Warm roof or cold roof: which should your flat roof be?
The standard is clearer than the sales pages, and the extra height is the real reason one quote is higher than the other.
Warm roof, in almost every case. BS 6229, the code of practice for flat roofs, does not recommend cold roof construction, because vapour control is hard to make effective, cold bridging is hard to avoid and the void is hard to ventilate properly. A cold roof is the fallback for when the extra build-up height genuinely cannot be found.
The short version
- BS 6229 does not recommend cold roof construction at all, and it gives three reasons: vapour control, cold bridging and ventilation.
- A warm roof adds roughly 100 to 150 mm above the existing deck, and thresholds, upstands, flashings and the roof edge all have to move up with it. That is why a warm-roof re-cover is priced above a straight re-cover.
- A cold roof needs a clear 50 mm air gap above the insulation, cross-ventilated through openings equivalent to a continuous 25 mm gap on each side, and a sealed vapour control layer on the warm side.
- In England the roof void is dealt with by Approved Document C, not Approved Document F. Anyone citing Part F for eaves vents is pointing at the wrong document.
- Boarding over an existing cold roof without stripping the insulation between the joists creates a hybrid, which needs a condensation risk analysis, not a rule of thumb.
The standards position
BS 6229 does not recommend cold roof construction
Worth knowing before anybody quotes you. BS 6229, the code of practice for flat roofs with continuously supported flexible waterproof coverings, does not recommend cold roof construction, and it gives three reasons.
Vapour control is hard to make effective. A cold roof depends on a continuous sealed layer under the insulation keeping warm damp air out of the void. Every downlighter, extract duct, loft hatch and joist hanger is a hole in it, and the ceiling gets fitted out by other trades after the roofer has gone.
Cold bridging is hard to avoid. Insulation between joists is interrupted by every joist and by the wall plate at each perimeter. Timber conducts heat better than insulation, so the ceiling runs colder in stripes above the joists and coldest at the edges. That is where the mould shows first.
Enough ventilation to the void is hard to provide. A cold roof is only safe if outside air moves right through the gap above the insulation and carries the moisture away. On a small extension there is often nowhere to put the second opening, and air in with no air out does nothing.
A warm roof removes all three by moving the insulation above the deck. The deck then sits on the warm side of it, stays near room temperature all year and gives water vapour nothing to condense on. No cold void, nothing to vent. The moisture side sits under BS 5250, the code of practice for management of moisture in buildings.
The three build-ups
Warm, cold and inverted, side by side
The only thing separating the three is where the insulation sits, and everything else follows from that. Warm roof: insulation on top of the deck, under the waterproofing. Cold roof: insulation between the joists, below the deck, leaving a void that has to be ventilated. Inverted roof: insulation on top of the waterproofing, held down by ballast or paving.
| Dimension | Warm roof | Cold roof | Inverted roof |
|---|---|---|---|
| Where the insulation sits | Above the deck, under the waterproofing | Between the joists, below the deck | Above the waterproofing |
| The standards view | The preferred construction | Not recommended by BS 6229 | Accepted, with ballast or paving to hold it down |
| Deck temperature | Warm and dry all year | Cold. Condensation forms under the deck | Warm and dry |
| Ventilation needed | None to the structure | Yes. A clear 50 mm void, cross-ventilated, 25 mm equivalent opening each side | None |
| Vapour control layer | Yes, below the insulation, on the warm side | Yes, on the warm side, fully sealed. This is where cold roofs fail | Not needed in the same way |
| Build-up height | Adds roughly 100 to 150 mm above the deck. Affects thresholds, upstands, flashings and the roof edge | Adds nothing above the deck. Its one real advantage | Adds the most of the three |
| Cold bridging | Continuous, straightforward to detail | Bridged at every joist and every perimeter | Continuous |
| Where it fails | Rarely, provided the falls are right | At the vapour control layer, at penetrations, and wherever the void gets blocked by insulation pushed too tight | Ballast displacement |
| Relative cost | Higher. New insulation, upstands, often new flashings | Lower up front | Higher |
Read the height row and the cost row together. They are the same row.
