The terms warm roof and cold roof describe where the insulation sits and, as a result, what temperature the structure runs at. In a warm roof the insulation lies on top of the deck, so the joists and boards stay close to the temperature of the room below. In a cold roof the insulation lies between or beneath the joists, so the deck above it is as cold as the weather outside. That single difference decides how the roof is ventilated, where condensation can form, how thick the build-up is at the edges, and what goes wrong in later years. This guide sets out both constructions, and a third that mixes them, so you can follow what a roofer means when they recommend one.
Not to be confused with the pitched roof loft choice, which is a different decision and is covered in our guide to a warm loft versus a cold loft.
How a cold roof is built
A cold flat roof is the traditional arrangement found on most houses with an older flat roof. Working from the ceiling upwards, there is the plasterboard, then a vapour control layer (a sheet that slows the movement of warm, moist air from the house into the roof), then the insulation laid between the joists. Above the insulation is an air gap, then the deck, then the waterproof covering on top. The gap is the point of the whole design. It has to be ventilated, usually through vents or slots in the soffit or fascia at opposite edges of the roof, so that moist air that does get past the ceiling is carried away before it can condense on the cold underside of the deck.
In theory, this works. In practice it depends on three things that are hard to guarantee: an unbroken vapour control layer, a clear air path from one side of the roof to the other, and enough gap depth for air to move. Fill the void with insulation to meet modern thickness, and there is little room left for air. Block a vent with paint or insulation, or add a ceiling light cut-out that leaks warm air upwards, and condensation begins to form on the deck.
How a warm roof is built
A warm roof reverses the order. The vapour control layer sits on top of the deck, the rigid insulation boards lie above it, and the waterproof covering goes on the insulation. The joists and deck are below the insulation, on the warm side, and there is no need for a ventilated void. Moist air from inside the house meets the vapour control layer and is stopped from reaching a surface cold enough for condensation. The structure stays dry, and the thickness of the insulation is no longer limited by the depth of the joists.
The insulation boards are usually PIR (polyisocyanurate), a rigid foam with a high resistance to heat flow for its thickness. They are fixed with adhesive or mechanical fasteners, and the covering is then bonded or fixed over them, so the choice of covering has to suit the substrate. Some boards come with a built-in fall, which solves a second problem: the roof can be sloped to its outlets without having to rebuild the structure.
The hybrid roof
A hybrid, sometimes called a combination or part-warm roof, mixes the two. Some insulation sits between the joists and some above the deck. The idea is to gain thickness without raising the roof surface too much, which helps where the roof meets a window cill or a door threshold with no room for a deep build-up. A hybrid still needs the ratio of insulation above and below the deck to be calculated, so the deck temperature stays above the point where moisture condenses. Done by guesswork, it recreates the problems of a cold roof. Done by calculation, it is a useful solution to a tight detail.
Condensation: the reason the choice matters
Warm air holds more moisture than cold air. When it meets a cold surface, the excess comes out as water. In a cold roof the cold surface is the underside of the deck, and the water is hidden in a void. It can soak the timber for years before the first stain shows on the ceiling, and by then the boards or the ends of the joists may be rotten. We see this on flat roofs where the surface has never leaked at all, and the covering looks sound, but the deck beneath is black and soft.
Our short answer on whether a flat roof needs ventilation covers the ventilation side. The key point here is that warm roof construction avoids the question. With the insulation above the deck and a sealed vapour layer beneath it, there is no cold void to ventilate and no cold timber to collect water.
Cost and disruption compared
A warm roof asks for more insulation on the outside, which means more material above the deck, a thicker build-up at the edges and some adjustment to upstands, flashings and trims. A cold roof avoids that, but making it work means opening up the ceiling or working from inside to fit insulation between the joists. Where the ceiling is intact and decorated, a warm roof can be done entirely from above, leaving the room undisturbed. That is often a significant point in its favour on an occupied house.
The difference in price between the two is real but modest in the context of the whole job, and a warm roof is usually repaid by a drier structure and a longer-lived covering. Our guide to what a flat roof costs sets out how insulation affects a quote. For a lasting roof, it is rarely the place to save.
