From the pavement, a flat roof looks like a single surface. Underneath, it is a stack of layers, and the order of that stack has a name: warm, cold or inverted. The terms describe temperature. In a warm roof the structure stays on the heated side of the insulation. In a cold roof it sits on the outside of it, and the roof void above the ceiling is cold. In an inverted roof the insulation is laid on top of the waterproof layer.
This guide explains each build, where condensation risk sits in it, and which one tends to suit an extension or a replacement. It does not cover the Part L trigger for insulating when you re-cover, which is dealt with in our guides to flat roof insulation and regulations. For the rest of our flat roofing work, see the flat roofing page.
Why the order of the layers matters
Warm air indoors carries water vapour from cooking, washing, breathing and drying clothes. If that vapour passes through the ceiling and reaches a cold surface, it turns back into liquid. The name for the effect is condensation, and inside a roof structure it is called interstitial condensation because it happens in the gaps between layers where nobody can see it.
A flat roof has far less room for error than a pitched one. A pitched roof has a large ventilated loft in which moisture can disperse. A flat roof's void, if it has one, is narrow, and the covering above is waterproof, so vapour has nowhere to go if it gets in. The three constructions are three answers to a single problem: how to keep warm, wet air from meeting a cold surface where it can do damage.
The two controls
Every design uses two tools. The first is a vapour control layer, a sheet that slows vapour travelling up from the room. The second is either ventilation, which carries moisture out, or placement of the insulation so the structure never gets cold enough to condense water on. Which tool carries the load is what separates the three types.
The warm roof
In a warm roof the insulation sits on top of the deck, directly under the waterproof covering. From the bottom upwards the layers are: the ceiling, the joists, the deck (the boards the roof is built on), a vapour control layer, rigid insulation boards and the membrane. It is the natural choice when a flat roof is being replaced and the old deck is exposed.
Where the condensation risk sits
The deck and joists lie below the insulation, so they stay close to room temperature and do not get cold enough to collect moisture. The vapour control layer, sitting on the deck, stops the warm air reaching the cold insulation face above. The point to guard is the vapour control layer itself. If it is torn, or poorly lapped at a pipe, rooflight or wall junction, vapour can get through and condense in the insulation layer. The detail at these places is where the quality of the job shows.
Why it suits replacements
A warm roof needs no ventilation path. That is a real convenience on an extension that butts against a wall or sits beneath a parapet, where there is no place to let air in. It also leaves the joist depth free for ceiling-side insulation if wanted. The cost is height. The new build-up sits on top of the deck, so the roof edge, the upstand at the wall and the door threshold all rise. We check this on the survey, because a high threshold or a low window cill can limit the thickness of insulation that can go on top. Our page on upgrading flat roof insulation goes through the Part L side of it.
The cold roof
A cold roof keeps the insulation low, between or beneath the joists, with an air space above it and the deck on top. From the bottom upwards: plasterboard, a vapour control layer, insulation between the joists, a ventilated gap, the deck and the membrane. The deck and upper face of the joists stay at close to outside temperature, which is why the roof is called cold.
Where the condensation risk sits
The risk is in the void. Any warm, damp air that gets past the ceiling meets a cold deck underside and condenses on it. The design relies on two things: a vapour control layer that is continuous and well sealed, and a flow of outside air through the gap above the insulation to carry vapour away. The moisture-management reference for buildings is BS 5250, and it is for the designer to set how much ventilation is needed. We do not quote a figure here, because it varies with the span and the layout.
Where it goes wrong
Ventilation only works if air can move. It needs openings on opposite sides, and a clear path between them. On an extension where one side meets a house wall, a parapet blocks the other side, or the joists run the wrong way for air to flow, a cold roof can end up with trapped pockets of damp air. Insulation can also be pushed hard against the deck by a careless fitter, blocking the gap entirely. A cold roof that has been fitted with spray foam raises its own questions, which we cover below.