Height and price
The build-up height is why the two quotes differ
If one quote is noticeably higher than another, the warm roof build-up is the usual explanation, and it is not padding. Adding 100 to 150 mm on top of the deck moves everything that meets the roof.
The upstand at every abutment rises with it, so the flashing comes out of the brickwork and gets reset higher. A door threshold onto a balcony may lose the height it needs above the new finished level. Outlets and edge trim move up, and the fascia often has to be remade to hide the thickness. On a dormer, the cheek and the window cill above are usually what stops it happening at all.
A warm-roof re-cover is a different job from a straight re-cover, not the same job with better boards. Comparing two prices, check whether one includes new upstands, flashings and edge trim and the other does not. That should be itemised, and our note on what a roofing quote should include covers the rest.
Height buys one more thing: the chance to build the fall in. Tapered insulation above the deck makes the slope and the thermal performance in one layer, often the neatest way out of a roof that has always held water. What goes on top is a separate choice, in our comparison of GRP, EPDM and felt.
The thickness is not a number off a shelf. Approved Document L works in U-values rather than millimetres, and it applies whenever you renovate a thermal element, which a flat roof re-cover usually is. So the thickness falls out of the U-value the build-up has to reach and the conductivity of the board used. The same target gives one thickness in rigid board and a greater one in mineral wool, which is the subject of PIR board against mineral wool. Ask what U-value the roof is designed to and what thickness that makes it, because that decides whether your threshold still works. The wider picture is in Part L on a re-roof or loft conversion.
If it has to be cold
Building a cold roof properly, and the numbers that matter
Sometimes the height is genuinely not there. A dormer cheek under a window cill will not take another 120 mm. A cold roof is then the honest answer, and three parts of it are not optional.
A clear 50 mm air gap between the top of the insulation and the underside of the deck, across the whole roof. Not 50 mm in the middle and nothing at the edges. Insulation pushed tight to the deck to squeeze in extra thickness blocks the void and turns a ventilated roof into an unventilated one.
Cross-ventilation through openings equivalent to a continuous 25 mm gap on each side of the void. Both sides. Air comes in one side and leaves the other, so vents on one elevation are decoration. A roof with a wall on three sides probably cannot be ventilated at all, which is a reason to go back and find the height for a warm roof.
An air and vapour control layer on the warm side, above the ceiling and below the insulation, sealed at every lap, penetration and abutment. This is where cold roofs fail. Not the membrane on top, not the insulation, the sealed layer under it. Every downlighter, extract fan, cable and pipe through the ceiling is a route for warm damp air, and warm damp air in a cold void is a wet deck by February.
Before you sign anything for a cold roof. Ask where the air comes in and where it goes out, and have the answer pointed at on the building rather than described. If nobody can show you two opposite openings, the void will not ventilate, and extra insulation between the joists does not fix that. A roof that cannot be ventilated has to be built warm.
The document people cite is usually the wrong one
In England the roof void is dealt with by Approved Document C, not Approved Document F. Section 6 of AD C covers roofs and accepts that a roof designed and built to BS 5250 satisfies requirement C2, resistance to moisture from inside the building. Approved Document F deals with ventilating the rooms people sit in: trickle vents, extract fans, background ventilation, not the space above the ceiling.
Anyone citing Part F for eaves vents has the right idea and the wrong document, and it matters because the two point at different fixes. AD C takes you to BS 5250 and a construction that manages moisture. AD F takes you to a fan in the bathroom, worth having and no use at all on a wet deck.
The two traps
When the answer is neither
Neither a warm nor a cold roof is the answer when the job is re-covering a cold roof and changing nothing else, or adding boards on top and leaving the old insulation in. Both make a wet flat roof worse, and both look reasonable on the quote.
Re-covering a cold roof and changing nothing else
The membrane comes off, a new one goes on, the roof looks new and the deck underneath is still wet. If the old covering failed because of condensation rather than a hole, a new membrane over a damp deck traps the water in the timber and the deck rots faster than before.
