Key points
- An overlay only works on a sound, dry deck with a well-bonded, compatible existing covering.
- Core samples and moisture readings, not the look of the surface, show whether a roof can be overlaid.
- Overlays add weight and height, and overlaying more than half of a roof over a heated room usually triggers the thermal upgrade rules.
- Wet insulation, a soft deck, poor falls or previous overlays mean the roof should be stripped.
What does overlaying a flat roof mean?
Overlaying means putting a new waterproof layer over the existing covering instead of removing it. The old felt, asphalt or membrane stays in place and becomes the base for the new system, which can be a liquid-applied coating, a new membrane bonded or fixed over it, or, on some roofs, new insulation and a membrane on top of everything.
Stripping means removing the old covering, and often the insulation and deck too, back to the joists or concrete, then building a new roof up from there. It is more work, but it lets the roofer see and fix whatever is underneath.
When does overlaying a flat roof work?
An overlay works when three things are true: the deck is sound, the roof is dry inside, and the existing surface is suitable for the new system to go on. In practice that means:
- A firm deck with no soft, spongy or sagging areas.
- Dry insulation and deck. Wet layers stay wet under an overlay, rotting timber and losing insulation value.
- A well-bonded existing covering, with blisters, loose laps and lifted areas cut back and made good first.
- A compatible substrate. Some systems cannot go directly on others without a separating or carrier layer.
- Adequate falls. An overlay follows the shape of what is underneath, so it does not cure ponding unless falls are added in the build-up.
- Room for the extra height at upstands, doors and edges.
When those boxes are ticked, an overlay can be a sound, cost-effective option. When they are not, it is just a new skin over an old problem.
How do roofers find out whether the deck is sound and dry?
By taking core samples and moisture readings, not by looking at the surface alone. A roof can look tidy from above while the insulation underneath is saturated.
- A close inspection by the roofer, with safe access, looking for soft spots, blisters, ponding and failed details.
- Moisture readings across the roof to map damp areas.
- Core samples. Small holes are cut through the build-up at chosen points to see each layer: covering, insulation, vapour control layer and deck. They show whether insulation is wet, what the existing build-up is and how thick it is.
- Inside checks of the ceiling below for stains, sagging or condensation.
- Adhesion checks on the existing surface where a liquid or bonded overlay is planned.
Core holes are sealed after sampling. A flat roof survey with a written report is a guide £150–£300; if you are already planning a replacement, start with the free survey and the roofer will advise whether cores are needed.
Can a liquid roof coating go over an existing roof?
Yes, this is one of the commonest overlays, provided the roof underneath is sound, dry and properly prepared. Cold liquid-applied systems are usually reinforced with a fleece and can be applied over many existing felt, asphalt and some other surfaces, following round upstands, outlets and awkward details without joints.
Preparation decides how long it lasts. The surface is cleaned, loose material removed, blisters cut out and repaired, and a primer used where the manufacturer specifies one. Some systems need a test patch or adhesion test first.
Service life varies by product, from around 10 to over 25 years, with one reinforced system BBA-assessed at 25 years or more. A liquid coat over a wet roof traps moisture in and fails early, so the survey matters as much as the product.
Does an overlay add too much weight or height?
It can, so both are checked before an overlay is specified. A liquid coating adds little weight. A new membrane over the old one adds more, and adding insulation, a second deck or a green roof build-up adds a lot more.
In England, a new roof covering that adds 15% or more to the roof's weight brings the work under building regulations, and the structure may need checking. Older garage and extension roofs often have little spare capacity.
Height is the other limit. Each layer raises the finished roof, so upstands may end up below the usual 150 mm above roof level, door thresholds and window sills may be too close, and edge trims and gutters may need resetting. On a roof already overlaid once or twice, there is often simply no room left.
Do building regulations apply to an overlay?
They can. Leaving the old covering in place does not take a job outside the rules: in England, Wales and Northern Ireland, overlaying more than half of a roof over a heated room is usually treated as renovating a thermal element, just like a full re-roof. In Scotland, check with the council's building standards service.
That matters for the overlay decision, because the roof should then normally be upgraded towards the renovation U-value where it is technically, functionally and economically feasible. On a roof with little or no insulation, that usually means new insulation on top of the old covering, which adds exactly the height and weight discussed above, and the old covering then often has to act as the vapour control layer, which it can only do if it is sound and continuous.
Small patch repairs are different, and an unheated detached garage is normally outside the thermal rules. We handle building control or building standards where a job needs it; see the building regulations guide for the targets in each nation.
When should a flat roof be stripped instead?
When the problem is underneath the covering, or the roof has no room left for another layer. The survey findings usually point clearly one way or the other:
| Survey finding | Overlay? | Usual approach |
|---|---|---|
| Dry cores, firm deck, well-bonded covering, good falls | Often yes | Prepare and overlay with a compatible system |
| A few local blisters or loose laps, otherwise dry | Often yes | Cut out and repair locally, then overlay |
| Wet insulation or damp deck in places | No | Strip at least the wet areas, usually the whole roof |
| Soft, rotten or sagging deck | No | Strip to joists, repair, new deck |
| Ponding from poor falls or backfalls | Rarely | Strip and re-fir, or tapered insulation |
| Already overlaid once or more | Rarely | Strip back to the deck |
| Cold roof with condensation | No | Strip and rebuild as a warm roof |
A stripped roof is also the only way to meet some manufacturers' specified build-ups for a material guarantee. The ponding water guide covers how falls are rebuilt.
Older roofs sometimes contain bitumen products or boards that may include asbestos. If that is suspected, it is surveyed and removed by a licensed specialist before roofing work starts.
How much does overlaying cost compared with stripping?
An overlay usually costs less than a strip because there is less labour and less waste, but only when the roof is genuinely suitable. Guide prices, including VAT, for an 18 m² single garage on a sound deck, plus the items that are added when a roof has to be stripped back further:
| Item | Guide price |
|---|---|
| Liquid-applied garage roof | £2,050–£3,250 |
| EPDM garage roof | £1,600–£2,800 |
| Strip-out waste and skip | £250–£400 |
| New deck, per m² | £20–£35 per m² |
| Warm-roof insulation, per m² | £35–£60 per m² |
These are guide prices; the fixed price comes after a free survey. An overlay over a wet roof can end up costing more in the long run, because the roof still has to be stripped later. See the flat roof cost guide or use the guide-price tool.
Who decides whether to overlay or strip?
The decision should come from what the survey finds, set out in the written quote, not from the price alone. Ask the roofer to tell you what the core samples showed, why they recommend an overlay or a strip, and how the choice affects falls, height, insulation and the guarantee.
If you are comparing quotes and one firm proposes an overlay while another proposes a strip, ask each to explain the condition of the deck and insulation. The repair or replace guide covers the earlier question of whether the roof needs replacing at all. For a flat roof replacement we offer a free survey and a fixed written quote with no obligation.
More questions
Can you put EPDM over an existing felt roof?
Sometimes, where the felt is sound, dry and well bonded and the system allows it, often with a separating layer or new insulation board between them. Many roofers prefer to strip old felt so the deck can be checked.
How many times can a flat roof be overlaid?
There is no fixed number, but each layer adds weight and height. Once a roof has been overlaid before, stripping back is usually the better option.
Will an overlay stop ponding?
Not on its own, because it follows the shape of the roof underneath. Ponding needs the falls correcting with firrings or tapered insulation.
Is a liquid coating over my old roof a permanent fix?
On a sound, dry, well-prepared roof, a reinforced liquid system can last many years. Over a wet or failing roof it will not, because the problem underneath remains.