The short answer
A natural slate roof is lighter than most people think. Thin slates laid with a generous overlap usually weigh less per square metre than concrete tiles, and a bit less than or similar to clay, while man-made slates are lighter again. The figure depends on slate thickness, size and the overlap. What matters is the change: a new covering that is 15% or more heavier than the old one needs building control approval.
What decides the weight
The weight of a slate roof comes from the slates, the battens that carry them, the nails and the overlap. A slate is laid so that each one covers the joint between two below it and overlaps the course beneath by a set head lap, which means that several layers of stone cover most of the roof. Thicker slates, such as the rougher Welsh or Cornish types, weigh more than thin, finely split Spanish or Brazilian ones. A smaller slate with a larger overlap uses more stone per square metre than a big slate with a small overlap. The battens and counter-battens add a little, and so does wet weather, because slate absorbs very little water, which is one reason its weight stays steady.
For comparison, a concrete tile is a thick, dense block and is heavier per square metre than a typical natural slate. Clay plain tiles vary. Fibre-cement and composite slates are the lightest of all, which can make them appealing on a roof structure with limited capacity.
Why weight matters to the structure
The rafters, purlins, walls and foundations were designed to carry the roof that was on the house when it was built, plus snow and wind. If you change to something heavier, you load all of that more than the designer intended. Old timbers that have been softened by damp, or that were slender to begin with, may not cope. Swapping a light covering for a heavy one on a roof that has already sagged is how some owners discover the problem. Swapping a heavy one for slate often reduces the load, which can be a benefit to the structure.
Our guide to natural versus man-made slate compares the options.
The 15% rule
Under the Building Regulations, a new roof covering that is 15% or more heavier than the old one needs building control approval, and the structure may need strengthening. The reason is simple: more weight can overstress the timbers. Going from concrete tile to slate rarely triggers this, but going from slate to a heavier concrete or stone-coated tile might. Re-roofing also has to meet the thermal rules where the work affects more than a set share of the roof. A like-for-like re-cover on a small part of the roof may sit below the threshold where building control gets involved, but a larger share of the roof can mean approval and an insulation upgrade for the whole roof, which is a separate rule from the weight test.
Where the job is notifiable we deal with building control for you, and we say early if the weight change calls for an engineer. If you are weighing materials against each other, slate or tile compares them across more than weight.
Checking your roof before a change
If you are thinking of changing the covering, ask for the roof structure to be looked at from the loft. The roofer looks for sagging ridges, bowed rafters, split or rotten timber, spread at the eaves, and any previous repairs. They check the spans and the spacing against the new load. In older houses, hand-sawn timbers often have more capacity than they look because they were generously sized, but damp, beetle or alterations can undermine that. A chimney removed or a loft converted can also change how the roof carries load.
Do not treat a published weight per square metre as exact for your roof, because it depends on the specific slate and fixing. Ask the supplier for the data sheet for the exact product and calculate the difference against the existing covering.
Slate on top of battens, and the hidden extras
The slates are only part of the dead load. Battens and counter-battens, underlay, nails, ridge and hip pieces, lead work and the mortar bedding at the ridge all add weight, and so does insulation laid on or between the rafters. If a roof is being reinsulated at the same time as a re-slate, the total change is the sum of all of these. On a house with a single-skin loft, the rafters may carry more than the slates and underlay alone suggest. Ask for the proposal to be set out layer by layer, so that you can see how the new build-up compares with the old.
Our survey includes pictures of the rafters and the covering and an honest view of whether the structure can carry the change. The price is fixed once the survey is done. Slate re-covering and repair are covered under our slate roofing service.
Every roof is different. A free survey with photos settles it for yours.
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