ridge and verge rather than mortar that will crack again in fifteen years. Lead at a chimney or an abutment gets renewed while the scaffold is already standing, which is far cheaper than returning for it later.

It is also the only moment the roof is open. Insulation at rafter level, or a roof window, happens now or not for another few decades.

One piece of paperwork matters. The threshold is a quarter: past that, re-covering is notifiable building work, and past a half Part L treats the roof as a renovated thermal element, so the insulation has to come up to the current standard as part of the job — a real cost that belongs in the quote rather than being discovered halfway through. Whoever does the work either self-certifies under a competent person scheme or notifies building control before starting. We are not on the Competent Roofer scheme, so ours goes to the borough and the completion certificate comes back from them. Keep it; a buyer's solicitor will ask for it.

Ask to be shown a photograph of a batten with a slate lifted, not a photograph of the roof from the garden. Ask which slope and why not the other one. Ask what happens to the slates that come off, because sound reclaimed slate has real value and should either go back on or come off the bill. Ask for the guarantee in writing, with a term on it and a plain description of what it covers.

If someone cannot show you the underside of the roof and will not put the extent of the failure in writing, what you have is not an assessment. It is a preference.