Read this if you are re-slating, or thinking of changing to a different covering. It sets out the weight test building control applies, who checks the structure, and why this page quotes no stock weights.
The rafters under a slate roof were sized for the covering that was on the house when it was built, together with its battens and underlay. Put a different covering on the same timbers and the load they carry changes. Sometimes it changes by very little. Sometimes it changes enough that the law treats the job differently, and that is the question this guide is about.
Two sets of words matter. The Planning Portal gives the point at which a heavier covering needs Building Regulations approval, and Approved Document A, the structural volume of the regulations, uses a similar figure for what counts as a significant change in roof loading. Between them they draw the line.
Slates differ in size, thickness and origin, so the weight that matters to your rafters is the one for the slate actually chosen. What we can do is set out the test, the order of checks, and who signs off the structure. This guide belongs with our slate roofing pages, which cover repair, re-slating and replacing another covering with slate.
What the roof timbers are carrying
A pitched roof is a stack of layers, and the timber structure carries every one of them. Rafters slope away from the ridge to the eaves, where they rest on the wall plate, a timber laid along the top of the wall. Across the rafters go the battens, which hold the covering, with the underlay between. Ridge pieces, hips, leadwork and anything fixed to the roof add to the total as well.
Lead is the one layer where published weights are to hand. The Royston Lead brochure lists each code by thickness and by weight per square metre, and the three below show how quickly the figure climbs.
| Code | Thickness | Weight per m² |
|---|---|---|
| Code 3 | 1.32 mm | 14.97 kg |
| Code 5 | 2.24 mm | 25.40 kg |
| Code 8 | 3.55 mm | 40.26 kg |
Lead covers small areas on a house roof, at valleys, around chimneys and beside walls, so it does not decide the total. What it does show is that weight belongs to the product and is set by the maker, and that it changes from product to product.
The same holds for slate. A large, thick slate and a small, thin one are both natural slate, yet they are far apart in weight per square metre. The only dependable number for your roof is the supplier's published weight for the exact slate chosen, read against the weight of the covering it replaces, with battens and underlay added to each side of the sum.
The 15 per cent test in the Planning Portal's words
The Planning Portal names two ways a new covering brings in the need for approval. One is a covering whose weight is 15 per cent or more above that of the one it replaces. The other is a covering whose behaviour in a fire will be significantly different from that of the covering already there.
Approved Document A deals with the structure itself. In its terms, a significant change is a roof load raised by more than 15%. The two wordings differ by a hair, 15 per cent or more in one and more than 15% in the other. We plan around 15 per cent, because landing on it exactly is easy and being caught out is costly.
The test sets the new covering against the old, and says nothing about how heavy either one is. A roof that already carries a heavy covering can be re-covered with something of similar weight and meet the test easily. A roof carrying a light covering can fail it with a modest change. Only the two data sheets can say which side of the line a particular job falls on.
For a covering that is heavier, the Planning Portal's advice is to involve a structural engineer or a surveyor ahead of the work. That advice is the one we follow, and the next section turns it into a set of steps.
A structural check before switching coverings
When the new covering is a different product from the old one, the weight comparison comes first, and it is done before a price is written. The steps run in a set order.
- Identify the existing covering, and any extra layers beneath it, from the survey and from the loft.
- Take the proposed covering's weight per square metre from the supplier's data sheet.
- Compare the two, adding the battens and underlay each would need.
- Where the increase reaches 15 per cent, or the result is close enough to be doubtful, ask a structural engineer or a surveyor to assess the rafters, their spacing, their span and their condition.
- Settle the building control route with the council, or with a private approver, ahead of the strip.
The engineer's check looks at more than a total. Rafter size, span and spacing matter, and so does the state of the timber, because a rafter with rot or old insect damage is weaker than its section suggests. Purlins, the horizontal timbers that hold up rafters at mid-span, and the wall plates they bear on are part of the same assessment.
If the structure needs strengthening, that is done before the new covering goes on, never afterwards, and the extra work is written into the one total price you receive. Private approvers have had to be on the Building Safety Regulator's register since 6 April 2024, although the private route itself is older than that date.
Same covering back on: the 25 and 50 tests
Weight is one test. A second looks at the size of the job against the house. The national test sets two limits for work that normally needs no application: under 25 per cent of the building envelope touched, and under 50 per cent of the roof touched. The envelope is the outer shell of the house: its walls, its floor, its roof and every window, door and rooflight.
A full re-roof of a whole house takes in every slope, so it falls outside the 50 per cent test. Re-slating one slope of a four-slope roof might stay inside it. For that reason the route is confirmed with the council for any strip of a slope, and the weight comparison is made beside it.
The councils word their advice differently, and the wording follows the address:
- For Margate, Ramsgate and Broadstairs the national test is the one in force, because Thanet District Council refers owners to the Planning Portal.
- Dover District Council puts roof coverings on its approval list, including a replacement that is like for like. Only Dover addresses are covered by that line.
- At Folkestone & Hythe District Council, replacing roof coverings is among the alterations for which approval may be needed, and the same page says most repair, replacement and maintenance jobs need no approval.
- Canterbury and Swale both use STG Building Control, on 01634 331133.
- Ashford addresses go to Ashford Borough Council's building control.
None of the councils listed gives a percentage of its own, so none of those wordings moves the 15 per cent figure. If you are unsure which applies to your address, the council's building control team can answer that in a short call.
Weight, wind and fixings are separate questions
A heavier covering is not a better-fixed covering, and a lighter one is not a loose one. How a slate is held down is a different calculation from how much it weighs. BS 5534:2014+A2:2018 sets out good practice for how slates and tiles are laid and held on, and Marley says it includes the wind uplift formulas used to calculate each fixing.
The standard is not the law, and Marley is plain that fitting to BS 5534 is not a legal requirement. It is nonetheless the benchmark a re-roof should be built to, and since the 2018 amendment ordinary re-roofing and repairs fall inside its scope. In practice that means the fixings are calculated to suit the exposure of that roof, so a tall house or an open position gets more than a sheltered one.
Pitch and fixings are linked as well. Cupa, the slate maker, reads BS 5534 as setting 20 degrees as the minimum pitch for natural slate, and says hooks have no place on pitches below 25 degrees. A shallow roof can therefore narrow the choice of slate and the way it is held, whatever the weight.
At the ridge, hips and verges, dry-fix kits made to BS 8612:2018 hold the capping on with mechanical fixings and not mortar alone. Our page on dry ridge on a slate roof goes through how they work and when they suit.
Why there are no stock weights here
A page that offered a weight for slate in kilograms per square metre would be inventing it. Natural slate comes in a range of sizes and thicknesses, from different quarries and countries, and one product can be supplied in more than one grade. The figure that matters is the one on the data sheet for the slate you have chosen, and it is read alongside the weight of the covering you are taking off.
This is also why a weight comparison between slate and another covering belongs in a survey of your own roof and not in a general rule. Which is heavier depends on the exact products and on how the roof is laid, including the headlap, the overlap between courses, which changes how many slates cover each square metre. Because of that, two roofs of the same slate can differ.
What we do instead is work from your roof. The survey records what is on it, and the loft shows what holds it up. If the job is a straight re-slate with a like-for-like covering, the comparison is quick. If you are considering a change, the data sheets and the structural check come first, and you see the findings before anything is ordered.
If you want to compare running costs of different coverings, our slate against tile cost page covers it, and our survey guide shows what we check at the start.