What the regulations expect
A roof is a thermal element of the building, and when its covering is stripped and replaced, the insulation normally has to be brought up to the standard in Approved Document L. That is why a replacement flat roof comes with insulation, whichever construction is used. Overlaying an existing covering is generally treated as a repair and leaves the existing insulation alone. Approved Document C and BS 5250, which deal with moisture, set out the ventilation of cold roofs and the need for a vapour layer in a warm one. We deal with the Building Regulations paperwork where it is required, and you will find the detail on our page about flat roof building regulations. For a specific project, your building control body or the council's team can confirm what applies.
If you are wondering how thick the insulation has to be, the answer depends on the product and the design, and it is set out in our short answer on flat roof insulation thickness.
Converting an existing cold roof to a warm roof
If your roof is a cold roof, you do not have to keep it that way when the covering comes off. In most cases we can convert it. We strip the old covering, check and repair the deck, and fit a new vapour control layer and rigid insulation on top, then cover it with the new waterproofing. The old insulation between the joists can stay or go, depending on its condition. The ventilation gap is sealed or left, but it is no longer doing a job, and the vents are closed off so air cannot carry moisture into the void.
There are two cautions. First, the deck has to be dry before the vapour layer goes down. A damp deck trapped under a new warm roof has nowhere to dry out, so we check moisture content, and where a deck is wet, it is replaced. Second, the edge details have to be thought through, because the roof surface rises by the thickness of the new insulation. Rooflights, door thresholds and gutters all need to be checked against the new height.
| Feature | Cold roof | Warm roof |
|---|---|---|
| Insulation position | Between joists, below the deck | Above the deck |
| Ventilated void | Required | Not needed |
| Deck temperature | Cold, close to outside air | Warm, close to inside |
| Condensation risk | Higher, if the air path or vapour layer fails | Lower, if the vapour layer is continuous |
| Disruption inside | May involve the ceiling | Done from above |
The vapour control layer, in plain terms
The vapour control layer is the least glamorous part of either roof and one of the most important. It is a thin sheet, often foil-faced or a heavy polythene, laid with taped or sealed laps. Its job is to stop the moist air from kitchens, bathrooms and living rooms from reaching a place where it can cool and turn to water. In a warm roof it lies on the deck, under the insulation. In a cold roof it lies on the warm side of the insulation, usually just above the plasterboard.
The layer only works if it is continuous. Every pipe, cable and light fitting that goes through it is a hole, and every hole has to be taped or sealed. On a warm roof we lay it on a clean, dry deck and seal it up the upstands, so the insulation is wrapped on the warm side. If you spot a recessed spotlight in a ceiling beneath a cold roof, it is worth knowing that each of those is a potential route for warm air into the void.
Common mistakes with either build
Whatever the construction, the faults repeat. A vapour control layer with holes in it, cut for a pipe or a light fitting and never sealed, lets moist air through. Insulation boards laid with open gaps cause cold bridges, where heat escapes and condensation forms. A warm roof built on a damp deck traps the moisture. A cold roof with blocked vents cannot dry. The covering is bonded to the wrong substrate, or the roof is left without a proper fall. Most of these cannot be seen once the roof is finished, which is why we photograph the layers as they go on and show you the record.
Which we recommend, and when
When a flat roof covering is being replaced, we normally recommend a warm roof. It keeps the structure dry, allows the insulation to meet the standard without losing ceiling height, is built from above and sidesteps ventilation problems. A cold roof can be right where a roof is being repaired rather than replaced, where the existing ventilation works and the insulation is in good order. A hybrid is a tool for a tight detail, not a default.
The way to settle it for your roof is a survey. We look at the deck, check for moisture and signs of condensation, and measure the heights at the doors and walls. You receive photographs and a plain recommendation with a fixed written quote, and our work carries a guarantee for ten years. To arrange a visit, ring 01256 213956 or read more on our flat roofing in Basingstoke page.
