The short answer
On a pitched roof, no: the old covering comes off first. Laying new tiles or slates over old ones hides the battens and rafters you need to inspect, adds weight to timbers that were sized for one layer, and leaves the fixings with nothing sound to hold. A covering that is 15 per cent or more heavier needs Building Regulations approval, and two layers go well beyond that.
It sounds like a short cut, and it is not a safe one. Here is why a pitched roof is stripped back to the timbers before anything new goes on.
Why a pitched roof is stripped first
A tiled or slated roof is a system. Battens, the thin laths the covering is fixed to, run across the rafters, which are the angled beams spanning from wall to ridge. Beneath the battens, underlay acts as a back-up barrier against anything that slips past the tiles. A new covering needs fresh battens, fresh underlay and sound timbers underneath it. Putting it over the old one means none of that can be checked.
What do you find only when it is stripped?
Rot in the rafter feet, damp battens, woodworm, torn underlay, and sagging purlins, the heavy horizontal timbers that hold up the rafters, all hide under an old covering. They only show once the old slates or tiles are lifted. Fixing a new roof to timber that is damp or soft is how a new roof ends up failing early.
Our page on rotten roof timbers explains what turns up and how it is put right, and the re-roofing process shows where stripping sits in the sequence.
The weight problem and the 15 per cent rule
The Planning Portal puts the Building Regulations trigger at a heavier covering: an extra 15 per cent of weight or beyond. Approved Document A, the structural guidance, words its own test as a significant change in loading being an increase of more than 15%. Adding a second covering on top of the first makes the roof carry the weight of both.
Is that a Building Regulations matter or a planning matter?
Building Regulations. The 15 per cent rule is not a planning rule, and planning permission is not what stops you. The Planning Portal's advice is to bring in an engineer or surveyor for the sums first, because the rafters were designed for a particular load, and a second layer takes them well beyond it.
A second trigger, according to the Planning Portal, is a covering whose fire performance differs significantly from what it replaces. Separately, building control is normally not involved where under 25 per cent of the building envelope and under 50 per cent of the roof is affected, though the weight and fire tests still apply. Our answer on who notifies building control for a re-roof explains who does what.
Why layering fails the fixings and the details
Tiles and slates are held by nails and clips into battens. Over an old covering, the nails would need to be long enough to pass through the old tiles, the old battens and into the rafters, and the old tiles are brittle and uneven. Ridge, hips and verges are the worse for it: they need a firm base and a clean edge.
What happens at the eaves, verges and chimneys?
Every edge of the roof is built to a set height. A second layer lifts the covering above the gutter line, above the verge, and above the base of the chimney flashing, so lead has to be altered and water can run behind it. The details were made for a single layer.
Does the same apply to flat roofs?
This page is about pitched roofs. A flat roof is judged on its own terms, by its structure and falls, and the answer for one is not the answer for the other. See our flat roofing page.
A roof built for one layer should carry one layer. Ask any roofer who proposes two to put the weights and the rafter check in writing.
What to ask and what to expect
If a roofer offers to lay a new covering over the old one on a pitched roof, ask three things. What does each covering weigh per square metre, old and new? Who checked the rafters? Where are the photographs of the timbers under the old covering? A genuine answer to all three takes a survey, and a quote without one is a guess.
What do we do?
We survey first. We go up and photograph your roof, and show you the pictures of the covering, the ridge, the valleys and, where we can reach them, the timbers in the roof space. If the covering has to come off, you see why. If it can be repaired instead, we tell you so.
You then have one total price in writing. Scaffold is arranged with the scaffolder, and waste leaves the site with us.
A stripped roof is also the moment to add things a layered roof cannot take, such as new underlay, ventilation and leadwork. Sarking boards are one such upgrade, and the roof replacement page gives the whole picture.
More on stripping and weight
Is there any case where layering is allowed on a pitched roof?
The rule of thumb is no, because the timbers cannot be inspected and the weight rises. If a structural engineer says the rafters can carry the load, it becomes a Building Regulations question. A survey tells you whether any of that matters for your roof, and photographs tell you why.
Does stripping the roof cost much more than covering it over?
Stripping adds labour and waste removal, and it gives you a roof you can trust. Covering over adds weight and hides faults that cost more later. Cost guides give typical UK ranges, but your own figure comes from the survey, once we have seen what sits under the old covering.
The full picture: Roof replacement and new roofs in Taunton ยท More short answers