A slate roof is a covering made from thin, overlapping pieces of natural stone laid over timber battens and a waterproof layer, fixed to the rafters of a roof. Renewing one — often called a re-slate or re-roof — means stripping the old covering back to the timber, repairing or replacing any structure that has failed, and building the roof up again from underlay to ridge. On a period property this is usually a full strip-and-recover rather than a patch repair, because slates of similar age tend to reach the end of their life together.
What makes up a slate roof?
The visible surface is the slate itself. Natural slate is a metamorphic rock that splits into flat, durable sheets; Welsh, Cumbrian and imported slates each have their own colour and thickness. Each slate overlaps the two courses below it, so water sheds down the surface without reaching the timber underneath.
Beneath the slates sit several layers that do the real waterproofing work:
- Timber battens — horizontal strips of treated wood fixed across the rafters. The slates are nailed or hooked to these, and the batten spacing (the gauge) sets how much each slate overlaps.
- Breathable underlay — a membrane laid over the rafters and under the battens. It catches any wind-driven rain that gets past the slates and, on modern installations, lets moisture vapour escape from the roof space rather than trapping it. Older roofs may have bitumen felt or no underlay at all.
- Ridge tiles — the capping pieces along the apex where two slopes meet. They are usually bedded in mortar or fixed with mechanical dry-ridge systems, and they seal the most exposed line on the roof.
Flashings — usually lead — seal the joints where the roof meets chimneys, walls and valleys. Together these elements form a system, and a weakness in any one of them can let water in even when the slates themselves are sound.
When does slate reach the end of its life?
A slate roof is a covering made from thin, overlapping pieces of natural stone laid over timber battens and a waterproof layer, fixed to the rafters of a roof.
Natural slate often outlasts the fixings holding it. A common failure on period roofs is "nail sickness", where the original iron nails corrode and slates begin to slip even though the stone is still good. Once a roof starts shedding slates regularly, individual repairs become a losing battle and a full renewal is usually the more economical course.
The slate itself can also delaminate — splitting into flaky layers as water works into the stone over decades of frost. Cheaper or fibre-cement slates degrade faster than good natural slate. Signs that a roof is nearing the end include:
- Slipped, cracked or missing slates appearing after each storm.
- Slates that flake, sound dull when tapped, or show a powdery surface.
- Damp patches, staining or daylight visible in the roof space.
- Failed or absent underlay, leaving the slates as the only line of defence.
- Sagging in the roof line, which can point to decayed battens or rafters.
A surveyor or roofer will usually inspect from inside the loft as well as outside, since the condition of the battens and underlay is not visible from the ground. An isolated slipped slate is a repair; widespread slipping across a roof of one age is a renewal.
The order of work in a re-slate
A re-slate follows a consistent sequence, working from the timber outwards. Scaffolding goes up first so that operatives can work safely and move materials. The existing slates are then stripped off; sound natural slates are sometimes set aside for reuse, though many are too brittle to salvage.
With the roof open, the structure is checked. Any rotten or split rafters are repaired or replaced, and decayed timber around chimneys and valleys is made good. Breathable underlay is then laid up the slope, lapped so that water always runs over the joint rather than into it, and dressed into gutters and valleys.
New timber battens are fixed over the underlay at the correct gauge for the size of slate being used. Slating then begins at the eaves and works up to the ridge, each slate set with the right headlap and side gap so the courses break joint with one another. Lead flashings are dressed around chimneys and abutments, and the ridge tiles are bedded or mechanically fixed last to close the apex. Finally the work is checked, debris cleared and the scaffolding removed.
How long should renewed slate last?
Good natural slate has a very long working life — well over a century is realistic for the stone itself on a roof that was correctly laid and maintained. The limiting factor is rarely the slate but the fixings, underlay and flashings around it, which age faster.
A properly renewed roof using quality natural slate, correct nails or hooks and a sound underlay should give many decades of service before attention is needed again. Lower-grade or reclaimed slates, or work where the underlay or battens were not replaced, will not last as long. Periodic inspection — clearing gutters, checking flashings and replacing the occasional slipped slate — extends the life of any covering.
What drives the cost of a slate roof
The cost of re-slating a period property varies widely, and there is no single figure that applies. The main factors that move the price are worth understanding before comparing quotes.
- Roof size and pitch — larger and steeper roofs need more material and more time, and very steep or complex shapes are slower to work.
- Choice of slate — new natural slate, reclaimed slate and synthetic alternatives sit at different price points, and Welsh slate typically costs more than imported equivalents.
- Structural repairs — rotten rafters, failed valleys or chimney work found once the roof is open add to both labour and materials.
- Access and scaffolding — tall, terraced or awkwardly sited properties cost more to reach safely.
- Lead and detailing — chimneys, dormers and valleys all need careful flashing work that adds labour.
- Heritage constraints — listed buildings or conservation areas may require like-for-like materials and consent, which can limit choices and raise costs.
Because so much depends on what lies beneath the slates, written quotations should set out the slate type, the underlay and battens included, and how unforeseen structural work would be handled. Comparing like with like matters more than comparing headline figures alone.