The short answer
A roof can take heavier tiles only if its timbers and walls can carry the extra load, and that has to be checked, not assumed. The Planning Portal puts the line at a new covering that is 15 per cent heavier or more, at which point Building Regulations approval is needed and a structural check follows. Swapping slate or clay for concrete is the usual case that triggers this.
Why weight matters
A pitched roof is a carefully balanced load. The covering sits on battens, the battens on rafters, and the rafters pass their load down through purlins, wall plates and the walls themselves. Each piece was sized by the person who built the roof, for the weight of the covering that was meant to go on it.
Change that covering for something heavier and every part carries more. The rafters flex a little further, the battens bear more, and the load on the walls rises. A roof that has sat happily for a century on light slate may not cope with a covering that is considerably heavier, especially if the timbers have been weakened by damp, rot or beetle.
Wind and snow loads sit on top of the covering's own weight, so a roof that is already close to its limit has less in reserve. This is why a heavier covering is a decision for the structure, not a choice of look.
The 15 per cent rule
The Planning Portal is plain about the trigger. Fit a covering that raises the roof's covering weight by 15 per cent or more, and you need approval. It also flags that a covering far heavier or far lighter than what is there deserves thought in its own right.
The 15 per cent is calculated on the weight of the covering per square metre, comparing what comes off with what goes on. You do not guess it. You take the manufacturer's laid weights for both materials, once battens and fixings are accounted for, and compare them.
If the result is below 15 per cent, approval for the weight alone is not required, though a replacement can still need approval for other reasons, such as the amount of roof affected. Above 15 per cent, building control must be involved, and the structure must be shown to cope. The structural reference is Approved Document A.
For the planning side of re-roofing in general, the Planning Portal has a page on Building Regulations and re-roofing. Ask your building control body about the details of your roof.
Which swaps usually add weight
The common cases follow a pattern.
- Slate to concrete tiles. Concrete is the heavier of the two, so this change commonly crosses the 15 per cent line. It changes the appearance of the roof as well.
- Clay plain tiles to concrete. Again the concrete is heavier, and the old roof framing may have been sized tightly.
- Slate to stone slates, or reconstituted stone. Stone coverings are heavy. One reconstituted Cotswold-style slate, according to its manufacturer Bradstone, weighs about 84 kilograms per square metre laid.
- Fibre-cement to natural stone or clay. The lightweight slate-effect sheets are often well below what a traditional covering weighs.
Going lighter has its own concerns. A much lighter covering can be more easily lifted by wind, and the fixing standard must be raised. Whatever direction you move, the weights per square metre from the maker are the numbers to start with.
For the two materials compared, read clay or concrete tiles.
What a structural check looks at
A proper check starts in the loft. Someone competent looks at the size, spacing and span of the rafters, the purlins and their supports, the wall plates and how the roof is tied together. They look for decay in all of them, since a sound timber and a wet, soft one carry very different loads.
The outside matters too. A bowed ridge or a dip along the slope may mean the structure is already under strain. Our page on why roof tiles slip shows how some of those signs first show at the covering.
Where the check shows the structure cannot take the load, the options are to choose a lighter covering, or to strengthen the timbers, which may involve new rafters, sistering existing ones or adding support. A structural engineer's calculation is normally what building control wants to see. A survey lets us report what is visible, but calculations come from an engineer, and you should appoint one where the check calls for it.
Other things that change when the covering changes
A heavier tile is more than a load question. Several other parts of the roof respond.
The battens and their spacing change with the tile, so the roof is re-battened to the right gauge. The fixings change, since heavier tiles need a different nail or clip pattern. The underlay may need renewing, and our guide to roof underlay explains the options. The ridge, hips and verges need matching fittings, which have to be mechanically fixed under BS 5534:2014.
The pitch can also rule a tile out. Each tile has a minimum pitch set by its maker, so a flat-looking roof may limit your choice.
Then there is the building itself. On a listed building, the covering cannot be changed without consent from B&NES Council, and the conservation area, covering roughly the inner half of Bath by area, may take an interest in a new material. Confirm with B&NES planning first.
How we approach a change of covering
We start by looking at what is there. Our free survey covers the covering, the battens, the timbers in the loft and the walls where we can see them, with photographs of your roof so you can follow what we find.
If you are set on a heavier tile, we say what the structure shows and whether a check or an engineer is advisable. Where the roof was built for a lighter covering, we usually suggest matching the existing material, which spares the structure and often the planning conversation.
If approval applies, we set out what building control will ask for and look after the submission where the job is notifiable. Once the survey is done, a fixed price in writing is yours, and it shows the cost of any extra timbers or rafter strengthening as a separate item you can question. The pitched roofing page covers the wider service.
