Key points
- A warm roof puts insulation above the deck and under the waterproofing, and is the preferred build-up for most homes.
- Cold roofs risk condensation on the underside of the deck and need a ventilated gap that is often hard to achieve.
- Inverted roofs place insulation over the membrane and suit terraces and green roofs.
- Adding insulation raises the roof, so upstands, door thresholds, sills and fascias must be planned at survey.
What is the difference between a warm, cold and inverted flat roof?
All three describe where the insulation sits relative to the deck and the waterproofing, and that position decides whether the timber structure stays warm or cold.
| Type | Insulation position | Deck temperature | Typical use |
|---|---|---|---|
| Warm roof | On top of the deck, under the waterproofing, with a vapour control layer below it | Warm | Most homes and extensions; the preferred approach |
| Cold roof | Between or below the joists, with a ventilated gap above | Cold | Older roofs; industry guidance advises avoiding it where possible |
| Inverted roof | On top of the waterproofing, held down by ballast, paving or planting | Warm | Roof terraces, green roofs and some concrete decks |
Terms such as deck, upstand and vapour control layer are defined in our glossary.
How is a warm flat roof built up?
From the inside out, a warm roof is joists, a structural deck, a vapour control layer, rigid insulation boards, then the waterproof membrane bonded or fixed on top.
The insulation is usually PIR board, laid tight with staggered joints, sometimes tapered to create the fall. Because the deck sits on the warm side of the insulation, its temperature stays close to that of the room below, so moist indoor air has nowhere cold to condense against. The membrane can be EPDM, GRP, torch-on felt, single-ply or liquid-applied, as long as the manufacturer approves it over that insulation. A warm roof upgrade adds £35–£60 per m² to the cost of a re-roof, and it is the build-up we normally specify on extension and kitchen roofs.
Why are cold flat roofs discouraged?
Cold roofs carry a real risk of condensation, so current industry guidance advises avoiding them where possible.
In a cold roof the insulation sits between the joists, leaving the deck above it at close to outside temperature. Warm, moist air from the room rises through gaps around light fittings and plasterboard joints, meets the cold underside of the deck and condenses. Over time that moisture can rot the timber, wet the insulation and drip back through the ceiling, which can look like a roof leak when the waterproofing is sound. To work at all, a cold roof needs a clear, cross-ventilated air gap above the insulation and openings on opposite sides of the roof, conditions that are hard to achieve on an extension built against a wall or under a parapet.
When is an inverted flat roof the right choice?
An inverted roof suits roofs that will be walked on or planted, because the insulation shields the waterproofing from foot traffic, sunlight and temperature swings.
The membrane goes directly on the deck, then water-resistant extruded polystyrene boards are laid on top and held down with gravel, paving on supports or a green roof build-up. Rainwater passes through and over the insulation to the membrane below, so the design has to allow for some heat loss from that cooling water, and the deck must carry the extra weight. It is common on roof terraces and concrete roofs, and less usual on lightweight timber extensions.
What does a vapour control layer do?
A vapour control layer is a membrane laid on the warm side of the insulation to stop moist indoor air reaching the cold parts of the roof, where it would condense.
In a warm roof it is laid on the deck under the insulation, with sealed laps and turned up at the edges so the insulation is fully enclosed between it and the waterproofing. A torn or poorly lapped vapour control layer lets moisture into the insulation, where it can build up unseen for years. Kitchens and bathrooms below a flat roof put the most vapour into the structure, so attention to this layer matters most there. Where an unusual build-up is proposed, such as insulation both between and above the joists, a condensation risk calculation should be done before work starts.
What U-value does a flat roof need in each UK nation?
The target depends on the nation and on whether you are renovating an existing roof or building a new extension, with lower numbers meaning better insulation.
| Nation | Renovating an existing roof | New extension roof |
|---|---|---|
| England | 0.16 W/m²K (domestic), 0.18 (non-domestic) | 0.15 W/m²K (domestic) |
| Wales | 0.16 W/m²K | 0.13 W/m²K |
| Scotland | Section 6 of the building standards technical handbooks; domestic roof benchmark 0.12 W/m²K, pursued as far as reasonably practicable | Section 6 applies; agree the target with building standards |
| Northern Ireland | 0.18 W/m²K (Technical Booklets F1 and F2) | Set by the current Technical Booklets; check with building control |
On a renovation, the upgrade applies where it is technically, functionally and economically feasible, which is covered in our building regulations guide. Reaching these figures with PIR in a warm roof often means boards well over 100 mm thick, depending on the product and the rest of the build-up. The insulation supplier's calculation for your roof gives the exact thickness.
How much higher will the roof be after adding insulation?
Adding a warm-roof build-up raises the finished roof level by roughly the thickness of the insulation plus the deck and membrane, so on most upgrades the roof ends up noticeably higher than before.
That height has to be found somewhere, and it affects several details:
- Upstands at walls. The waterproofing usually needs to rise about 150 mm above the finished roof surface, so flashings and cavity trays may need moving up the wall.
- Door thresholds. A balcony or roof door can end up with too small a step to keep water out; the threshold may need raising or a drainage channel adding.
- Window sills and vents. Low sills and air bricks must stay clear of the new roof level.
- Edges and fascias. Fascias and edge trims are usually deepened to suit the new build-up, which changes how the roof edge looks from below.
- Rooflights. Kerbs are often rebuilt taller so the frame still sits clear of the surface; see rooflights in flat roofs.
We check these at survey, because they shape both the design and the price.
Can an existing cold roof be turned into a warm roof?
Yes. Converting a cold roof to a warm roof is usually done during a re-roof by stripping the old covering, repairing or renewing the deck, and laying a vapour control layer, insulation and new membrane on top.
The old insulation between the joists is often removed or reduced so the deck stays on the warm side, and the ventilation openings that served the cold void are closed. If the ceiling below stays in place, the room is undisturbed apart from any work at the edges. On a 20 m² extension, a new deck and warm-roof insulation with EPDM typically come to £2,800–£4,900 including VAT, as shown in our cost guide. That is a guide price; the fixed figure follows a free survey, and you can try other sizes and systems in the guide-price tool.
More questions
Is a warm roof better than a cold roof?
For most homes, yes. A warm roof keeps the deck at room temperature, which greatly reduces condensation risk, and it avoids the need for a ventilated void.
Do I need a vapour control layer on a flat roof?
In a warm roof, yes. It sits under the insulation and stops moist indoor air reaching cold layers where it could condense.
Can insulation go inside the ceiling instead of on top of the roof?
It can, but that creates a cold roof, which needs careful ventilation and carries a higher condensation risk. Industry guidance advises avoiding cold roofs where possible.
Will a warm roof upgrade raise my roof?
Usually, by about the thickness of the insulation plus the deck and membrane. Upstands, door thresholds and fascias are planned around the new level at survey.