Warm roof vs cold roof construction
If a flat roof keeps getting damp without an obvious leak, the build-up is usually the reason — and this is the distinction that explains it.
Cold roof
Insulation sits between the joists, below the deck. The deck stays cold, and any warm moist air reaching it condenses. Cold roofs must be ventilated above the insulation, and in practice that ventilation is often blocked or was never there.
Warm roof
Insulation sits above the deck, under the waterproofing. The deck stays warm, so condensation doesn't form on it. It's the build-up we recommend on almost every re-cover.
The height problem
A warm roof raises the finished level. Where a door threshold or window sill is close above the roof, this needs designing around — sometimes with a tapered or high-performance insulation to keep the build-up thin.
Vapour control
A warm roof needs a vapour control layer below the insulation. Leaving it out is the single most common warm-roof mistake and it undermines the whole point of the build-up.
Inverted roofs
Less common domestically, but occasionally right where a terrace finish sits over the waterproofing on ballast or paving supports.
FAQs
Not always, but a cold roof with blocked ventilation is a very common cause and worth checking before chasing a leak that isn't there.
Yes, during a re-cover. It's the natural time to do it because the covering is coming off anyway.
The insulation adds cost, but the labour is similar and the result is a roof that doesn't sweat.
Related reading
More on comparisons and the jobs that usually come with it.
- Dry-fix vs mortar-bedded ridgesComparisons
- Felt vs EPDM vs GRP: choosing a flat roof systemComparisons
- Flat roof systems compared and how long each lastsComparisons
- Soffit vents, over-fascia vents and airflow at the eavesMaterials & systems
Looking for something specific? See our roofing services, cost guides, common roof problems or the areas we cover.
Get a fast quote
Family-run and Hampton-based. Tell us what you're seeing on the roof and we'll come and look properly before quoting.