For these reasons a cold roof is more at home on a simple shape with open eaves on two sides than on a hemmed-in extension. It is the right choice when the roof cannot get any taller, because it keeps the whole build within the joist depth.
The inverted roof
The inverted roof turns the warm roof upside down. The waterproof membrane is laid directly onto the deck, and the insulation goes on top of it, held down by a layer of ballast: stone or paving slabs. From the bottom upwards: ceiling, joists, deck, membrane, insulation boards, a filter layer and the ballast.
Where the condensation risk sits
Because the membrane sits close to the heated structure, it stays warm and so does the deck beneath it, which removes the usual condensation concern within the build-up. The insulation boards above it get wet whenever it rains, so they must be a type designed to sit in water without absorbing it. The ballast and the filter layer protect the membrane from sunlight and from weather, and that helps the membrane to last. A rooflight or upstand detail still matters, because the membrane has to be sealed at every penetration.
The trade-offs
An inverted roof is heavy. The ballast adds a load that the structure must carry, and the Planning Portal names a covering at least 15 per cent heavier as a trigger for Building Regulations approval, which ballast can easily cause. The weight is why an inverted roof usually appears on a purpose-designed deck, a terrace or a balcony, and less often as a bolt-on replacement over an old timber roof. It can make sense where the roof is also a terrace that people walk on, since the ballast doubles as the surface. Tracing a leak is also harder, because stone, filter layer and boards all come up before the membrane is exposed. Our guide to EPDM rubber roofs covers the membrane that is often chosen for this kind of build.
Choosing for an extension or a replacement
The choice follows the building. For most flat roof replacements on a timber deck, a warm roof is the natural answer. For a new extension with a sound structure and a clear ventilation path, a cold roof is possible but needs a careful design. An inverted roof suits terraces and heavier decks.
| Construction | Insulation sits | Condensation risk sits | Best fit |
|---|---|---|---|
| Warm | On the deck, under the membrane | At the vapour control layer and its joints | Replacement of a timber deck, extensions with no ventilation path |
| Cold | Between joists, below a ventilated void | In the void, if air cannot move | Simple roofs with open eaves, or where height is limited |
| Inverted | Above the membrane, under ballast | Low within the build, but the structure carries the weight | Terraces, balconies and purpose-built decks |
Converting a cold roof to a warm one
An old cold roof can often be converted when it is re-covered. The ventilation gap is filled, a vapour control layer goes onto the deck and rigid insulation is laid on top. Done properly, this removes the dependence on ventilation. It does mean a deeper build, so it has to be matched against thresholds and wall details.
Spray foam under a flat roof, and what to ask before choosing
You may meet a cold flat roof that was filled from below with sprayed foam, which seals the void the design relied on for ventilation. The RICS consumer guide from March 2023 links poor installation with dampness and uncontrolled condensation, and wants condensation risk calculated by a qualified professional beforehand. It also tells owners to ask their mortgage provider and their insurer whether the product is acceptable. By November 2024 a BBC investigation found some lenders refusing homes with spray foam and others taking applications one at a time, as Today's Conveyancer reported. None of that means spray foam must always come out. It means the checks come first, and a surveyor's view is worth having before any decision on such a roof.
Questions for any roofer
Whichever build you choose, ask for the answers in writing.
- Which build is proposed, and why does it suit this deck?
- Where is the vapour control layer, and how is it sealed at penetrations?
- For a cold roof, where are the ventilation openings, and is the path clear?
- What U-value will the finished roof reach?
- How tall will the new roof edge be at the doors and cills?
What we do on the survey
We photograph the roof edge and the ceiling below, check the heights at thresholds and the wall abutments, and look for signs of moisture such as staining or soft boards. The written quote then names the build, the insulation thickness and the vapour control layer, which lets you set it beside another firm's quote. Our workmanship guarantee runs for 10 years in writing. For the U-value targets and the rest of the rules on this work, read our guide to flat roof Building Regulations.