This is a construction detail rather than a visible design choice — both build-ups can look identical from below. But getting it wrong is one of the more common causes of hidden timber decay in flat-roofed extensions, so it is worth understanding even if you never touch the detailing yourself.
How a warm deck is built
In a warm deck, the structural timber or concrete deck sits at room temperature, with rigid insulation laid above it and the waterproof membrane on top of that. Because the deck itself stays warm, there is no cold surface inside the roof void for moist internal air to condense on.
This is now the default specification for new flat roof work, because it removes the ventilation void as a point of failure entirely.
How a cold deck is built, and where it goes wrong
In a cold deck, insulation sits between the joists, leaving the deck itself cold and exposed to outside temperatures on its upper face. Warm, moist air from inside the building will condense on that cold deck unless a ventilated air gap above the insulation carries the moisture away before it can do so.
The failure mode is almost always the same: the ventilation path gets blocked, either at the design stage by insufficient gap, or later by insulation being topped up without checking the airflow. Moisture then condenses silently inside the void and rots the joists from above, often for years before it shows as a stain on the ceiling below.
When a cold deck is still chosen
Cold deck construction has a shallower overall build-up in some configurations, which occasionally makes it the only option under an existing window or door head that cannot be raised. It is also sometimes retained on refurbishment work where converting to warm deck would mean stripping the roof back further than the budget allows.
Cost difference between the two build-ups
Warm deck construction generally costs a little more upfront because rigid insulation above the deck and the additional upstand height it creates both add material and labour, but that cost is offset over the roof's life by removing the ventilation-dependent failure mode entirely.
Cold deck construction can look cheaper on a quote, but that figure rarely accounts for the risk of hidden decay if the ventilation path is ever compromised, which is a repair cost that does not show up until years later.
What a quote for either build-up should state
A warm deck quote should specify the insulation thickness, the vapour control layer position, and how upstands at doors and abutments are being raised to accommodate the extra depth of the build-up.
A cold deck quote should specify the ventilation gap size above the insulation, how continuous airflow is being achieved across the whole roof void, and how that ventilation will be inspected or maintained over time.
Common questions
Can an existing cold deck roof be converted to warm deck?+
Yes, generally by stripping back to the deck and rebuilding with insulation above it, which also removes the ventilation risk permanently. It adds height to the build-up, which needs checking against door and window heads.
How do I know if my cold deck is ventilating properly?+
This needs a qualified inspection rather than guesswork — signs like ceiling staining, a musty smell in the room below, or visible mould at cornices are worth acting on quickly rather than waiting for them to worsen.
Is warm deck always the right answer for new work?+
For new flat roofs, yes, it is now the standard approach precisely because it avoids ventilation-dependent detailing. The exceptions are genuinely tight headroom situations where a shallower build-up is unavoidable.
Get your flat roof checked
We'll assess the existing build-up, tell you honestly whether it is ventilating as designed, and quote for either a warm deck upgrade or a correctly detailed cold deck repair.