People who ask how long a slate roof lasts are usually looking for a number. Honest answers do not come in one, because a slate roof is a system with parts that age at different speeds. The slates themselves, if they are good natural slate, can last for generations. The nails that hold them, the battens they hang on, the underlay beneath and the lead around the chimney do not. Most of the slate roofs we are asked to look at have not worn out as slates. They have run out of fixings. This guide takes the parts in turn, explains what shortens a slate roof's life and what extends it, and sets out how to tell where yours stands.
Slate itself: the long-lived part
Natural slate is a metamorphic rock that splits into thin sheets along natural planes. It does not rot, it does not burn, and it absorbs very little water. A good slate laid correctly in a suitable climate can stay on a roof for a very long time, and it is common to see slates on period buildings that have been up for much of their history. This is the reason slate has a reputation for permanence, and it is deserved.
But slate is not all the same. Quality varies with the quarry, the seam and the grade. Welsh slate is known for its density and long life, and Spanish slate is widely used as a lower-cost alternative, with a performance that depends on the grade chosen. Our guide to Welsh versus Spanish slate compares them. Man-made and fibre-cement slates have a shorter and less certain life than natural stone, and our page on natural versus man-made slate sets out the differences. A poor-quality natural slate can delaminate, flaking into layers after some decades, and our answer on why slates delaminate explains why.
Nails and hooks: the usual weak point
Slates are held by nails driven through holes near the top, or by hooks that clip over the bottom edge. The nail is the single most common reason that an old slate roof begins to shed slates. Iron nails rust. In a damp, polluted or coastal setting they rust faster, and eventually the nail shank thins and the slate slips. The slipped slate leaves a gap, water gets in, and the neighbours around it may follow. The slate may still be perfect, and one that has dropped to the ground can often be refixed with a better nail.
Modern roofs use copper, stainless steel or aluminium nails that do not rust, and these can last as long as the slate. Some slaters prefer hooks in certain situations, and the trade-offs are set out in slate hooks versus nails. If your roof was laid with galvanised or plain steel nails, expect trouble at some point, and treat the first handful of slipped slates as a warning. Our page on slipped and cracked slates shows how to read the pattern.
Battens, rafters and the timber
The timber behind a slate roof decides how long its fixings will hold. Battens are the horizontal laths the slates are nailed to, and old ones may have been softwood of variable quality, now dry, brittle or damp-stained. If the batten has gone soft, the nail has nothing to hold. Rafters and the wall plate can suffer from rot where water has entered, or from beetle attack in old timber. Because slate is heavy, the structure has to be sound. When a slate roof is stripped for any reason, the timbers are checked, and sometimes the battens are renewed, so the roof is good for another long stretch. See re-slating a roof for how that process works, and how heavy a slate roof is for what it asks of the structure.
Underlay and ventilation
Older slate roofs often have no underlay, or an early bitumen felt that has since rotted or dried out. The underlay is the secondary defence: it catches wind-blown rain and drifting snow that gets under the slates and carries it to the gutter. Without it, water can reach the loft and the timbers. Replacing it means taking the slates off, which is why it usually happens during a re-slate. The air space under the slates also matters, and our guide on slate roof underlay and ventilation explains how modern breathable membranes and vents help to keep the timbers dry.
Lead, flashings and the details
The slates may be fine, and the roof may still leak at its edges. Chimney flashings, valleys, abutments and ridges are made of lead or other metal, and lead is lifted by the wind, cracked by movement and sometimes stolen. A leaking valley or chimney apron is often mistaken for a worn-out roof, when the repair is local. Lead lasts a long time when dressed correctly, but not as long as slate, so expect to renew it at least once in the life of the roof. Our section on lead roofing and leadwork covers it. Ridges and hips, which on slate roofs may be bedded in mortar, also need periodic attention.
What shortens a slate roof's life
A few things make slate roofs age faster than they should:
- Rusting nails. The most common cause, as above.
