Two flat roofs can look identical from the garden and be built in opposite ways. In one, the insulation lies on top of the deck, directly under the waterproof covering. In the other, it is tucked between the joists, with an air space above it. A third version lays the insulation over the waterproofing itself. The three layouts are called warm, cold and inverted. This page is part of our flat roofing advice, and it starts with the layouts themselves.
The difference is not academic. It controls where the dew point falls, which is the position in the build-up where moist indoor air turns to water. Get that wrong and a roof that looks perfectly sound outside can rot from the inside, with damp ceilings, mouldy plaster and dark patches that appear in winter and are blamed on a leak.
This guide explains the three layouts in plain terms, why a cold roof is the one that most often suffers from condensation, and why the layout matters when a worn covering is replaced. It is written for anyone working out what they already have, and what the roofer should be proposing for the next roof.
Three roofs, named for where the insulation sits
Start with the deck, the timber or board surface carried by the joists. Everything else is arranged around that layer.
With a warm roof the insulation lies over the deck, and the waterproof covering is the final layer above it. The deck and joists sit sitting on the room side of the insulation, close to the temperature of the room. A vapour control layer sits between the deck and the insulation to hold moist air back.
A cold roof puts the insulation below the deck, between or under the joists. The deck and the space around the joists are on the cold side, so they have to be ventilated with a clear air path. That keeps the timber dry, in theory, but it relies on the air being able to move.
An inverted roof reverses the usual order. The waterproof layer lies directly on the deck, and rigid insulation boards sit on top, held down by gravel, paving slabs or a similar weight. The waterproofing stays warm and protected, which spares it from the wide swings in temperature that age a covering exposed to the sun.
How to tell which you have
From above, you often cannot. A felt, EPDM or GRP surface looks the same over all three. The loft or ceiling void gives better clues: ventilation grilles at the eaves or soffit suggest a cold roof, a ceiling with no void and a thick build-up at the roof edge suggests a warm one, and loose gravel or paving over a membrane suggests an inverted layout. Where you are unsure, a roofer can lift a small section to check, and you get pictures of the layers as they come up.
Why cold roofs are the ones that suffer condensation
Air inside a house carries moisture from cooking, bathing, drying clothes and breathing. Warm air holds more moisture than cold air, so when that air rises through the ceiling and meets a cold surface, some of the moisture turns to water. In a cold flat roof, the underside of the deck is exactly that kind of surface.
The ventilation space exists to carry the moisture away, but a flat roof makes this harder than a pitched one. There is no steep slope to encourage air movement, and the air path can be blocked by insulation pushed too far in, by a boarded-up soffit or by a poorly planned extension in which the roof void ends at a wall. Once the path is broken, the damp air sits and condenses, and timber that stays damp for long enough starts to decay.
On pitched roofs, BS 5250 (as Marley summarises it) sets an eaves ventilation gap that depends on pitch and underlay. A flat roof void has its own sizing, shaped by span, depth and design, and it should be specified by whoever designs the roof, not guessed.
Signs of trouble
The tell-tale pattern is damp that spreads across the ceiling rather than coming in under one spot, appears in the cooler months, and does not follow the rain. Black spotting on ceilings, damp patches on joists seen from the loft or a musty smell all point the same way. Our page on telling condensation from a leak helps you separate the two before you call anyone out.
Warm roofs: the usual choice for a new covering
When a flat roof is replaced, a warm roof is the layout most roofers propose, and the reason is the dew point. With the insulation above the deck, the structure stays warm and dry, and any moisture that gets into the build-up has to cross a continuous vapour control layer first. There is no ventilated void to keep clear, so the layout does not depend on air paths that the next owner might block.
The trade-off is height. Insulation boards add depth on top of the deck, so a warm roof lifts the roof edge. That can create a step at a door threshold, push the covering against a window cill or leave less upstand at a wall, and it can change how the edge trim and the guttering sit. These are practical details worth checking before the work, not afterwards.
