Can you put new felt over an old flat roof?

Why an overlay is tempting

Stripping a flat roof is the messy, noisy part of the job. Old felt comes off in heavy sheets, there may be several layers, and the deck underneath is sometimes in a worse state than anyone hoped. Laying a new layer straight over the old one skips all that. It is quicker, it produces less waste and it costs less in labour, which explains why it is offered so often.

The idea is not unreasonable. Built-up felt is a layered system by design, and a new top layer is in some ways the same thing the roof was built with. A felt roof in fair order, with tired or crazed surfaces but a dry body, can take a new cap sheet and gain several more years of service. Typical lifespans for felt run from around 10 to 20 years, with longer for good three-layer systems, and the quality of the work counts as much as the date.

The question is not whether overlaying is allowed. It is what lies under the old surface, and whether the roof underneath can honestly be called sound.

When going over the old roof works

An overlay has a fair chance of lasting when several conditions are all true at once.

• The old felt is dry throughout. No soaked patches, no water between layers, no sponginess underfoot.
• It is stuck down firmly. It lies flat to the deck, with no big blisters, splits or lifting laps.
• The deck is firm. Boards are sound and fixed, with no soft areas and no stains from below.
• The surface drains. There is no persistent pooling that signals a dip in the deck.
• The details are fit. Upstands, flashings and outlets can be redone properly over the top.

If every item holds, a good roofer can clean the surface, treat any minor faults and lay the new system over it. Surveys usually include cutting a small test hole, called a core, to look at the layers and the deck, and checking moisture from underneath where the ceiling allows.

One further point is weight. Building control treats a new covering that is 15 per cent heavier than the existing one as needing approval and a structural check. An additional felt layer is light, but the point matters on a roof that is already carrying several layers.

When an overlay traps the problem

Water is the enemy of an overlay. If the old roof has been leaking for a while, the felt may be sealed on top and wet beneath. Insulation boards soak up water and lose their performance, and the timber deck starts to soften. Laying new felt on top of that traps the moisture in place. In warm weather it turns to vapour and pushes at the new layers, which is how blisters form, and in cold weather it keeps the deck damp so that rot spreads.

Visible signs include large spongy areas, bubbles that sag when pressed, a roof that feels warm and soft in summer, and ceiling stains below. Another sign is a long history of patching. A roof that has needed repeated repairs has usually been letting water in for longer than anyone realised.

A roof that has too little fall is another poor candidate. A new layer does not change the shape of the deck. If water pools now, it will pool on the new surface, and standing water shortens the life of any covering. Our answer on flat roof insulation explains why the layers below the surface matter just as much as the ones you can see.

Where a roof has several old layers already, adding another increases the weight and the build-up height, and makes the edges, the upstands and the flashings harder to detail correctly.

What the Building Regulations say about re-covering

The Planning Portal guidance on re-roofing sets two figures. A re-covering generally escapes approval when it touches below 25 per cent of the building envelope and below 50 per cent of the roof. Past those figures the job is notifiable, and the roof can be asked to perform better thermally. For existing dwellings, the Part L table for a new roof element gives a U-value of 0.15 W/m²K.

In practice, a small extension roof sits under those thresholds, so overlaying it falls into the re-covering category. A large roof, or a job that replaces the whole of a major flat roof, can cross them. At that point insulation becomes part of the specification, and an overlay of felt alone no longer meets the brief.

That is why an overlay sometimes costs more in the long run. A roof renewed to current standards gets insulation, a new vapour layer and a covering that is chosen for the building. Overlaying a worn felt roof without that work delays the decision. We discuss the costs and trade-offs on our page about the pros and cons of flat roofs.

How to decide for your own roof

The sound way to decide is to open the roof and look. A reputable roofer will cut a test section, check the layers, probe the deck and tell you what is there. If everything is dry, flat and firm, an overlay can be a sensible, economical choice. If the layers are wet or the deck is soft, the right answer is to take the roof back to bare deck, mend or renew whatever has failed, and rebuild upwards.

Ask for the reasoning in writing, not just a price. A quote that says only "new felt over existing" tells you nothing about what lies beneath. A quote that records the test results, the state of the boards and the way upstands and outlets will be dealt with gives you something to hold the work to.

It is also worth asking what else a different covering offers. EPDM and GRP are laid on a clean, sound surface and have their own strengths, which we compare in EPDM or GRP. A new membrane over a stripped deck often gives a longer life than a felt top-up.

We look at the layers on survey day, tell you openly whether an overlay is a reasonable choice, and fix the price in writing afterwards. Our wider work on flat roofs around Bath is described on the flat roofing page.