The short answer
Yes, when the new tiles add enough weight. By the Planning Portal's test, a new covering lifting the roof's weight by 15 per cent or more needs Building Regulations approval, and an engineer ought to be consulted before the old tiles come off. This is a Building Regulations question, not a planning permission one.
Swapping lightweight tiles for heavier ones, or concrete for slate or clay, loads the roof structure differently. The rules turn on how much weight is added.
The 15 per cent test
The Planning Portal lists the cases where a re-roof calls for building control approval. One trigger is a covering weighing at least 15 per cent above the one it replaces. Another is a new covering whose performance in a fire is significantly different from the old one.
Is it 15 per cent or more than 15 per cent?
Two official sources word it a little differently. The Planning Portal says 15 per cent or more. Approved Document A, the structural part of the Building Regulations, calls it a significant change when roof loading rises by more than 15%. The gap only matters if your roof lands exactly on the line, which is rare, and a structural check settles it. This page quotes the Planning Portal figure and names it each time.
The test is about the covering. It compares the weight of what comes off with the weight of what goes on, battens included if they change. Slate, clay and concrete differ, and each profile has its own weight, so the tile maker's data sheet gives the numbers for your choice.
Why weight matters to the structure
Rafters, purlins, walls and foundations are sized for the roof they carry. Add weight and every one of them takes more load. A heavier covering on rafters already weakened by rot or movement can push a marginal roof into sagging.
What does the structural check involve?
The Planning Portal's advice is to ask a structural engineer, or a surveyor, to assess the roof before anyone starts work. They look at the size and spacing of the rafters, the span, the condition of the timber and the walls, and they compare that with the load of the new covering. The result is a short report or calculation that your building control body can read.
Sometimes the answer is that the roof is fine. Sometimes the rafters need strengthening, which is the subject of the answer on whether roofers replace rafters. Either way, you want that answer before the old covering comes off, not after the new one is on. Our survey records the existing covering and what can be seen of the timbers, and you see photos, which gives an engineer a head start.
If weight is a worry, a lighter tile of similar appearance may avoid the question altogether, and a roofer can show you samples of both so the choice is made with the numbers in front of you.
Building Regulations, not planning permission
These are two separate systems, and they get mixed up. Planning permission looks at how a building appears and fits its surroundings. Building Regulations look at whether the work is safe, structurally sound and energy efficient.
Does changing the tile type need planning permission?
Under Class C of the permitted development order, alterations to a house roof are covered, so a similar re-roof is normally permitted development. That sits apart from the weight question. A listed house keeps Class C but also needs listed building consent too, should the work change its character, and a conservation area or an Article 4 direction can alter the position, so ask Somerset Council's planning team, or Exmoor National Park Authority inside the Park.
Approval means one application to Somerset Building Control (0300 303 7790) or to a registered building control approver. Where a re-roof is notifiable, it falls in Somerset Building Control's fee category for renovation of a thermal element, which is a good reason to ask early. The answer on roof tile profiles shows how profiles differ, and a heavier profile is usually where the question starts.
When the question comes up and what we do
It tends to arise in three places: putting natural slate or clay back on a house that was re-covered in concrete, re-roofing an old house that carries thin clay or stone slates, and swapping a lightweight sheet or tile for a heavier one. A listed or conservation-area home often needs the original material back, and we hold to slate for slate and clay for clay where it matters.
What should I do before choosing a heavier tile?
Ask for the weight per square metre of the old and new coverings, in writing, and for the structural check. Then talk to your building control body. A tile that looks right on a sample board may fail the weight test on your rafters.
The cost side of a re-tile is in roof tile replacement cost, and the wider work is on our pitched roofing page. We survey the roof, measure up the existing covering, show you photos of the timbers and tell you where weight could become an issue. If a heavier covering is your choice, one written price follows once the structural check is in hand.
More on tile weight
Does a lighter tile need Building Regulations?
Not under the 15 per cent weight test, which is about added load. The other trigger can still apply: if the new covering performs significantly differently in a fire, approval is needed. Insulation and ventilation rules may apply too, so ask your building control body before you start.
Who pays for the structural check?
You do, as the owner, because the check is part of the project. The cost depends on the engineer or surveyor and the roof. A small outlay beforehand is cheaper than finding out afterwards that the rafters needed strengthening, and the report is useful paperwork when you sell.
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