- Foot traffic. Slate cracks when walked on carelessly. The reasons for roof ladders and crawling boards are in can you walk on a slate roof.
- Poor repairs. Slipped slates stuck back with mastic, or odd-looking replacements in the wrong size, can trap water and cause more damage than they cure.
- Heavy moss and neglected gutters, which hold water against the slates and the fascia.
- Frost, which attacks slates that absorb moisture, and low-grade stone suffers most.
- Pitch below the safe minimum for the slate and lap, which allows wind-driven rain under the slates. Our answer on the minimum pitch for a slate roof sets out the limits.
- A mix of old and new slates of different thickness, which can lift and let in water, as in mixing old and new slates.
Signs your slate roof is getting tired
Watch for slates dropping into the garden or gutters, a rash of slipped slates on one slope, slates with a flaky, powdery or laminating edge, a sagging line in the roof, and daylight or damp stains in the loft. Several of these together point to the end of the roof's life, and our guide on signs a slate roof needs replacing goes through them. A single slipped slate does not mean much, and our answer on how a single slate is replaced shows how simple the repair is.
Repair or re-slate
If the slates are sound and the nails are the problem, a roof can often be stripped, the battens renewed, new underlay fitted and the same slates re-laid on copper or stainless nails, adding a very long life for a fraction of the price of new stone. Reclaimed slate can top up the gaps. Our page on reclaimed slate explains when it is a good option. If the slates themselves are delaminating, new slate is the answer. Where the roof is only partly failing, repair may be sensible, as in is it worth repairing a slate roof.
On cost, one trade source reports natural slate re-roofing at £120 to £180 per m² and man-made slate at £90 to £130 per m², and a second gives Welsh at £140 to £180 and Spanish at £100 to £130 per m². Those are typical UK ranges and not our prices, which are set only after we have seen the roof. For more, see slate roof cost.
What a re-slate with the old slates involves
Re-laying sound slates is a skilled and slow job, and it helps to know the steps. The roof is scaffolded and the slates are lifted off in sections, then sorted by size and condition on the ground or on the scaffold. Cracked, laminated or chipped ones are put aside. Old battens and underlay come off, and the rafters are inspected, with any rotten timber cut out and replaced. New underlay is laid with proper laps, new treated battens are fixed at the gauge set by the slate size and head lap, and the sorted slates are re-hung in courses, starting with the largest at the eaves and working up to the smallest at the ridge, a practice known as diminishing courses on traditional roofs. Each slate is fixed with two copper or stainless nails, and the bottom edge is checked for a consistent line. Any shortfall is made up with reclaimed or new slate of a matching size and colour.
The saving over new slate is real, but it is not total. Labour makes up a large share of the cost of a slate roof, and labour is the same either way. What you save is the cost of the stone itself, and you keep the weathered appearance of the original roof, which matters on a period house. If many of the old slates are found to be damaged during the strip, the saving shrinks, and we tell you before the work goes ahead.
Listed and period slate roofs
On a listed building, like-for-like natural slate is usually expected and changes of material may need consent, so speak to the council's conservation officer before any change. Our page on slate roofs on listed homes sets out the practical points. Swapping slate for concrete tile changes the load on the rafters as well, so the structural rules in Approved Document A need checking before any change of covering.
Thinking about the next owner
A slate roof in good order adds to a house in a way that a tired one subtracts. A buyer or a surveyor looking at slate wants to know whether the fixings have been checked and whether the lead is sound, and photographs of the roof from a past survey can answer both. Keep any records of work done, including the guarantee on a re-slate and any receipts for repairs. If you are selling an older home with original slates, a survey before you list can turn an unknown worry into a documented position.
How we judge a roof
We look at the slates, the fixings, the battens, the underlay and the details, and we photograph what we find. Often the answer is that the slate roof has plenty of life and needs a handful of repairs and perhaps new lead. When it needs re-slating, we show you which part has failed. Our slate roofing page lists the full service, and 01256 213956 is the number to ring to book a survey.
