Is slate too heavy for my roof?

Structure is dealt with in Approved Document A of the Building Regulations. Paragraph 4.4 puts the threshold at 15% extra load on the roof, and adds that it is worth knowing if the covering being lifted is the house's original one.

The Planning Portal puts the threshold slightly differently. Its re-roofing guidance includes, among the triggers for approval, a covering that adds 15 per cent or more to the weight on the roof. The two sources disagree at exactly 15%, so we say around 15% and ask building control to confirm for a borderline job. It is the weight of the finished covering that counts, including battens, underlay and fixings, not the slate in isolation.

A swap that stays clearly under the line is treated like any like-for-like re-roof. A swap above it brings in an application and a check of the structure.

The calculation compares what is on the roof now with what is going on. Manufacturers publish a weight per m² for each product, usually on the data sheet, and we use those figures rather than rules of thumb. A slate roof laid on battens will not weigh the same as a stone slate roof, a clay pantile roof or a concrete interlocking tile roof, and two slates of different thickness differ too.

Cotswold District Council's 2000 guidance gives a sense of the top end: it calls stone slate very heavy, putting a roof of it at about 1,000 kg per 10 sq m. That makes stone one of the coverings where going to a lighter material is far more common than going to a heavier one. We hold no verified weight for natural slate, so we ask for the data sheet of the slate you have chosen and do the sum from it.

The sum is only half the job. The timbers have to carry whatever the figure turns out to be.

Where a new covering is markedly weightier or lighter than what it replaces, the Planning Portal tells owners to take advice from a structural engineer or surveyor first. We agree with that. A roof that has carried one covering for a century may be sagging in places. Rafters, the sloping timbers, and purlins, the horizontal beams that support them, can be overstressed, rotted at their ends or already deflecting.

Approved Document A paragraph 4.5 deals with exactly that. If the existing frame cannot take the extra load, for example because members are overstressed or sagging badly enough to pond water, strengthening follows. In practice an engineer might specify new rafters, a doubled-up timber or additional support, and those costs belong in the quote before the roof is stripped.

Going lighter can bring its own questions, since it changes the wind uplift and fire performance. The same Planning Portal page adds a further trigger: a covering with fire performance significantly different from the old one.

When approval is needed, the application goes to a registered building control approver or to your council's service. In Stroud District that means the Gloucestershire Building Control Partnership, which the council shares with Gloucester City. Where the job is notifiable, we set out the consent route for you and make the Building Regulations notification ourselves.

Where the roof is in a conservation area or the house is listed, a change of covering is a different matter from like-for-like repair. Ask Stroud District Council's planning team first, before any slate is ordered. On a listed building, a new covering that changes the roof's character may need listed building consent.

Insulation rules can follow too: under Approved Document L a thermal upgrade comes in when more than 50% of the roof is renovated, or more than 25% of the building envelope. A weight change and an insulation change often arrive together, so we cover both at the survey.

With a torch in the loft we check for:

• Rafters that sag between purlins, or a ridge line that dips when viewed from the street.
• Dark, soft or crumbling timber near the eaves, where water has been getting in.
• Cracks or splits in rafters, and old repairs where one timber has been bolted to another.
• Walls that have moved or bulged under the roof's outward thrust.

None of these means slate is out of the question. It means the structure should be dealt with at the same time as the covering, and priced as its own line. The guide to the advantages and drawbacks of a slate roof touches on weight in passing; the structural question gets its answer here.

Thinking of moving to slate, or off it? Photographs of the roof and loft are a good start, or book a survey. We photograph your own roof, check the existing timbers, and write down the covering weights we are working from. Where the numbers are close to the 15% line we ask the engineer or building control to confirm rather than guess.

The covering also depends on what lies beneath: see our answer on underlay for a slate roof, and for access once it is laid read can you walk on a slate roof. Our slate roofing page sets out the rest of the service, and the work carries a 10-year workmanship guarantee in writing.