Repair or replace: how to judge whether a roof is finished

Count the failures and map them. Three slipped slates in one area after a gale is localised damage. Three slipped slates in three unrelated places, twice a year, in ordinary weather, is systemic failure of the fixings.

Localised damage is a repair, and a good one will outlast the rest of the roof. Systemic failure is a reroof, and repairing it repeatedly costs more over five years than doing it once.

Slate roofs rarely fail because the slate wears out — good natural slate outlives the people who laid it. They fail because the nails corrode and snap, and the slate then slides out under its own weight.

Nail fatigue is age-related and affects the whole roof at roughly the same time, because every nail was fitted on the same day from the same batch. Once it starts, the pattern is unmistakable: random slippage, no weather trigger, accelerating over a few winters.

Different coverings tell you they are finished in different ways, and it is worth knowing which signs are cosmetic and which are terminal.

Older roofs were laid on bitumen felt that becomes brittle and tears along the batten line. Once the underlay has perished, the roof has no secondary defence: any water past the covering goes straight into the structure.

You cannot replace underlay without stripping the covering, which is why a roof with sound slate but perished felt still ends up as a strip-and-relay — reusing the original slate where it is in good condition.

Stripping back to the rafters is the only opportunity to correct everything hidden: new breathable membrane, new treated battens at the correct gauge, stainless or copper fixings, proper eaves and verge details, insulation upgrades and ventilation.

It is also the only honest way to price the job, because rotten timber found under a covering is the most common cause of a mid-job cost increase. A survey that includes lifting units in the worst areas reduces that risk considerably.

Plenty of roofs get replaced when they did not need to be. If the covering is broadly sound, the underlay intact and the failures confined to specific details — one chimney flashing, a cracked valley, a run of ridge mortar, a handful of storm-slipped slates — repair it and re-inspect in a few years.

Ask for photographs of the actual defects on your roof and a written explanation of why the recommendation is repair or replacement. Anyone unwilling to provide either is not giving you enough to make the decision.