The short answer
Sometimes. A new covering can go over an old one when the existing roof is dry throughout, the deck is firm, the falls are right and the old layers are compatible with the new one. It saves on stripping and disposal. It is the wrong choice where the insulation is wet, the deck is soft, or the old roof already has more than one layer. Trapping a problem under new membrane only delays it, so the decision should follow a proper survey.
When an overlay makes sense
An overlay suits a roof whose covering has worn but whose build-up is still healthy. The deck is firm underfoot, the insulation (if there is any) is dry, the roof drains properly and the old layer is still bonded down. In that condition a new membrane can be laid over it, and the result is a sound roof with less mess and a shorter job. The existing covering also acts as an extra barrier.
It works best where the old roof is a single layer of torch-on felt and the new one is a compatible system, or where an old roof can take a specially formulated bonded membrane or liquid coating. It does not suit every combination, because some adhesives and solvents attack old bitumen, and some old surfaces do not accept a bond at all.
When it is the wrong choice
Wet insulation is the commonest reason to refuse. If water has got into the layers underneath, covering it over seals the moisture in and lets it carry on rotting the deck. Blisters, soft spots that give under foot, and a roof that has been leaking for a long time are all warning signs. Our guide on flat roof blisters explains what the bubbles mean, and it is usually a case for stripping rather than overlaying.
Ponding is the second. A dip that holds water will hold water under the new membrane too, so the fall has to be put right first. Our page on ponding on a flat roof goes into that. A roof that has already been overlaid once is also a poor candidate, because the added layers add weight and make it impossible to tell what lies beneath.
Building Regulations and insulation
There is a regulatory twist. The Planning Portal says that repairs and re-covering do not normally need an application where they affect less than 25% of the building envelope and less than 50% of the roof. Above that threshold, approval is needed and the whole roof's insulation may have to be upgraded under Approved Document L. Replacing a membrane, as opposed to overlaying it, generally brings the insulation upgrade into play, while an overlay is usually treated as a repair. That can make the overlay look cheaper, but it can also mean a poorly insulated roof stays poorly insulated. We tell you which route we think is right and, where work is notifiable, we handle the Building Regulations side.
What it saves and what it hides
An overlay saves the labour of stripping, the cost of taking old material away and the exposure of the deck to rain while the roof is open. Against that, it hides the condition of the deck. A strip lets us see the boards and joists, replace rotten ones and correct the falls. An overlay gives you no such chance. If the roof has a hidden fault, you carry it forward under a new surface.
It also adds a little weight and raises the roof edge, which can leave trims and flashings sitting low against the new level. Those details need adjusting, so they are checked at survey.
Checks that come before an overlay
A good overlay starts with finding out what is already there. We lift a corner or take a small core where the covering allows, to count the layers and see what the insulation is. A moisture reading, or a simple check for dampness on the underside from the room below, tells us whether water has been getting in. The roof edge is examined too, because an overlay raises the surface and the trim, gutter and any wall upstand have to cope with the extra thickness.
It also matters how the new layer will be fixed. A fully bonded membrane needs a clean, sound surface, while a mechanically fixed one goes through the old layers into the deck and needs to find good timber. Where the old roof has a skin of loose chippings, they are swept off first, or the bond fails.
How we decide
The survey is free. We look at the roof from above, test the deck by feel and look for moisture, then photograph what we find so you can follow our reasoning. Where an overlay will do the job honestly, we quote for that, and where it will not, we explain why and quote for a strip. The quote is fixed and in writing, and the work carries a 10-year workmanship guarantee. For the full range of systems, see flat roofing in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
Get a free survey