Slate roof weight
Is a slate roof heavier than a tiled roof?
The short answer
It depends on what you compare it with. Natural slate is cut thin, so a slate roof is often lighter than the same area of concrete interlocking tile, while thick or large slates and clay tiles can come out heavier. The number that settles it is the weight per square metre on the maker's data sheet. If a new covering is 15 per cent heavier or more, building control and a structural check come into it.
How slate compares with tile on weight, why the figure matters when one covering replaces another, and who checks it.
Slate against tile: why no single weight fits every roof
People expect stone to be the heavy option, so the answer surprises them. A natural slate is split into a thin sheet, and each one only has to cover a small part of the slope. Concrete interlocking tiles are moulded blocks with real thickness. Area for area, thin slate is often the lighter covering.
Often is not always. Slate is sold in different thicknesses depending on the quarry and the grade, and a roof of big, thick slates weighs more than a roof of thin ones. The slates are only part of the load, too. The battens they hang from, the underlay beneath and the overlap between courses all add to what the rafters carry. Clay plain tiles are laid with a double lap, two layers over most of the slope, and that can push them past slate. For that reason we never hand out a flat verdict on weight. We look at the product proposed for your roof and set it beside what is on the roof today, because the comparison only means something for two named products.
Where a reliable weight figure comes from
The figure to trust is the one the manufacturer prints for the exact product, usually as kilograms per square metre of finished roof. Ask for it by product name. A general saying such as slate weighs about the same as tile is no use when you are choosing between two real products with different thicknesses.
Welsh, Spanish and other imported slates differ in thickness and finish, so the weight differs with them. Our guide to Welsh and Spanish slate explains what the origin names tell you. Reclaimed slate is a separate case: thickness varies from slate to slate, and old nail holes and wear change how many are fit to go back. When we write the quote we name the covering, so the load can be checked against the roof before anything is ordered. If a supplier cannot give a weight, that tells you something about the product.
The 15 per cent line when one covering replaces another
Swapping the covering is where weight turns into a rule. One of the Planning Portal's triggers for a building control application is a new covering adding 15 per cent or more to the roof's weight. Approved Document A sets a threshold of its own, describing a load increase of over 15 per cent as significant. The wording differs slightly between the two, so we treat 15 per cent as the point at which to stop and check. This sits under the Building Regulations and is dealt with by building control. It is not a planning permission question.
The Portal adds some practical advice: when the replacement tiles or slates differ markedly in weight from the old ones, have a structural engineer or surveyor look at the roof before the job begins. Our guide to re-slating and Building Regulations goes through the other triggers, such as insulation.
From slate to concrete tile, or back to slate
Owners raise this because a neighbour has done it. Arun's conservation appraisal for Aldwick Road notes that many of its slate roofs were swapped for concrete interlocking tiles over the years, and it asks for the original material to be restored. Moving from slate to concrete tile can add a good deal of load. Moving back from concrete to slate often takes some off, although the frame still has to be sound beneath the new covering, and the battens and fixings change.
Where the house sits in a conservation area, raise it with Arun District Council's planners first, before you settle on a covering. Our guide to slate against tile compares the two on price, lifespan and fixing, and the choice should rest on the whole picture and not the weight alone.
What gets checked on the frame before the covering is chosen
Whatever goes on top is a load, and the frame has to carry it. On the survey we sight along the ridge for dips, look at the rafters and purlins from the loft where we can reach them, and photograph anything that has moved, bowed or gone soft. A roof that has carried slate for a long time and still runs straight rarely has a weight problem when the new covering is a similar one. A sagging slope or damp timber is a different conversation, and you hear about it before any price is given.
The photographs come with a written quote, set out as a single total. If a repair would hold, we recommend it in place of a new covering. Our slate roofing page describes the whole range of work.
Related questions
Weight and the roof frame
Do the rafters need strengthening before a slate roof goes on?
Not by default. A frame that has carried slate for years usually copes with a similar covering, and weight only becomes an issue when the replacement is far heavier, or the timbers have decayed or begun to sag. We check at the survey. If the load rises by 15 per cent or more, building control and a structural check follow.
Who can tell me what my slates weigh per square metre?
The maker of the product, through its data sheet. Your roofer should name the exact slate and underlay in the written quote so the figure can be looked up. For old or reclaimed slates there is no data sheet, so the load is judged from the roof itself and from the survey photographs.