Warm Roof vs Cold Roof: BS 6229 Says Avoid Cold Deck

The warm roof vs cold roof question sounds like a choice. On a flat roof replacement in 2026 it mostly is not. The British Standard that governs flat roofing advises against cold deck construction, and the reason is not fashion: a cold deck depends on a ventilated void that works, and on an existing roof that void is very hard to achieve and impossible to inspect once the covering is on. Warm deck has become the default because it removes the failure mode rather than managing it.

That said, warm deck creates a problem of its own, and it is the one nobody mentions until the roofer is on site: the whole build-up gets thicker, and that extra height has to go somewhere. Usually the somewhere is a door threshold or an abutment detail, and the answer needs to be worked out before anyone orders materials.

Here is the difference, the U-values the regulations actually ask for, and what to check on your own roof.

What the two build-ups are

Cold deck. Insulation sits below the structural deck, between or under the joists, with a ventilated air space above it and below the deck. The deck and the waterproofing stay cold. Warm moist air from the room below has to be stopped by a vapour control layer, and whatever gets past it has to be carried away by cross ventilation of the void.

Warm deck. Insulation sits above the structural deck, with a vapour control layer beneath the insulation and the waterproofing on top. The deck stays warm and close to room temperature, so the dew point sits inside the insulation rather than on the underside of the deck. There is no void to ventilate because there is nothing cold for moisture to condense on.

There is a third, the inverted or “upside down” roof, where the insulation sits above the waterproofing and is held down by ballast. It has its uses on concrete decks and terraces, and it is uncommon on domestic timber flat roofs.

Why cold deck is now the exception

BS 6229 is the code of practice for flat roofs with continuously supported flexible waterproof coverings, and the 2018 revision is unambiguous in its direction of travel. It does not recommend cold roof design, and where a cold roof is unavoidable it sets demanding conditions: cross ventilation of the void, ventilation openings kept close together, an air open breather membrane on the cold side of the insulation, and an air and vapour control layer below.

The reason a cold deck fails in practice is that all of those conditions have to hold across the whole roof, forever, and on a refurbishment you can rarely prove even one of them. Consider what a typical rear extension flat roof actually is: joists spanning between a wall plate and the house wall, with an abutment at one end and possibly a party wall or a parapet at the sides. To ventilate that void you need free air path in at one edge and out at the opposite edge, uninterrupted along every joist bay. In real buildings the path is blocked by noggins, by services, by insulation stuffed tight to the deck by a previous owner, and very often by the fact that there is no opposite edge to ventilate to at all.

When the ventilation does not work, moisture from the rooms below condenses on the cold underside of the deck. It does not show up as a drip. It shows up as staining months later, or as a deck that has quietly rotted while the waterproofing above it looks perfect. That is the single most common reason a flat roof that “has no leak” still needs replacing, and it is covered further in our page on flat roof problems and repairs and on loft condensation.

A warm deck removes the argument. There is no cold surface, so there is no need to prove ventilation. That is why it is now the default specification on a flat roof replacement, whatever covering goes on top; see EPDM, GRP fibreglass, and choosing a flat roof material.

The U-values, from the source

This is where most advice online goes wrong, because it quotes one number when there are three, and which one applies to you depends on what you are doing.

All of the following come from Approved Document L, Volume 1, Dwellings, the 2021 edition incorporating 2023 amendments, which is the version currently in force in England.

A brand new dwelling. Table 4.1 sets a maximum of 0.16 W/(m²·K) for all roof types.

A new roof on an existing dwelling, meaning a new element such as an extension. Table 4.2 sets a maximum of 0.15 W/(m²·K) for the roof. Note that this is tighter than the new build figure, which surprises people.

Renovating the roof you already have. Table 4.3 gives two figures: a threshold of 0.35 and an improved value of 0.16. Paragraph 4.11 requires that a renovated thermal element is both no worse than it was before, and meets the Table 4.3 limiting standards. Section 11 sets out when renovation triggers this, and in practice stripping a flat roof back and re-covering it does.

So the number to design to on a flat roof replacement is 0.16 W/(m²·K). That is the one to look for on a quote, and its absence tells you something about the quote; see how to read a roofing quote.

What if you cannot get there? The Approved Document anticipates this. Paragraph 4.13 says that if achieving the improved value is not technically or functionally feasible, or would not achieve a simple payback of fifteen years or less, then the element should be upgraded to the lowest U-value that both is feasible and achieves a payback within fifteen years, and generally an upgraded thermal element should not end up worse than 0.7 W/(m²·K).

There is also a note attached to the roof row of Table 4.3 that is written for exactly this situation: “If there are problems with the load-bearing capacity of the frame or height of the upstand, for a flat roof or roof with integral insulation, a lesser standard may be appropriate.” In plain terms, if the existing upstands are not tall enough to take the full insulation thickness, the standard itself allows a reduced specification rather than requiring you to rebuild the parapet. That clause is worth knowing about, because it is the honest answer to a roof that physically cannot take 150mm of PIR.

Does the Future Homes Standard change this? Not for roofs. The 2026 edition of Approved Document L, published on 24 March 2026 and coming into force on 24 March 2027, keeps the same figures: 0.16 for all roof types in a new dwelling, 0.15 for a new roof element in an existing dwelling, and a 0.35 threshold with a 0.16 improved value for existing elements. If you are being told to hurry a flat roof job because the roof U-value is about to change, it is not.

The build-up height problem

Here is the practical consequence nobody puts in a brochure. To hit 0.16 W/(m²·K) with a typical PIR board you are adding roughly 120mm to 150mm of insulation, plus the deck and the covering, on top of the existing structure. The roof surface finishes noticeably higher than it did.

