With a warm roof, the insulation lies across the top of the structural deck, immediately beneath the waterproof layer. In a cold roof, it sits underneath the deck, between the joists, and a ventilated gap lies above it. That single difference changes how the roof handles moisture, how much it needs ventilation and how it behaves when a roof is renewed.
The names describe the temperature of the structure. In a warm roof, the deck and joists stay on the heated side of the insulation, so they are warm and dry. In a cold roof, they sit outside the insulation, in the cold, and rely on a flow of air to carry away the moisture that rises from the room below. If that flow fails, damp air condenses on the cold timber and the roof rots from inside.
This guide explains how each build-up works, why condensation is the central risk, what Approved Document C says about ventilating cold decks, the Part L insulation figures and how to decide which build-up suits your roof. It is a general guide, and a survey is what settles the answer for a particular roof.
Where the insulation sits in each roof
A flat roof is a stack of layers, and the order of the layers is what separates the two types. Looking at each from the room below upwards shows the difference, and it is the same difference whatever the covering on top: felt, EPDM or GRP.
Cold roof, from below
- Ceiling board.
- Insulation between the joists.
- An air gap, kept open by ventilation at the edges.
- The deck.
- The waterproof covering.
The deck and the waterproof layer both sit on the cold side. The gap above the insulation is supposed to let air pass through and carry away any moisture that has crept up from the room.
Warm roof, from below
- Ceiling board.
- Joists.
- The deck.
- A vapour control layer, which slows damp air rising from the room.
- Insulation boards laid over the deck.
- The waterproof covering.
Here the structure is wrapped inside the insulation. The joists and deck are warm, so moisture has no cold surface to condense on, and no air gap needs ventilating.
Why it matters when you replace a roof
Before anyone chooses a new covering, the old build-up has to be understood, because the existing build-up decides what you can do. We lift a corner or look from below to see what is there. Where the old roof is a cold deck, you can convert it to a warm roof by placing insulation over the deck, which is a common upgrade. Our guide to flat roof Building Regulations explains how a replacement membrane counts as renovation.
Condensation: the risk that separates them
Everyday life puts moisture into the air in a house: cooking, bathing, washing and breathing. Warm air holds more water vapour than cold air, so when it meets a cold surface, some of the vapour turns to liquid. That is condensation, and in a roof it can happen inside the layers where nobody sees it.
Interstitial condensation
Approved Document C expects the layers of a roof to keep their strength and insulating value, and not to be spoiled by condensation that gathers within the build-up itself. In a cold roof, the cold timber deck is the surface at risk. If vapour reaches it, water collects, the timber becomes wet and decay follows. The first sign may be a stain on the ceiling, or nothing at all until the deck is soft.
Why a warm roof is safer by design
In a warm roof, the insulation keeps the structure warm and a vapour control layer slows vapour rising from the room. The deck stays above the temperature at which vapour condenses. The risk is not removed, because a damaged vapour layer or wet insulation can still cause problems, but it is lower by design.
Why a cold roof is harder to get right
A cold roof depends on three things all being right, and they are easy to undo with later work: a vapour barrier at the ceiling, no gaps through which warm air leaks upwards, and a clear ventilated void. A pipe hole, a downlight or an unsealed loft hatch lets humid air into the void. Insulation pushed into the edge blocks the vents. Each is easily done during later work. Our answer on condensation under a flat roof explains the signs.
What Approved Document C says about ventilation
Approved Document C (2013 edition) is the Building Regulations guidance on moisture, and a handful of its paragraphs bear directly on flat roofs. They are short, and the wording is worth knowing.
The aim
Paragraph 6.2 sets two aims for any roof. It must keep rain out of the building, and it must be designed and built so that damp forming inside its layers does not damage its strength or its insulation.
Routes to meeting it
Under paragraph 6.10, a roof meets the condensation aim if it follows Clause 8.4 of BS 5250 (the 2002 edition) together with BS EN ISO 13788:2002. Paragraph 6.11 then deals with cold deck roofs directly: the aim is met by ventilating them. The same paragraph allows that ventilation may not always be needed on a very small roof, such as over a porch or bay. Paragraph 6.12 adds a draught seal on loft hatches, a detail that matters to a cold roof above a hatch.
A note on the standard
The guidance cites the 2002 edition of BS 5250, but the current edition is BS 5250:2021, which the British Standards Institution lists as current and under review. A designer today reads the current standard alongside the regulation guidance as it stands.
