Warm Roof vs Cold Roof: Which Flat Roof Build-Up?

In a cold roof, the insulation is packed between the joists underneath the structural deck, and the deck itself sits out in the cold, above the insulation. Warm, moist air from the room below finds its way up through gaps around light fittings and hatches, hits the cold underside of the deck, and condenses. The timber then stays damp for months at a time.

In theory a cold roof can work if there's continuous cross-ventilation clearing that moisture — a 50mm ventilated void above the insulation, with uninterrupted airflow from eaves to eaves. In practice, on real UK garages and extensions, that ventilation is almost never achieved: the void gets blocked by insulation slumping, by stored items, or was never detailed properly in the first place. The result is the classic cold-roof failure: black mould on the deck underside, wet insulation, and joist ends rotting where they sit in the wall.

A warm roof flips the build-up. The insulation — usually rigid PIR boards — sits on top of the structural deck, and the waterproof covering goes on top of the insulation. The deck is now inside the warm envelope of the building, at roughly room temperature, so there's no cold surface for moisture to condense against. No condensation means no mould, no wet insulation, and no rotting timber.

A vapour control layer between the deck and the insulation stops room moisture migrating upward at all, and because the insulation is above the structure, the boards can be laid to falls — building in the slope the roof needs to drain, rather than hoping the joists happened to get it right. This is why modern Building Regulations effectively require a warm deck build-up on new extensions: U-values have to be demonstrated, and the warm build-up is the reliable way to hit them.

Rarely, but occasionally. If there's a specific structural reason the insulation can't go above the deck — a height restriction at a door threshold or an abutment that can't be raised, for instance — a cold build-up with genuinely engineered ventilation may be the only option. It needs to be designed properly, with the ventilated void detailed and protected, not just hoped for.

For a like-for-like replacement where height allows, we won't build a cold deck. When we strip an old cold roof, the correct job is to convert it: the old insulation comes out from between the joists, the deck is checked and replaced where it's damp-damaged, and the new roof goes back as a warm build-up with the covering on top.

Ask one question: is this a warm roof or a cold roof? If the answer is vague, or the quote makes no mention of insulation position, vapour control or falls, be cautious — a cheap flat roof quote is usually cheap because the insulation has been left out or left below the deck, which recreates the exact problem you're paying to fix.

A proper quote will state the build-up layer by layer: deck, vapour control layer, insulation type and thickness, and the covering system on top. Ours do, along with the U-value the build-up achieves, so Building Control can be satisfied without a fight.