The signals are on the underside: staining, dark patches, a musty smell, or drips on cold mornings, not during rain. If the underside of your deck has been wet, the deck is the question and the covering is a detail. Our guide to telling condensation from a roof leak goes through the differences. The quote should then say that the covering and the deck are coming off, soft boards are being replaced and the roof is being rebuilt warm.
Adding boards on top and leaving the old insulation in
This is the tempting one. Somebody suggests laying insulation above the deck of a cold roof and calling it a warm roof, while the quilt between the joists stays where it is. What that builds is a hybrid roof, and a hybrid roof is not a warm roof.
The deck now sits between two layers of insulation. Whether it stays dry depends on how much sits above it against how much sits below, how airtight the ceiling is, and how much moisture the rooms produce. Get that balance wrong and the deck is still cold enough to condense, only now it is sealed on both faces and cannot dry in either direction. BS 5250 addresses hybrid construction specifically, which tells you it is a case to be worked out, not assumed.
There is no rule of thumb here. A hybrid needs a condensation risk analysis on your roof and your build-up. If somebody proposes one and cannot say whether that has been done, the safe version is to take the old insulation out so the roof is genuinely warm.
Next steps
What to do next
Before choosing a warm or cold flat roof, look at the underside of the deck, measure the height available at thresholds and cills, and work out where ventilation air would come in and go out. The first three cost nothing and settle most of it.
- Look at the underside of the deck, from the room below or through the hatch. Staining, damp timber or a musty smell means a re-cover on its own is not the job.
- Measure the height you have, from the top of the existing covering to the underside of the door threshold, the window cill or the damp proof course at the abutment. With 150 mm a warm roof is straightforward. With 40 mm, say so early.
- If a cold roof is proposed, work out where ventilation air would come in and go out. Two opposite sides, or it does not work.
- Ask every quote which construction it is pricing, warm, cold or hybrid, and what U-value the build-up reaches. Both answers in writing.
- If a hybrid is proposed, ask for the condensation risk analysis. If there is not one, ask for the old insulation to come out instead.
- Check that new upstands, flashings, outlets and edge trims are itemised. That is where the gap between two prices sits.
If you would rather somebody looked at it, that is what our insulation work and our roofing work cover between them, and on a flat roof they are the same job. Thomas Hannen comes out and prices it himself: which construction you have, whether the deck is sound, what height there is to play with, and a free written itemised quote out of the visit. If the roof does not need touching, that is what it will say.
Questions
Common questions
Is a cold roof allowed under the Building Regulations?
Nothing bans it, but BS 6229 does not recommend cold roof construction, on the grounds that vapour control, cold bridging and ventilation of the void are all difficult to get right. In England, Approved Document C section 6 accepts a roof designed and built to BS 5250 as satisfying requirement C2.
How much height does a warm roof add?
Roughly 100 to 150 mm above the existing deck, depending on the U-value being targeted and the insulation being used. That extra height has to be found at the door threshold, the upstands, the flashings and the roof edge, which is why a warm-roof re-cover is a bigger job than a straight re-cover.
Can I turn my cold roof into a warm roof by adding insulation on top?
Not by itself: leaving the insulation between the joists in place creates a hybrid roof, where the deck sits between two layers of insulation and can still be cold enough to condense. BS 5250 addresses hybrid construction specifically, and it needs a condensation risk analysis, not an assumption.
Does Part F cover flat roof ventilation?
No, in England the roof void is dealt with by Approved Document C, which covers roofs in section 6. Approved Document F deals with ventilating the occupied rooms below, meaning extract fans and background ventilation. Somebody citing Part F for eaves vents has the wrong document.
My flat roof is wet underneath. Do I just need a new covering?
Probably not: drips and staining that appear on cold mornings and not during rain point at condensation on the underside of a cold deck. A new membrane over a wet deck traps the moisture and the timber rots faster, so the deck and the construction have to be dealt with first.
Read next
Related advice
The guides below pick up where this one leaves off, and the insulation page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Not sure whether your flat roof is warm or cold?
Thomas comes out, works out what you have, checks whether the deck is sound and measures the height you have to play with. What he finds goes in writing, including if the answer is to leave it alone.