What this means in practice
For a cold deck, ventilation is how the roof earns its approval. The void must be clear, vented at opposite edges and kept open through the life of the roof, which means nobody can bury it in insulation later. For a warm roof there is no void, and the design leans on the vapour control layer and the position of the insulation instead. Either way, the detail decides the outcome. The survey records the existing build-up with photographs before a choice is made, because you cannot design a safe roof around layers you have not seen.
The paragraphs describe the aim, not a single design, so your building control body is the right place to confirm a particular build-up.
Part L: the insulation figures
Approved Document L, the conservation of fuel and power, sets the insulation standard. For a flat roof being re-covered, the question is whether the work counts as renovation, and Volume 1 of the document says that renewing a flat roof's waterproof layer does.
| Item | Figure |
|---|---|
| Existing roof, threshold (Table 4.3) | 0.35 W/(m²·K) |
| Target after improvement (Table 4.3) | 0.16 W/(m²·K) |
| Payback test (paragraph 4.13) | 15 years |
| General upper limit after upgrade | 0.7 W/(m²·K) |
Reading the table
If an existing roof is worse than 0.35, renovation brings it towards 0.16. A lower U-value means less heat lost. Where full improvement is out of reach, or the simple payback runs past 15 years, the roof is improved as far as that allows, and the usual ceiling after an upgrade is 0.7.
Which build-up reaches it more easily
A warm roof puts the full thickness of insulation above the deck, and because it is not squeezed between joists, its thickness is not limited by joist depth. A cold roof is limited by the joist depth, because the air gap must stay open above the insulation. That can make reaching the target harder, and it is one reason warm roofs are often chosen at renewal.
The date to watch
The 2026 edition takes effect on 24 March 2027, with the tables renumbered 3.2 and 3.3. Until then, the 2021 tables apply. See our flat roofing page for the work this covers.
Choosing between them for your roof
Several practical points steer the choice, and they are worth weighing together. The survey tells you which apply.
Choose warm when
- You are replacing the covering and can lay insulation above the deck.
- Joist depth is too shallow to hold enough insulation with a ventilated gap.
- The roof is over a room with high moisture, such as a kitchen or bathroom.
- You want to avoid maintaining ventilation paths.
Cold may be right when
- Raising the roof level is not possible, for example at a door threshold or against a window sill.
- The existing build-up is sound, ventilated and dry, and the work is limited.
- Only the ceiling is being re-insulated from below, with the ventilation and vapour layer checked.
Heights and details
A warm roof adds thickness above the deck, which raises the surface, sometimes by a hand's breadth or more. That can affect door thresholds, upstands at walls, window sills and gutter heights. We measure each one on the survey. Where height is tight, a thinner, higher-performance insulation board can help, though it costs more.
Falls
The roof still needs a fall. Tapered insulation can create one, as our answer on tapered insulation explains. The finished fall under BS 6229:2025 is at least 1:80 for general areas.
None of this is a recommendation without seeing the roof. We photograph the existing layers, measure the heights and say which build-up fits.
Converting a cold roof, and what to check first
Plenty of existing flat roofs are cold decks, and their owners often want to improve them. There are three routes, and each has a caution.
Convert to a warm roof
Strip the covering, lay a vapour control layer over the deck, fix insulation boards above it and re-cover. This is the most complete answer, and it is the one most often chosen when the covering is due for replacement. The old insulation between the joists can be left or removed, and if left, the vapour layer above it needs care so that moisture is not trapped.
Insulate from inside
Adding insulation under the ceiling is possible but carries a risk. If the added insulation is not sealed from the room, warm damp air can reach the cold deck and condense. The ventilation gap also has to stay open.
Leave as it is
If the roof is sound and dry, and is not being renovated, it may be left. Checking for signs of damp is sensible, and the survey does it. A roof that has never shown a stain is not proof that the deck is dry, so we look at the underside and at the ceiling edges for signs of hidden moisture, and photograph them.
What to check before any of them
- The state of the deck: soft, stained or dark boards mean moisture is already at work.
- Whether the vents at the edges exist and are clear.
- Whether there are gaps at pipes, lights and hatches.
- Heights at thresholds, sills and upstands.
Our survey records the build-up, photographs the layers and gives one written price after the visit.
