Question answered
Can you put a new roof over an old one?
The short answer
On a pitched tiled or slated roof, a new covering does not go over the old one. The old tiles come off so the battens, underlay and rafters can be inspected and the new covering fixed to sound timber, and the extra load of a second layer is a further problem. Overlays are used on some flat roofs and on fragile asbestos cement sheeting, where the aim is to leave the old surface alone.
On a pitched roof the answer is almost always no. Flat roofs and some old sheet roofs are the exceptions, and each comes with conditions worth understanding first.
What lies under old tiles that you cannot see
Tiles and slates are only the visible skin. Underneath sit the underlay, the battens that carry the covering and the rafters that carry the battens. Those are the parts that fail quietly: battens rot where water has tracked along them, underlay perishes with age and rafter feet decay at the eaves. A second covering laid on top hides all of that and buries a problem you have just paid to cover up. If those timbers then fail, the cost of reaching them includes lifting two layers of tile instead of one.
Fixings matter as well. Marley explains that BS 5534 moved the trade towards screws and clips for the ridge, the hips and the single-lap tiles, with the outer border of the roof fixed twice. Those nails and clips need to land in sound battens at the right spacing, and you cannot guarantee that through an old layer. A stripped roof lets new tiles be set out afresh, with correct headlaps and flashings, rather than copying whatever was there before and inheriting its mistakes.
Extra weight is a Building Regulations question
A second layer adds load to timbers that were sized for one. The Planning Portal puts approval on the table once a new covering weighs 15 per cent or more than the roof carried before. Approved Document A frames it as loading rising by more than 15%, with strengthening or replacement of a structure that cannot cope. Strengthening of that kind counts as a material alteration. Each source is named because their wording differs by a hair.
Doubling up tiles on an old timber roof, or laying concrete over lighter slate, crosses that line fast, and the rafters may never have been sized for it. For a covering far heavier or lighter than the one coming off, the Planning Portal suggests consulting an engineer or a surveyor before starting. Our page on heavier roof coverings goes further into how the check is made, and the survey is where we weigh up what your own roof can carry.
Where an overlay does make sense on a flat roof
On flat roofs, bonding a new waterproof layer over a sound existing one is sometimes done. It can suit a roof whose deck is dry, firm and well drained, and whose surface is level enough for the new membrane to lie properly. It stops being a good idea when the old layer has trapped moisture, the deck has gone soft or water ponds, because the new covering inherits each of those faults and hides them, leaving the next leak harder to trace.
Insulation enters the picture too. Under Approved Document L, laying a fresh waterproofing layer on a flat roof is renovation work on a thermal element. Once the thresholds are passed, better insulation has to go in alongside it. So before we call an overlay sensible, we ask whether the existing build-up can be inspected, and the answer shapes both the method and the written quote.
Asbestos cement sheets and the overlay option
Garage, shed and outbuilding roofs of corrugated sheet from before 2000 may be asbestos cement. For buildings put up or refurbished before 2000, HSE says to assume asbestos is there, and its notes call such roofs fragile every time. One option in its guidance A14 is a new non-asbestos sheet laid on top, which keeps the old material untouched.
Taking such a roof off is a separate process. Sheets lifted out whole are non-licensed work, yet once they crack up substantially, the job becomes notifiable, and waste from a home needs a consignment note. Nobody should walk on, drill into or pressure-wash a roof like this, and a roofer should refuse if you ask. None of it applies to the tiled roof of a house, which is stripped and re-laid. The stripped roof is kept dry in stages, as set out in what happens if it rains during a replacement.
How the decision is made on your roof
We look first at the covering, the timbers beneath it and the pitch, then at how the roof has aged and what it has to cope with, from its exposure to any heavy chimney or extension junctions. Where a roof is genuinely sound apart from one fault, we say that plainly. Photographs from the survey show what turned up on your slopes, and the quote follows in writing as one total. If a flat roof is sound enough for an overlay, you will be told so; if it is not, you will be told why. For a pitched roof the route is a strip and a fresh lay of the covering, with the insulation that goes with it planned in the same visit.
If you cannot tell how long the old covering has to run, how to tell how old a roof is lists the clues. For the whole service, from survey to clear-up, see our roof replacement page.
Overlays and roof layers
Can new slates or tiles go straight onto the old battens?
Rarely. Old battens may be rotten, split or set to a different gauge from the new covering. Sound battens are sometimes kept where they pass inspection and match, but renewing them is the normal route because the new fixings need solid timber to grip.
Is covering an old flat roof cheaper than stripping it?
It can be, as less is removed and less waste leaves the site. But a wet deck beneath a new membrane just repeats the leak. What the survey finds in the existing layers decides the method, and the written quote reflects it.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.