Old slate roof: repair it or strip and re-slate?

A slater sitting on roof ladders dressing a slate, with stacks of slates beside him on a Scottish terrace roof — Photo: Karora, Public domain, via Wikimedia Commons. Cropped.

Repair while the failure is local. Re-slate when fixings are going across a whole slope and repeat repairs have stopped making sense. SPAB's technical note on south-west slating puts the reason neatly: the slate is durable and may last hundreds of years, but a roof's life is set by its fixings, because pegs and laths rot and nails rust. It adds that the decision to re-slate is often triggered by the repeated cost of scaffolding for repairs.

A slate on the path therefore tells you little by itself. One or two slips after a storm are maintenance. Slates sliding from all over the slope, year after year, suggest the fixings are coming to the end of their working life.

Fig. 1 A slope in section. Slates lap over battens; ridges and fillets belong in soft lime mortar.

What to look at, from the ground and the loft

SPAB's roof maintenance advice is to check the roof twice a year. Binoculars from the pavement are safer than a ladder and will show most of what you need. Look for slipped, broken or missing slates, loose ridge and hip pieces, and split or lifting lead at chimneys and abutments. Cracked mortar fillets deserve attention because junctions are the usual trouble spots.

Then go into the loft on a bright day and again after rain. Look for water staining on rafters and laths, bearing in mind that the mark may be some distance from the point of entry. Underlay only became usual from the mid-20th century, so seeing the underside of the slates is not itself a fault. Wet timber is another thing. Check too that loft insulation has not been pushed down to the eaves, where it blocks ventilation and promotes damp and decay. If damp is showing on bedroom walls below, work through our damp checklist as well.

Repairs that make sense

Most small repairs on a slate roof are simple in principle:

Slipped slates. True slates can be refixed with copper wire or strip, known as tingles. Heavier stone slates have to be nailed or pegged instead.
Ridges and hips. Rebed loose pieces in lime mortar.
Fillets. Repoint cracks, or re-form them if the cracking is extensive, in soft lime mortar rather than cement. SPAB prefers keeping a traditional fillet over swapping it for lead.
Lead flashings. Replace a damaged length in matching material. Vertical splits can be covered with a small new section wedged and pointed above the old.
Moss. Brush down seasonally if it is excessive. Moisture retention can damage coverings and acidic run-off can corrode some metals.

Repairs to turn down

Be wary of anyone offering to coat or foam the roof. SPAB's advice on bitumen-coated fabric outside and spray-on foam underneath is plain: a false economy that in some reported cases has caused serious damage to the structure. These treatments prevent inspection, make the slates hard or impossible to reuse, and reduce ventilation, raising the risk of timber decay. Grouting or slurrying with mortar falls into the same category, and the south-west note says it can eventually cost far more than re-slating would have. Treat a doorstep offer of this kind with particular suspicion.

SPAB also warns about underlay used to cover for poor workmanship: the slates must still shed water properly on their own.

If it does need re-slating

Ask for a survey of the existing roof before stripping, then a written statement of how it will be done. The south-west note says repairs and re-slating should retain as much historic fabric as possible, and sound slates should be reused or dressed down to remove damaged parts. The number that can be reused will not be fully known until the slope is opened up, so expect a provisional sum. New slate should match the local source wherever one exists, since slate from elsewhere looks different and regional styles vary a great deal. SPAB's technical notes cover several regions. Roof timbers, once exposed, can be checked and repaired at the same time, so allow for it.

Permission: three separate questions

Before any work starts

Planning. Re-roofing that does not materially affect external appearance will usually not need planning permission. A switch to a different-looking material may be a different matter, particularly in a conservation area. Listed buildings. The south-west note says most changes to construction details will need listed building consent, including a change of slate type, fixing method or ridge material, and re-slating where renewal would affect character.

Building regulations. The Planning Portal says repairs or re-covering will not normally need an application if under 25% of the building envelope and under 50% of the roof is affected. Above that, or with structural work, a markedly different fire performance, or a covering 15% or more heavier, you need approval. Re-covering can also trigger insulation upgrades, and a heavier or lighter covering may call for a structural check. Confirm with your building control body, since these figures can change and older guidance quotes different ones.

Bats. Repairing or replacing a roof is named by GOV.UK as an activity that can affect bats, which are fully protected. If you suspect a roost, stop and ring the National Bat Helpline first. Timing and licensing may affect when the work can happen.

Insulation deserves a slow decision on an old roof. The south-west note warns that care is needed where insulation goes in, and SPAB's damp note says manufacturers' instructions for vapour-permeable underlays must be followed closely to avoid condensation in the roof. Ask how moisture will leave the roof space before you agree to a specification.

Sources

Opened and read by the editors in November 2026. Rules and guidance change, so check the current version.