Can you lay a new flat roof over the old one?

Stripping a flat roof means lifting the old covering, taking it away and disposing of it, and that takes time. Laying over what is already there looks like a shortcut, because it avoids the stripping and shortens the job. It is a fair question, and in the right conditions it is a fair answer.

It is the right answer only if the existing roof gives the new one a sound base. A new covering is only as good as what lies beneath it. If that base is damp, loose or uneven, the new layer inherits every one of those faults and hides them from the next person who looks. Fewer days on site, less noise and less waste leaving the property are real gains, but they are worth having only when the roof underneath has earned the chance to stay.

Three things need to be true. The deck underneath must be dry and firm, with no soft or rotten patches. The existing covering must be well stuck down, with no blisters or wet insulation trapped in it. And the new system must be one that suits being laid over the old one, which depends on the maker's instructions for that product.

A roof with a single layer of felt that is dry, flat and firmly stuck can fit that picture, and the new covering is then chosen to match. Even so, the extra layer adds weight and a little height at the edges, so trims, upstands and flashings may need reworking to suit. An old roof left in place may also have details that are due for renewal, and burying them under a new layer only delays the day they fail. A moisture survey and a few small cut inspection holes are the way to confirm it, which is far more reliable than an opinion from the ground. The holes also tell us how many layers are already there and what sits under them, so the verdict rests on what we have seen and photographed, not on a guess from the ground. The photographs are yours to keep, and you can question any of them before you decide.

Strip it when the felt is blistered, the insulation is wet, the boards are soft, or the roof already has several layers. Every extra layer adds weight to the structure and makes it harder to find faults later. Where water has already got into the old layers, an overlay seals it in, which is the main reason overlays fail. Covering a wet deck traps moisture, which then turns to rot and may push the new covering up in bubbles, as our answer on blisters on a felt roof describes.

Stripping also lets us correct the falls. A roof that holds water needs tapered boards or insulation to put the slope right, and you cannot do that on top of an old covering. BS 6229:2025 keeps a minimum finished fall of 1:80, and roofers design steeper, around 1:40, so the finished roof still drains well. Our answer on flat roof minimum fall explains the figures.

Under Approved Document L, relaying the waterproof layer of a flat roof is renovation work. Once more than 50% of the roof's surface is renovated it should be upgraded if feasible, and renovating over 25% of the external building envelope counts as major renovation. A new membrane laid on top of the old one is not exempt, so the position needs checking before work starts, with your building control body or ours. If the roof belongs to an extension built some years ago, we ask for any building control papers you hold, because they show what was approved at the time.

Insulation is the usual consequence, and a roof overlaid with no extra insulation may not comply where the rule applies. We explain where your roof stands and handle the Building Regulations notification where the work is notifiable. The wider rules are set out on the page about flat roof Building Regulations, which is worth reading before you decide.

We survey the roof, test the deck and photograph what we find. You get a plain verdict: overlay, strip and replace, or repair. The leanest job that cures the fault is the one we recommend, and when stripping is the only sensible route we tell you so. A written quote with one total comes next.

If you are comparing coverings as well, the page on flat roofing lists the options we lay, and the typical lives of each covering are worth a look first. An overlay shortens the job, but it does not extend the life of the roof beneath it, so weigh the saving against what you will be left with in ten or fifteen years.