Read this before you agree a flat roof specification for an extension or a replacement. It explains each build-up in plain terms, what Approved Document C and BS 5250 say, and how the choice affects the rest of the job.
Every flat roof has to do two things at once: keep the rain out and keep the heat in. The insulation is what keeps the heat in, and where it sits decides how the roof handles the warm, damp air that rises from the rooms below. Put it in one place and the deck stays warm and dry. Put it in another and the space above the insulation has to breathe or it will collect moisture.
The two arrangements have plain names. In a warm roof the insulation lies over the deck and beneath the waterproof covering. In a cold roof the insulation is placed below the deck, usually between the joists, and a ventilated gap is left above it. The names describe the deck: it is kept warm in one and left cold in the other.
This guide sets the two side by side, looks at the Building Regulations wording and the moisture standard behind them, and then asks what it means for a rear extension or a garage conversion in Thanet. It belongs to our flat roofing work, where the build-up is settled at the survey, well ahead of the first delivery.
The difference in one table
Both build-ups can give a sound roof. They fail in different ways, and they go wrong when the wrong one is chosen for a particular room or detail. The table sets out what separates them.
| Question | Warm roof | Cold roof |
|---|---|---|
| Insulation | Above the deck | Between or below the joists |
| Deck | Stays warm | Stays cold |
| Void above insulation | None to ventilate | Ventilated gap needed |
| Ceiling below | Left untouched on a repair | Opened up if insulation changes |
A third arrangement, the hybrid, puts insulation both above the deck and between the joists. It is used where a thick warm layer alone would raise the roof surface too high at a door or a wall.
The waterproof covering sits on top in every case. Whether it is felt, rubber or GRP, that layer behaves the same way, so the choice between warm and cold is about what lies underneath it and how the insulation and the air in the building interact. A good specification names each layer, its position and the way it is sealed or ventilated, so the person on the roof is never left to guess.
One point is easy to miss. A cold roof depends on ventilation staying open for the life of the roof, and a warm roof depends on a continuous layer staying sealed. Each has a thing that can be broken later by a careless hand, a blocked vent or a pierced vapour layer.
How a warm roof is built
On a warm roof the order from the ceiling up is simple. The joists carry the deck, a vapour control layer lies over the deck, rigid insulation boards go over that, and the waterproof covering finishes the top. The vapour control layer is a sheet that holds back moist air from the room so it cannot reach the cold side of the insulation, and the insulation keeps the deck at room temperature.
Because the deck stays warm, the point where moist air meets a cold surface moves up to the waterproof layer, where it cannot cause trouble. There is no void to ventilate, so there are no eaves vents to leave clear and no gap to protect from being blocked in later years.
The build-up has two practical effects on an existing building. The finished roof surface sits higher, so upstands at walls, door thresholds and window cills need checking on the survey. And the ceiling underneath can stay as it is, which helps on a repair where the rooms below are decorated and in use.
A warm roof also suits a loft-style room that must stay habitable, because the whole depth of the joists remains open for services, wiring and light fittings. The price of that is the thickness above the deck, which has to be built into the roof's edge trims and its upstand heights from the start.
Cold roof construction and why it must breathe
On a cold roof the insulation lies between or beneath the joists, and the deck sits above it, exposed to outside temperatures. Warm moist air from the rooms will find any gap in the ceiling and rise into the space above the insulation. If that space is sealed, the moisture condenses on the underside of the cold deck, and the timber stays damp.
That is why the Building Regulations say what they say. Approved Document C, in paragraph 6.11, treats ventilating a cold deck roof as the way to meet its moisture requirement. In practice that means the air gap above the insulation needs a way in and a way out, usually through vents along opposite edges, so that a flow of outside air carries moisture away.
The air gap is also where a cold roof can be spoiled, through faults such as:
- insulation pushed hard against the deck, closing the gap
- vents painted over, blocked by cladding or covered by a new fascia board
- ceiling gaps round pipes and light fittings that let room air in
- a new extension joined to an old roof so the air path is cut
On an older cold roof being re-covered, the first question at survey is whether the ventilation can be checked and kept open. If it cannot, the conversion to a warm roof is a real option, and the covering and the insulation are then planned together.
What BS 5250 adds on moisture
BS 5250:2021 is the current British Standard code of practice for the management of moisture in buildings. It is a standard, and not the law in itself, and it sets out the detail behind vapour control layers, vents and condensation risk.
Moisture reaches a roof from the building's own air. A kitchen, a bathroom and a household of people all add water vapour, and warm air holds more of it than cold air. When that vapour meets a cold surface it turns to liquid. In a roof, the cold surface is the underside of the deck in a cold roof, or any point inside the insulation where the temperature drops below the dew point.
Two ideas follow from that. The first is to stop vapour getting in, which is the job of the vapour control layer and a sealed ceiling. The second is to give the vapour somewhere to go, which is what ventilation does in a cold roof.
A roof is tested by how it behaves over the winter, not on the day it is finished. Dampness appears as stained plasterboard, mould near the ceiling edges or drips at a light fitting, and our page on condensation under a flat roof covers the signs. Water vapour is not rain: a roof can be watertight above and still be wet from within. Fixing the covering on its own then achieves nothing, because the source is the air from inside the building.
Insulation and Part L on an extension
A rear extension is where this choice bites hardest. The roof spans a wide room, the ceiling is often a feature, and the Building Regulations ask for a standard of insulation. Approved Document L gives a U-value, a measure of how much heat passes through a building element. The lower the number, the better the element insulates.
| Situation | Table | Figure |
|---|---|---|
| New roof element | 4.2 | 0.15 |
| Existing roof, trigger value | 4.3 | 0.35 |
| Existing roof, improved value | 4.3 | 0.16 |
Approved Document L (2021, with the 2023 amendments) treats a fresh waterproof membrane on a flat roof as renovation work, and renovation is judged against Table 4.3. Paragraph 4.13 gives a way out where the upgrade cannot practically be done, or where the saving would take over 15 years to repay the cost. The 2026 edition takes effect on 24 March 2027, so the version in force depends on the date the work starts. Ask the building control team which edition applies to your job.
Which build-up reaches those figures depends on space. A warm roof can take a thick board above the deck without touching the ceiling. A cold roof is held back by joist depth, because the ventilated gap has to stay clear above whatever insulation goes in. Our guide to Building Regulations for flat roofs goes through the approval route.
Choosing for your own roof
No build-up wins everywhere. The answer follows from the building as it stands, the way the room below is used and the height available at the edges. These are the points weighed at the survey, taken in turn.
- Can the roof void be reached and checked? If not, a cold roof is hard to keep ventilated.
- Does the ceiling below need to stay untouched? A warm roof leaves it alone.
- How high can the finished surface go at doors, walls and window cills?
- How deep are the joists, and what insulation value does the work need?
- Does the roof have a rooflight or lantern, which changes the detail at its upstand?
On a new extension, a warm roof is often the simpler answer, because there is no ventilation path to protect and the insulation can be as thick as the upstands allow. On an older roof with sound, ventilated joists and a ceiling that suits the room, a cold roof can be re-covered and kept as it is.
Either way, the specification is written down before the work starts, covering the deck, the vapour control layer, the insulation and the covering. For a roof on a rear extension, our page on replacing a rear extension flat roof shows how that fits into the wider job.
If building control approval is needed, the specification goes to your council's building control team, which for Thanet is the council's own team.