Getting the build-up right
A warm roof only works if the layers are installed in the right order and sealed where they meet. A vapour control layer with gaps, an insulation board left damp, or a hole cut for a vent that is not sealed will each invite the very condensation the layout is meant to prevent. We check each of these at the survey and during the work, and take photographs at the stages that will be hidden once the covering is down.
A warm roof can carry felt, EPDM or GRP, and the choice of covering is separate from the choice of layout. Our guide to torch-on felt roofs covers how the most traditional covering goes onto this build-up.
Cold roofs: living with an existing layout
A great many flat roofs in Worthing and along the coast were built cold, especially on extensions, porches and garages from the decades when ventilated voids were standard practice. Some work perfectly well for years. Others have a history of staining at the ceiling that nobody has traced.
If you have a cold roof that is performing, there is no need to change the layout simply for the sake of it. The priorities are keeping the ventilation paths open, keeping insulation from squashing the air gap, and checking the vapour barrier at the ceiling for gaps where cables or light fittings pass through. A downlight cut into the ceiling can leak a surprising amount of warm air into the void if it has no cover.
Converting cold to warm
When the covering reaches the end of its life, converting from cold to warm is a common choice. The old insulation between the joists can often stay, but the new layers are laid over the deck, and the ventilation openings are closed off so the void no longer has to breathe. That is not simply a matter of laying boards on the roof. The void must be sealed correctly, and the vapour control layer has to go in cleanly, or the cold roof's weaknesses carry over into a layout that does not allow for them.
It is worth asking any roofer proposing the change to explain which layers will be kept, which will be removed and why. Our page on flat roof maintenance explains what to look at in a cold roof between replacements.
Inverted roofs and where they make sense
An inverted roof is a deliberate choice, normally made because of what sits on top. Paving for a terrace, a green roof, or a plant area is far more at ease on an inverted layout, as the ballast that holds the insulation down also carries the load. The waterproof membrane stays below, shielded from sunlight, frost and foot traffic, which is a real advantage for coverings such as EPDM or a liquid-applied membrane.
The design asks for more care than a warm roof. Water runs through the joints between insulation boards and reaches the membrane, so drainage must be planned to let it escape, and the insulation has to be a type that tolerates being wet. The ballast adds weight, so the structure under it needs checking, and the depth of the build-up is greater still than a warm roof. Outlets and upstands need to be set higher so that gravel does not block them.
Maintenance on an inverted roof
Because the waterproofing is hidden, faults are harder to spot. Moss, leaf litter and silt gather in the ballast and slow the drainage, so the surface wants a regular look. If a leak does start, tracing it means lifting ballast and boards to reach the membrane, which is slower than on a warm roof where the covering is exposed.
For most domestic extensions and garage roofs with no terrace above them, an inverted layout is not needed. It earns its place where the roof has a second job to do, and a terrace or green roof is the usual reason.
Part L when the membrane is replaced
A new flat roof covering can bring thermal rules with it, which is the part homeowners seldom expect. Approved Document L Volume 1 (Dwellings, 2021 edition incorporating 2023 amendments) treats renovation of a thermal element as including stripping down to the structure and rebuilding, and paragraph 11.2 also lists replacing the membrane of a flat roof.
Paragraph 11.3 then sets two triggers. The whole element must be upgraded where more than 50 per cent of its surface is being renovated, or where the job amounts to a major renovation, which means more than 25 per cent of the external building envelope (by surface area) being renovated. Table 4.3 gives a roof threshold of 0.35 and an improved figure of 0.16 W/(m²·K) for existing elements. Table 4.2, for new or replacement elements, gives a limiting roof U-value of 0.15 W/(m²·K).
What that means in practice
In plain terms, a full re-cover of an uninsulated or poorly insulated flat roof will usually mean adding insulation, and a warm roof is the simplest way to do it. The depth needed depends on the product chosen and the structure available, and that is something the survey settles.
When approval applies is covered in Building Regulations for a flat roof replacement. Check with your building control body, or with the Adur & Worthing building control team, before work starts. If the job is notifiable we deal with the notification and tell you what consent is involved.
Ask any quote to say which layout it proposes, what U-value it is aiming for and where the vapour control layer sits.
