Every flat roof is built in layers, and the order of those layers matters more than owners expect. The waterproof covering sits on top. The deck, the boards or sheets that carry it, sits below. The insulation can go in two places: under the deck, between the joists, or on top of the deck, directly beneath the waterproofing.
Put it under the deck and you have a cold deck roof. Put it on top and you have a warm deck roof. The labels refer to the temperature of the timber. In a cold deck roof the boards and joists sit outside the insulation and stay close to the outside temperature. In a warm deck roof the structure sits inside the insulation and stays at room temperature.
That one choice changes whether damp air from the rooms below can turn to water inside the roof, whether the roof needs a ventilated void, how deep the build-up must be and how the roof has to be brought up to date when it is replaced. This guide explains each point, with the rules that apply in England. Our flat roofing hub covers the coverings that go on top.
Cold deck: insulation below the boards
A cold deck roof is the older arrangement, found on many garages, porches and extensions built before insulation standards tightened.
How it is built
From the inside up, you have the ceiling, the insulation between the joists, a ventilated void, the deck and the covering. The void is a gap of air above the insulation, below the boards, with openings at the edges so air can pass through.
Why the void is there
Warm air from a kitchen, bathroom or living room carries water vapour. Some of it gets past the ceiling and rises. When it meets the cold underside of the deck, it can condense, leaving the timber damp. Ventilation carries that moisture away before it does harm. Approved Document C, the official guidance on moisture, treats ventilating a cold deck as the way to meet its condensation requirement (paragraph 6.11), while noting that a very small roof over a porch might not need it.
What goes wrong
The void must stay open. If insulation is stuffed into the gap, if the edge vents are painted over or blocked by a new fascia, or if a ceiling below leaks warm air, the moisture has nowhere to go. The first sign is often mould on the underside of the deck or a musty smell in the room, and the last is rotten joists. Our answer on condensation under a flat roof explains how to spot it before that.
Cold deck roofs can work well when built and maintained properly, but they depend on details staying clear for decades.
Warm deck: insulation above the boards
A warm deck puts the insulation in rigid boards on top of the deck, and the waterproof covering goes over it.
How it is built
From the inside up: the ceiling, the joists, the deck, a vapour control layer, rigid insulation boards, and then the covering. The vapour control layer is a membrane that holds back water vapour from the house so it cannot travel into the insulation and condense there.
What it does for the structure
Because the insulation is outside the structure, the boards and joists stay at roughly the temperature of the room. They are above the dew point for most of the year, which is the temperature at which moisture condenses, so there is little chance of condensation on the timber. The roof needs no ventilated void, and that makes it simpler to build around a ceiling that cannot be opened up, or where there is no room for a gap.
The cost to the build-up
The insulation, the vapour layer and the covering all sit above the deck, so the roof edge is higher. Upstands at walls, door thresholds and window cills must all allow for the extra depth. On an existing building that can mean altering flashings, trims and gutters. For a flat roof with a high threshold below, check the height before choosing.
For most replacements where the deck is being renewed anyway, a warm deck is the more forgiving choice.
Where condensation actually comes from
Condensation is the main reason this distinction exists, and it is easy to misunderstand.
Interstitial condensation
Approved Document C (England), at paragraph 6.2, asks for roofs whose strength and insulation are not harmed by interstitial condensation. Interstitial means inside the construction, between layers, where you cannot see it. Moisture collects within the roof build-up, soaks insulation, and rots timber with no outward sign until a stain appears.
The British Standard
AD C says a roof meets the condensation requirement if it is designed and built to Clause 8.4 of BS 5250:2002 and BS EN ISO 13788:2002. BS 5250 is the British Standard on controlling moisture in buildings, and its current edition is BS 5250:2021, which is listed as under review. The roof designer or installer should work to the current edition, and it is reasonable to ask which they used.
