The short answer
Natural slate is on the heavy side, and the maker's sheet for your slate gives its weight. In law the change is what counts: the Planning Portal ties Building Regulations approval to a replacement that adds 15 per cent or more to the covering's weight, and advises asking an engineer or surveyor first. Swapping tile for slate is a case to check.
Why weight is the deciding question
A roof is designed to carry its own covering, the weather on it, and the occasional person. The rafters, purlins and walls are sized for that, so adding load changes the margin. A roof that has carried slate for a century is fine as it is. The trouble starts when the covering changes.
Slate varies in weight with its thickness, size and source, and a thick hand-split slate weighs more than a thin machine-cut one. The product sheet states the weight per square metre, and that figure is the one to use, since the true load depends on the slate chosen, the battens and the underlay.
The weight on a roof is also more than the covering. Wet slate holds a little water, snow adds load, and fixings, battens and any insulation or boarding count. A designer works with the combined figure.
For a homeowner, weight is a structural matter and not a matter of opinion. If the answer is unclear, it needs measuring and calculating, and a structural engineer or surveyor is the right person to do it.
Our answer on slate or tile compares the coverings on other grounds.
The 15 per cent trigger
Beyond its area test, the Planning Portal adds two triggers for building control: a new covering adding 15 per cent or more to the weight, or with a significantly different fire performance. Approved Document A words it a little differently, calling an increase of more than 15% a significant change in loading.
So the portal says 15 per cent or more and AD A says more than 15%. They differ by a word, and the practical meaning is the same: a noticeably heavier covering is a matter for building control.
This is a Building Regulations point and not a planning one, so a 15 per cent increase does not create a need for planning permission. Class C of the permitted development rights normally covers re-covering a house roof, and the approval question runs separately.
The comparison is between the new covering and the old, so a heavier slate on a roof that carried a lighter slate counts, and so does concrete tile where there was slate.
Which council to ask depends on your address, and for Farnborough it is the Hart and Rushmoor Building Control Partnership.
Changing from tile to slate
This is the case that raises the question. A roof covered in lightweight tiles was designed for them, and natural slate can be heavier. Replacing the tiles with slate may cross the 15 per cent line, and the rafters, purlins, wall plate and walls would then need checking. The portal's advice to take structural advice before starting applies directly.
The reverse is the opposite. Swapping slate for a lighter covering lowers the load, which raises no weight issue, though the pitch and the look have to suit. A tile that needs a steeper pitch than the roof has is a separate problem, as our answer on the minimum pitch for slate notes for the slate side.
Concrete and clay differ from slate as well. HomeBuilding (Sep 2025) gives typical lives of "40-50 years" for plain concrete and "60-80 years" for clay, against "100+ years" for Welsh slate, and those are the figures people compare when weighing a change. Weight is the other side of the choice.
Where the structure is old, a check is sensible even when the weights are similar. Rafters that have sagged, a ridge that has dipped, or battens that have rotted show a roof already under strain.
We measure the rafters, spans and spacing from the loft, photograph the timbers and pass the facts to an engineer where it is warranted.
What the structural check involves
A structural check looks at the roof's frame and the walls under it. The engineer or surveyor considers the span and size of the rafters, their spacing, the purlins and ridge, the wall plate, and the condition of the timber. They calculate whether the structure can carry the new covering with its load from snow and wind, and they say what strengthening, if any, is needed.
Outcomes vary. The roof may be fine as it is. It may need some strengthening, such as extra timbers alongside the rafters or a stronger wall plate. Or the answer may be that the heavier covering is not suitable and a lighter one should be chosen.
Paperwork counts. Building control may want the calculations, and a completion certificate helps at sale. We keep our own survey photographs and measurements for the file, and we can give them to the engineer.
Finding out before the slate is ordered and the scaffold is up keeps a change of plan cheap.
You see the photographs and the findings in plain words, and each route comes as one written total. See slate roofing for the wider service.
