The short answer
Weight depends on the slate and the headlap, so there is no single figure for every roof. For comparison, Marley gives its plain concrete tile at 73.8 kg/m² laid, and Cotswold District Council says traditional stone slate roofs weigh about 1,000 kg per 10 m². The exact figure for slate depends on the stone and the lap. The real question is whether the roof structure can carry what you want to put on it.
Why there is no single slate weight
A slate roof is made of overlapping pieces, so the weight per square metre depends on how deep the overlap is. A roof laid at a shallow pitch needs a deeper headlap, which puts more slate on every metre of rafter. A steep roof can use a smaller lap and carries less. Slate thickness, size and the origin of the stone add to the variation.
Battens, nails and underlay add a little more. Taken together, they mean a figure quoted for one roof does not transfer neatly to another. If you need a number for a decision, the supplier's data sheet for the exact slate and lap is the place to look, and our survey records which slate you have.
Wet slate weighs more than dry, though the difference is small compared with the covering itself, and snow can add to the load for a few days. A roof that has stood for a century is not cause for alarm. The point is simply to ask before anything heavier goes on top, and to ask someone who has seen the timbers.
How slate compares with tile and stone
Published weights give a useful yardstick. Marley's plain concrete tile is listed at 73.8 kg/m² on a 100 mm gauge. Bradstone's reconstructed Cotswold slate is about 84 kg/m² at an 80 mm headlap. Cotswold District Council's technical guidance for stone slate says such roofs are very heavy, at roughly 1,000 kg per 10 m².
Natural slate is a thin, dense stone that is split fine, so a laid slate roof is normally a good deal lighter than the thick stone slates of the Cotswolds, though the exact figure depends on the slate and lap. Our page on Welsh against Spanish slate covers how the stone itself differs in thickness and finish.
If your house is near the Cotswold edge, the stone slate figure matters most, because those roofs were built on timbers sized to carry a very heavy covering. Replacing one with a lighter material is a design and conservation question as well as a structural one.
What weight means for the roof structure
Rafters, purlins and the walls beneath them were sized for the covering that went on at the time. A roof can carry what it was built for, and sometimes more, but it can also be pushed past its limit. Signs of strain include a sag along the line of the ridge, deflection between purlins and cracking where timbers meet the walls.
Rot weakens a roof further. Water that has run into the same corner for years can soften a rafter end or a wall plate, and then the covering that stood for a century becomes a load the timber can no longer share. If you see sagging, our slate roof leaking guide is worth reading, since moisture and weight often go together.
A sound frame with a little movement is common on old houses and is not a fault in itself. What we look for is change: new gaps, fresh stains, a line that has dropped. The photographs let you compare them with your own.
When a heavier covering needs approval
Approved Document A calls a loading change significant when it increases by more than 15%. The Planning Portal states the threshold a little differently: its test for Building Regulations approval is a new covering that weighs at least 15 per cent more than the one it replaces. Treat anything near that figure as a conversation to have with your building control body before ordering materials.
The Planning Portal also advises speaking to a structural engineer or surveyor before work starts on a significantly heavier covering. Swapping a slate roof for concrete tile may or may not cross the 15 per cent line, and a lighter covering raises a different question, but the check is quick to make while nothing has been bought.
If the work is notifiable, we explain the consent route and handle the Building Regulations notification ourselves. If a roof looks as though it needs an engineer's opinion, the survey says so and shows the photographs behind that advice. A roof that carries a visible dip, or has had a wall rebuilt beneath it, is the sort of case where the extra opinion is worth having.
Practical steps if you are unsure
Start with what is already above your head. In the loft, are the rafters straight, dry and tight at the walls? Photograph anything that looks bowed or stained and send the pictures to us, with a note of which room is underneath so we can place them on the plan of the roof.
Anyone planning solar panels, a roof window or a heavier ridge should bear in mind the extra load, and our guide to cracked slate shows what the covering itself may be telling you about movement underneath. For loose or damaged slates that are otherwise sound, a repair almost never changes the weight, so there is nothing to approve.
The survey records which slate is on the roof, how much headlap it has and what condition the visible timbers are in, and the written quote follows. The aim is a job no bigger than the fault needs, and workmanship carries a 10-year guarantee in writing. Our matching roof slates page explains how replacements are chosen so that they weigh and look like the originals. For the wider service, see slate roofing.