The short answer
Usually not, but the answer turns on which slate you choose and what is on the roof now. Slate is laid thin, so a slated roof is not always heavier than a tiled one, though thick slate can be. The Planning Portal's trigger is a new covering 15 per cent or more above the old, which brings the job under the Building Regulations.
Weight compared with tiles
People tend to picture slate as a heavy stone and assume the roof must be heavy too. The truth is more mixed. Natural slate is dense, but it is cut thin and each slate overlaps its neighbours by a set amount, so the load on the roof depends on thickness, size and head-lap, not on the rock alone.
Tiles differ too. Plain clay, interlocking concrete and Double Roman concrete tiles each weigh a different amount per square metre, and a tile and a slate of similar look may sit well apart. So there is no safe general rule that slate is heavier or lighter than tile. The numbers come from the exact product's own data sheet, usually quoted in kilograms per square metre of finished roof.
The fixings and extras count as well. A heavier slate often calls for stout battens, and a bedded ridge or mortar-fixed hips add their own weight. When we compare coverings for a house, we compare the finished roof, with its battens, underlay and ridge, and not only the slates. Our answer on slate or tile looks at the other differences.
The 15 per cent test
England's Building Regulations set a threshold for changes in roof loading. The Planning Portal sets the trigger at any covering that comes out 15 per cent or more above the old one in weight, as well as one with a very different fire performance. Approved Document A, the structure document, words it a little differently and treats a rise above 15% as significant.
Both are measuring the same thing, the new covering against the old. Sit the data sheets side by side. If the slate comes out at or above the threshold, the job becomes notifiable and needs approval from your building control body or a registered approver. If it comes out below, the national test does not require an application on that ground, though other triggers such as the share of the roof being replaced still apply.
Note what the rule is not. It is a Building Regulations matter, not planning permission,. Under the permitted development rules, Class C normally takes in swapping the covering on a house. A conservation area or a listed building adds checks, so ask your planning authority.
When a structural check is needed
Where the weight changes, the Planning Portal advises getting a structural engineer or a surveyor to look before works begin. That check looks at the rafters, the purlins, which are the horizontal beams that carry them, and the walls that take the load. Older timbers may have been sized for a lighter covering, or may have decayed where a previous leak ran through them.
This matters when a house with concrete tiles is changed to natural slate, or when a lighter covering is replaced with something heavier for a conservation area. The loft is where it shows: sagging between supports, a rafter that has bowed, a purlin with a split. A roof that already sags is a roof to have looked at before it gets a new weight on top.
On our survey we photograph the timbers and note any movement, and where the numbers are close or the structure looks tired we say a structural engineer should confirm it before ordering. That costs less than finding out after the slate is up. Our slate roofing page covers the full process.
Working it out for your house
The sum needs three figures: what is on the roof now, what you want to put on it, and the area. Start with the weight per square metre of the existing covering, which a survey can identify, and the same figure for the new slate from the supplier. Include the battens and the underlay on both sides of the comparison.
Then divide. If the new covering reaches the 15 per cent line, speak to building control and a structural engineer at that point. If it is clearly below, you will still want the roof timbers checked on a survey, but approval is not triggered by the weight alone.
One more point catches owners out: a lighter covering still has to be laid to its own rules. The slate's size and head-lap are set to the pitch, so see our answer on slate head-lap. Slate suppliers publish data sheets, so ask for the one for your slate before you compare quotes on weight.
