Roof weight
Is a slate roof heavier than a tiled roof?
There is no single answer, because every slate and tile has its own weight. The rule that matters is how much heavier the new covering is.
The short answer
It depends on which slate and which tile, so no single answer holds, and the weight to compare is the one on each maker's data sheet. The test that matters is the Planning Portal's: a new covering that comes out 15 per cent or more above what the existing roof covering weighs needs Building Regulations approval. Approved Document A sets a separate trigger, at loading increased by more than 15%. A survey compares the two products before any covering is chosen.
5 min read ยท Part of Slate roofing
Why no single figure settles it
"Slate" and "tile" each cover a family of products. Slate can be Welsh, Spanish natural slate, reclaimed slate, fibre cement or man-made, and tiles can be clay or concrete in plain, pantile and interlocking forms. Marley alone lists eleven concrete tile products in its range, each with its own minimum pitch.
A weight per square metre belongs to a product, not to a material. Two slates of different thickness differ, and so do two concrete tiles of different profile. The covering is also only part of the load: battens, fixings and the overlap all add to what the rafters carry, and a longer lap puts more slate on every square metre of the slope.
For that reason a blanket rule in either direction would be wrong for some pairs of products and right for others. The only sound comparison is between the two actual products, set side by side on their published data, with the lap and the batten size for each laid out beside them. That is the comparison made before a covering is recommended, and it is put in front of you in plain terms. If the two products turn out to weigh about the same, the weight rule drops out of the conversation altogether and the choice rests on the other facts.
The 15 per cent test
Weight is the rule that bites even on a small area of roof, since it concerns the covering itself and not the share of the roof the work reaches. Two documents word it slightly differently, so we quote them separately and name each one.
| Document | Wording |
|---|---|
| Planning Portal | A new covering carrying 15 per cent or more extra weight |
| Approved Document A | Loading increased by more than 15%, which counts as a significant change |
Approval is likewise needed where the new covering's fire performance would differ significantly from that of the existing covering. Before any work, the Planning Portal advises checking with a structural engineer or surveyor. A heavier covering is not taken past that step on a hunch, and no one should be asked to sign off a covering whose weight has not been checked against the structure that must carry it.
One is a householder's guide and the other is the technical document behind the regulations, so a quote that cites one is not citing the other. Our answer on changing to heavier roof tiles follows the structural check further.
What the survey checks in the loft
If the covering is changing, the loft is where the structure can be read. We look at the timbers that carry the load and compare the two products.
- The rafters, which are the sloping timbers under the covering.
- The purlins, the horizontal beams that support the rafters part way down the slope.
- The wall plates, the timbers laid along the tops of the walls.
- The weights of the old and new coverings, taken from the makers' data sheets.
Where the new covering is the heavier of the two, a structural engineer is brought in before a price is settled. Where it is the same weight or lighter, the Building Regulations weight trigger does not arise, though the other tests still apply, such as the share of the building envelope and the roof that the work reaches.
The Planning Portal sets the threshold for a re-roof at less than 25 per cent of the building envelope and less than 50 per cent of the roof; where a job falls under both figures, an application is not normally required. Our slate roofing page shows how a slate job is scoped.
Weight is one reason among several
Even when weight is not an obstacle, the choice between slate and tile turns on other published facts. Pitch is one: natural slate has a 20 degree minimum in the case Cupa describes, while Marley's tile minimums run from 12.5 degrees to 40 degrees depending on the product. Lifespan is another, as HomeBuilding gives it.
| Covering | Lifespan (approx) |
|---|---|
| Welsh slate | 100+ years |
| Clay tiles | 60 to 80 years |
| Plain concrete tiles | 40 to 50 years |
These are ranges for the covering, not promises for the roof. The price depends on the roof, the access and the product, and it comes as a single written figure after a survey. Because both coverings are described on published sheets, you can check every figure here against the maker's own pages, and ask us to show our working on any of them. A quote that skips the comparison and simply names a covering has left the main question unanswered, so it is worth asking for the comparison in writing.
Our answers on minimum pitch for slate and how long a slate roof lasts go further into the first two points.
More questions
Does changing the type of covering need Building Regulations approval?
A change of covering meets the same tests as any re-roof. Approval follows when the new covering carries 15 per cent or more extra weight or would differ significantly in how it performs in a fire, or when the work reaches more than 25 per cent of the envelope or 50 per cent of the roof. Dover District Council also lists like for like replacement.
Where do I find the weight of a slate or a tile?
On the manufacturer's data sheet for that product. We compare the sheet for the covering on your roof with the one for the covering you have in mind, then look at the timbers in the loft, before any question about structure is put to an engineer.
Tell us what's wrong with the roof.
You'll get one written total price.