The short answer
Sometimes, but only where the old roof is dry, firm and flat enough to carry a second layer. Whether it can be done depends on the deck underneath, not on how the old surface looks. Wet insulation, soft boards or ponding rule it out. A new membrane laid over the old one is still renovation under Building Regulations.
Laying new waterproofing over the old saves stripping time, which makes it tempting. The deck beneath decides whether it is a good idea.
When an overlay is a reasonable choice
An overlay means leaving the old covering in place and fixing a new one over the top. It can suit a roof where the old layer is firmly stuck down, the boards below are sound and dry, and the fault is only that the surface has aged. Garages and sheds with a single layer of felt are the usual candidates.
What has to be true before we would overlay?
Four conditions have to be met. The old covering must be bonded or fixed down firmly, with no loose, blistered or wet areas. The deck must be sound, which means pressing it underfoot and finding no softness. The surface must drain, because a roof that ponds will not be cured by adding a layer over the puddle. And the roof structure must be able to carry the extra weight.
If any one of those fails, stripping back is the safer course. That is a judgement made on the roof itself, with samples taken where a doubt exists, not from the ground. Pictures from the survey show what lies under the top layer, so you see the same evidence we do.
A roof that has already been overlaid once is a particular worry, since a third layer on top of two is seldom a good idea.
Why the deck decides
The visible covering is only the top of a build-up. Beneath it sit the deck, any insulation and, in an older roof, sometimes a vapour layer. Leaks find the weakest of these.
Can I see if the deck is sound from above?
Not reliably. A felt roof can look flat and intact while the boards beneath are saturated. Moisture trapped between layers cannot escape, so an overlay seals it in, and the timber rots from the inside. Blistering is a clue that water vapour is already present, as the page on felt roof blistering explains.
A moisture reading, a cut-out sample and a walk over the surface tell far more than a glance. Where the boards are rotten or the insulation is wet, only a strip-back will cure the cause. Covering it up delays the cost and adds weight on a damaged structure. A flat roof lifespan also resets only when the whole build-up is sound, which is covered at flat roof lifespan.
Overlaying does not repair falls, either. If water lies on the surface, the cure is a new fall, not a thicker covering.
Weight, fire and Part L
Even a decision to overlay brings regulation into the picture. The Planning Portal sets two triggers for Building Regulations approval: a covering that raises the weight of the roof covering by 15 per cent or more, or one whose fire performance is significantly different. The Portal also advises asking an engineer or surveyor to check before work begins.
Is overlaying a way to avoid insulation rules?
No. Approved Document L names renewing a flat roof's waterproofing as renovation. Sheet laid on top of the old covering is still renewed waterproofing, so the safe assumption is that the thermal rules apply as they would to a strip-back. For renovation that means a 0.35 threshold in Table 4.3 and a 0.16 improved target, with the 15-year payback relaxation at paragraph 4.13. Check your own case with the building control body, since that is the authority on how it reads.
Putting insulation over the old roof is possible, but it raises the surface and the edge details, so upstands and trims have to be extended. The page on insulating a flat roof when replacing it goes into the figures, and Somerset Building Control is on 0300 303 7790 for questions about your address.
Overlay or strip: how the decision is made
The choice comes down to risk. Overlaying is quicker and produces less waste going away to a tip, but it hides what lies underneath. Stripping costs more time, yet it lets every board be checked and every fault be seen.
Which is the better buy?
Overlaying earns its place on a small, young, dry roof that has simply worn out at the surface. Stripping wins when the roof has leaked, has ponded or has been covered before, because the repair is worth little if the structure under it is not sound. The cost of a second job to put right a failed overlay can outweigh what the first one saved.
With a quote that proposes an overlay, ask what was checked under the old layer, whether the boards were tested for moisture, and how the finished roof will drain. A sound answer to each is a good sign. A vague one is not.
The survey leaves you holding pictures of the roof, our view on which route suits it and why, and a single written price for that route. You can read about the full range of systems on our flat roofing page, and send a few photos of your roof first if you want an early view.
More on overlaying
Can a rubber roof be laid over old felt?
It can in some cases, provided the felt is firmly stuck, dry and free from sharp edges, and a separating layer protects the rubber from bitumen. Wet felt, loose felt or a ponding roof are reasons to strip it instead. The maker's data sheet sets the conditions, so check it before agreeing.
Will an overlay look different from the street?
Slightly, because the build-up is thicker. Extra thickness raises edges and upstands a little, and any visible change to the outline is worth raising with Somerset Council's planning team first, particularly on a listed home or inside a conservation area.
The full picture: Flat roofing in Taunton ยท More short answers