The short answer
Where a new covering weighs 15 per cent or more beyond the old, approval under the Building Regulations is required, and the Planning Portal advises a prior inspection by a structural engineer or surveyor. Approved Document A words the line slightly differently, as more than 15%. Either way the rafters were built for the old load, so the check protects the house. The figure comes from the maker's weight for each covering.
The 15 per cent line, and whose it is
Two official sources set the weight line, and they differ by a word. On re-roofing, the Planning Portal says approval is needed if a new material makes the covering at least 15 per cent heavier. Approved Document A, the structural part of the Building Regulations, puts a significant change in loading at a rise of more than 15%.
The difference is the case where a new covering is exactly 15 per cent heavier. The Portal brings it in, and Approved Document A does not. Rather than argue the point, a careful roofer treats 15 per cent as the line and checks anything near it.
The Portal also catches a covering whose fire performance would be markedly different. So weight is not the only trigger.
Approval here means Building Regulations approval, which is a separate regime from planning permission. A heavier covering does not need planning permission for that reason. Re-roofing a house normally sits within Class C permitted development, whichever tile is chosen.
It also helps to say which source you are using, since they are not identical. We name the Planning Portal's wording when we explain it to customers, and Approved Document A where the structural question is the point.
Why the structure is what counts
The rule is not an arbitrary number. A roof frame is designed for a particular load: the weight of the covering, battens, underlay and fixings, plus weather and snow. Rafters, purlins, ridge and walls are sized to carry that, with a margin.
Add weight and the margin shrinks. The first symptoms may not show for years, and then a ridge dips, rafters bow or a wall begins to lean. On the Portal, the instruction is to bring in a qualified structural engineer or a surveyor before the works begin.
Heavier coverings are not the only cause of overload. Old timbers may have decayed or been weakened by alterations, so a frame that coped with an old light covering may not cope with a new heavy one. Our page on what causes a sagging roof shows where overload ends up.
The comparison that matters is between the old and new covering per square metre, including battens and underlay where they change. Makers publish the weight for each product, and the existing covering can be identified or measured.
If the new covering is lighter, the weight rule is not engaged, though other things such as wind uplift and pitch need checking. If it is heavier by 15 per cent or more, the structural check is not optional advice but part of getting approval.
How the check works
A structural check is done by a structural engineer or a building surveyor with the right knowledge. They inspect the roof frame and the walls, often from the loft, note the sizes and condition of the timbers and compare what is there with what the new load will need.
The result is a view on whether the frame can carry the new covering as it is, or whether strengthening is required. Strengthening can be modest, a timber fixed alongside a rafter for instance, or larger, with new purlins or wall plate repairs.
The engineer's note then goes to building control with the application. For Farnborough and Fleet addresses, that body is the Hart and Rushmoor Building Control Partnership, 01252 398715. A registered building control approver offers the private route; other councils have their own teams.
Councils word their re-roofing pages differently and none gives a percentage, so ask the council for your address.
The cost of a check varies with the roof and is not something we quote from general figures. It is usually small beside the cost of a new roof, and cheaper than correcting an overloaded frame.
If the survey shows the frame is fine, you have a document that records it.
What we do before choosing a covering
The survey identifies the existing covering, measures the roof and examines the frame from the loft with photographs, so you see the timbers, any decay or past alterations, and the pitch.
If you want a heavier covering, we get the makers' weights for the old and the new, set them side by side and tell you which side of the 15 per cent line the choice falls on. Across the line, the approval and the check are explained, along with who arranges each.
We may also suggest a lighter option that looks similar. Where a like-for-like covering suits, it keeps the job simpler.
A quote then names the covering and states any structural work and approval costs, as one total in writing. Our page on changing to a different type of roof tile covers the planning side, and our page on the re-roofing 25 per cent rule explains the other threshold.
The roof replacement hub shows how the whole job runs across Farnborough, Aldershot and Fleet.