What drives the moisture
Water vapour comes from cooking, washing, drying clothes and breathing. A ceiling with gaps, recessed light fittings that open into the roof void, and a loft hatch that does not seal all let vapour reach the roof. A room that is heated and then left cold, such as a conservatory kitchen or a bedroom over a garage, cycles through damp and dry.
Why details matter
A well-sealed ceiling and a continuous vapour layer matter in both types of roof. Without them, the best ventilation or the thickest insulation will not keep a roof dry.
Part L when the deck is replaced
When a flat roof is stripped and recovered, the insulation question stops being optional.
Renovation under Approved Document L
Volume 1 of Approved Document L (England), the 2021 edition with 2023 amendments, counts re-waterproofing a flat roof as renovation of a thermal element, which is the part of the building that keeps heat in. Table 4.3 gives a threshold U-value of 0.35 W/(m2K) for an existing roof, and an improved value of 0.16. A U-value measures how much heat passes through, so a lower figure means a warmer roof. If the roof is above the threshold when it is renovated, it should be upgraded to the improved value.
When the upgrade is limited
Paragraph 4.13 allows a smaller improvement when the full upgrade cannot reasonably be done, or when it would take longer than 15 years to repay itself in saved energy. In that case the aim is the best figure achievable within a 15-year payback, with 0.7 W/(m2K) as a general ceiling. This relaxation is for real constraints, not a way round the rule.
The next edition
A new Approved Document L takes effect on 24 March 2027 for ordinary building work, meaning work outside the higher-risk building category. Its tables are renumbered 3.2 and 3.3, and the payback rule is paragraph 3.14. Until that date the 2021 edition applies.
Check with your building control body, as they decide how the rules apply to your roof. Our guide to flat roof Building Regulations sets out the wider picture.
Converting a cold roof to a warm roof
Many older flat roofs are cold decks. When the covering is renewed, it is natural to ask whether to keep the arrangement or change it.
Why people convert
A cold deck with a poor void, blocked vents or a leaky ceiling is the classic setting for hidden damp. Converting to a warm deck takes the timber into the heated envelope and removes the need for ventilation. It can also meet the Part L target in a thinner build-up than filling the joist depth alone, because rigid boards above the deck are not interrupted by timber joists.
What changes
The old covering is stripped. The deck is checked and repaired, a vapour control layer is laid, the insulation boards go on, then the new covering. Any insulation below the deck is left or removed depending on the survey. If the old void is sealed and the new insulation sits above the deck, the void becomes dead air, which is fine, provided it cannot take in damp. The edges, outlets and upstands are all raised to suit.
When to keep a cold roof
If the height cannot go up because of a door threshold or a neighbouring wall, a cold roof can be improved with insulation between the joists and a sealed ceiling. The void must remain ventilated and the vapour layer continuous. That is a careful job, and condensation is the risk if it is rushed.
We look at the build-up from above and below on the survey and explain which choice suits the roof you have.
Choosing for your roof, and what a survey checks
There is no universal answer, and a roof that works for a neighbour's extension may not suit yours.
Points that tip the choice
The height available at doors and walls, the state of the deck, how well the ceiling below is sealed, how the roof drains and the covering you want all play a part. A deck that is being renewed anyway makes a warm roof easy. A sound deck under a ceiling that is difficult to open may favour improving a cold roof from inside.
What the survey checks
We look at the roof from above and from inside. We check for stains, mould, smell and soft boards, look at whether the void is open, lift a small section where it is safe to see the layers, and note thresholds, upstands and outlets. You get photos of what we find. Our guide on flat roof insulation goes into the materials.
The written price
You receive one written total that states the build-up, the covering and the insulation thickness we recommend, so you can see exactly what you are agreeing to. The standard VAT rate is 20 per cent, and the government zero-rates energy-saving materials installed between 1 May 2023 and 31 March 2027, with 5 per cent from 1 April 2027, so ask which rate the quote has applied.
A roof built to Part L still needs a sealed ceiling and a vapour layer that is continuous. Insulation alone does not stop damp.