Three places that immediately matters:

The door threshold. If the flat roof abuts a set of patio doors or a back door, the finished roof level rises towards the threshold. There is a minimum upstand height needed between the finished roof surface and the door threshold, and if the roof rises and the door does not, you lose it. The outcomes are all bad: water standing at the threshold, a detail that relies on sealant instead of an upstand, or a door that has to be reset. Establish this before ordering, because the fix may be a tapered scheme, a different insulant with a better lambda value, or lowering the deck.

The abutment. Where the flat roof meets the house wall, the waterproofing turns up the wall and is protected by a flashing, usually lead, chased into a joint or run under the existing weatherings. Raise the roof by 150mm and that upstand loses 150mm of height unless the chase is cut higher. Sometimes the chase lands mid brick, or in a course that is a bad place to cut, or above a window head. This is a detailing decision, not a materials one, and our lead flashing and leadwork guide covers what a correct detail looks like.

The parapet, verge or edge trim. On a roof with a parapet, the parapet gets shorter relative to the roof. On an open edge, the fascia and trim geometry changes. Neither is difficult, both need thinking about at quote stage.

And the windows above it. If there is a first floor window sill above the flat roof, the clearance between the finished roof and the sill shrinks. It is worth measuring.

The reason the height issue matters so much on a warm deck specifically is that all the insulation is above the structure. A cold deck put the insulation inside the depth you already had, which is precisely why it was popular, and precisely why it fails.

When a cold deck is still the answer

Not never. Two situations come up:

You genuinely cannot gain height and the roof has a large, well ventilated void with a clear cross ventilation path at two opposite edges, no obstructions along the joist bays, and a properly installed air and vapour control layer below. If a roofer proposes cold deck, ask them to show you where the air comes in and where it leaves. If they cannot answer that in one sentence, the answer is warm deck.

A hybrid. Insulation above the deck plus a limited amount between the joists is a common compromise, but it is a calculation, not a guess. Put too much insulation on the cold side of the vapour control layer and you move the dew point back into the structure and recreate the problem you were solving. This needs a condensation risk analysis rather than a rule of thumb.

Pitched roofs are a separate conversation. A traditional cold loft, insulated at ceiling level with a ventilated loft space above, is still perfectly normal and is not what BS 6229 is talking about; BS 6229 covers flat roofs. Where a pitched roof becomes a warm roof is in a room in roof or loft conversion, where insulation moves to the rafter line; see loft conversion roofing and loft and roof insulation. Ventilation of a cold pitched roof has its own rules, covered in our roof ventilation guide.

What to ask before you sign anything

1. Warm deck or cold deck, and why? If the answer is cold deck, ask where the ventilation enters and leaves.
2. What U-value will the finished roof achieve? You want 0.16 W/(m²·K) on a replacement, or a stated reason why a lesser standard applies under paragraph 4.13 or the load-bearing and upstand note.
3. How much will the finished level rise, in millimetres?
4. What happens at the door threshold at that new level? Get an answer in writing.
5. How is the abutment being detailed, and does the chase need cutting higher?
6. Is the deck being replaced? If the existing deck is coming out anyway, the height and the vapour control layer can both be solved properly.
7. Is this notifiable, and who is certifying it? Renovation of a thermal element engages the Building Regulations; see planning permission and building regs for a new roof.

A flat roof done as a warm deck, detailed properly at the threshold and the abutment, with a stated U-value, is a thirty year roof. The same roof done as a cold deck with a ventilation path nobody can point to is a gamble on a void you will never see inside again.

Frequently asked questions

What is the difference between a warm roof and a cold roof? In a warm roof the insulation sits above the structural deck, so the deck stays warm and there is nothing cold for moisture to condense on. In a cold roof the insulation sits below the deck, between or under the joists, leaving a cold void above it that has to be cross ventilated to carry moisture away.

Is a cold roof still allowed in the UK? It is not banned, but BS 6229:2018 does not recommend cold roof design for flat roofs and sets demanding conditions where it is unavoidable, including a cross ventilated void, ventilation openings kept close together and an air open breather membrane on the cold side of the insulation. On a refurbishment those conditions are hard to achieve and impossible to verify later.

What U-value does a flat roof need in the UK? On a renovated existing roof, 0.16 W/(m²·K) under Table 4.3 of Approved Document L. A new roof element on an existing dwelling is 0.15, and a roof in a new dwelling is 0.16. Those figures are unchanged in the 2026 edition of Approved Document L, which comes into force on 24 March 2027.

How much thicker is a warm roof? Typically 120mm to 150mm of rigid insulation above the existing deck, plus the new deck and covering, so expect the finished level to rise by roughly that much. This is why the door threshold and the abutment flashing have to be resolved before work starts.

Can I put a warm roof over an existing flat roof? Sometimes, if the existing covering is sound, dry and firmly bonded, and the structure can take the load. More often the honest answer is to strip back, because overlaying traps whatever moisture is already in the old build-up and you inherit its defects. Insist on a proper look at the deck first.

What if my upstands are not tall enough for a warm roof? Approved Document L allows for it. The note to the roof row of Table 4.3 states that where there are problems with the load-bearing capacity of the frame or the height of the upstand, a lesser standard may be appropriate on a flat roof or a roof with integral insulation. In practice that means a thinner high performance insulant, a tapered scheme, or an agreed reduced U-value rather than rebuilding the parapet.

Sources

• Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Tables 4.1, 4.2 and 4.3
• Approved Document L, 2026 edition