Warm deck against cold deck, and where the rooflight or skylight sits in each

The two arrangements in one sentence each

A warm deck puts the insulation above the structural deck, so the deck is inside the heated envelope and stays warm. A cold deck puts the insulation between the joists, below the deck, so the deck is outside the envelope and stays cold.

Everything else follows from that single difference.

The warm deck, layer by layer

From the room upward: ceiling, joists, structural deck, vapour control layer, insulation, waterproof covering.

The insulation is a continuous unbroken layer over the whole roof. The vapour control layer sits directly under it, on the warm side, exactly where it needs to be. The deck and the joists are inside the insulation and therefore warm and dry.

There is no void, so there is nothing to ventilate. That is the arrangement’s greatest advantage: it has one fewer system to get wrong.

The cold deck, layer by layer

From the room upward: ceiling, vapour control layer, insulation between the joists, a ventilated air gap, structural deck, waterproof covering.

The insulation sits between the joists, so it is interrupted by every joist. The deck above it is cold, because it is outside the insulation. The air gap between the insulation and the deck has to be ventilated to outside air, so that any moisture reaching it is carried away.

It uses the roof depth efficiently, which is why it was standard for decades and why so many older Sussex extensions have one.

Which one you have

The vents are the reliable one. A flat roof with ventilation openings along the fascia or at the eaves is a cold deck, because a warm deck has nothing to ventilate. A roof with no vents at all is either a warm deck or a cold deck that was built wrong.

Why the warm deck won

The warm deck is the current standard for good reasons, and they are all about removing ways to fail.

• Continuous insulation, so no thermal bridging at every joist.
• No ventilation void, so nothing to block, and no dependence on air movement that may not happen.
• The vapour control layer is in one continuous plane immediately under the insulation, which is straightforward to detail.
• The structural timber is warm and dry for its whole life.
• Better real thermal performance for the same nominal insulation thickness, because there are no joists interrupting it.

The cold deck problem

A cold deck relies on ventilation, and ventilation on a domestic flat roof is difficult to achieve reliably.

The void is shallow, the air path is long, and it needs openings at opposite sides of the roof for air to actually move rather than sit. On a roof abutting the house on one side and a boundary wall on another, there may be only one open edge, and cross ventilation is then impossible.

When the void stops working, moisture that gets past the vapour control layer condenses on the underside of the cold deck and stays there. Deck decay follows, quietly, over years.

The inverted roof, the third arrangement

There is a variant worth naming because it turns up on some larger and newer buildings: the inverted roof, where the waterproofing sits below the insulation and the insulation is held down by ballast or paving.

It is rare on domestic extensions and common on terraces and podium decks. Where it appears, the rooflight kerb detail is different again, because the waterproofing terminates at a lower level and the insulation is loose laid around the kerb rather than bonded to it.

If you have a flat roof with gravel or paving on it, this is a possibility and it is worth establishing before anything is designed.

Where the rooflight kerb sits in a warm deck

On a warm deck the kerb is built up from the structural deck, and the roof insulation is carried up its outer face to meet the covering.

The vapour control layer comes up the inside face and is sealed to the rooflight frame. The insulation on the outer face laps into the main roof insulation with no gap. The covering goes over that and terminates under the trim.

Every layer is continuous round the kerb. It is the easier detail of the two by a considerable margin, and it is why we would build a warm deck wherever the roof is being formed or recovered as part of the job.

Where the rooflight kerb sits in a cold deck

On a cold deck the kerb passes straight through the cold zone, and there are two things to get right at once.

Converting a cold deck to a warm deck

Where a roof is being recovered anyway, converting is frequently the right call and it is less disruptive than it sounds.

The covering comes off, a vapour control layer goes down on the existing deck, insulation goes over that, and the new covering goes on top. The old insulation between the joists can generally stay where it is, and the void is no longer relied upon.

What changes is the roof height, by the thickness of the new insulation. That has consequences at the abutment, at the upstands, at the door and window heads below, and at any rooflight kerb, all of which now sit lower relative to the finished surface.

Which is precisely why a rooflight kerb on a roof about to be converted has to be raised at the same time, and why doing the two jobs together is far better than doing them two years apart.

Depth, and the constraint nobody mentions

Insulation thickness is set by the performance the roof has to achieve, and on a warm deck that thickness sits above the deck and adds to the overall height of the roof. That is frequently the practical limit on a retrofit.

The roof cannot rise past the sill of a first floor window above it, past the top of an existing upstand at the house wall, or past a boundary condition where height is constrained. On a single storey rear extension under a first floor window, there is sometimes only a hundred millimetres or so of headroom to work in, and that decides how much insulation is possible.

Where the depth is genuinely tight, higher performance insulation products achieve more in less thickness at a higher cost, and that trade is worth setting out plainly rather than discovering at the end. A rooflight kerb has to be raised to suit the finished level either way, which is the part people miss when they think of a recover as a like for like job.

What the ceiling tells you

The room below is evidence, and it is available without a ladder.

On a cold deck, the joists are thermal bridges through the insulation between them. In cold weather they run cooler than the plasterboard between them, and over years dust deposits preferentially on the cooler lines. The result is faint parallel stripes across the ceiling at joist centres, sometimes called ghosting, and it is a reliable indication of a cold deck with insulation only between the joists.

An evenly clean ceiling with no striping points at a warm deck or at a cold deck with an additional continuous layer under the joists. Neither is proof, and both are worth noting before anybody goes onto the roof.

Hybrid roofs, and why they are a bad idea

Adding insulation above a deck that already has insulation between the joists produces a hybrid, and hybrids can behave badly.

The risk is that the deck now sits between two insulation layers, colder than the room and warmer than outside, with a vapour control layer in the wrong place relative to at least one of them. Where the proportions are wrong, condensation forms on the underside of the deck and the void is no longer ventilated because the roof is now nominally warm.

It can be done, and it needs the proportions checked rather than assumed. Where we meet a roof somebody has already done this to, it is worth establishing before adding a rooflight to it.

What each one means for the rooflight itself

The rooflight is the same product on either roof. What differs is the kerb, the ventilation and the risk.

On a warm deck we can be confident in the detail, because every layer is continuous and inspectable while the work is open. On a cold deck we are working with a system that depends on air movement we cannot see, and the honest position is that the detail is as good as it can be made while the rest of the roof remains a cold deck.

That is a real difference in the confidence we can offer, and we say it rather than implying both are equivalent.

What we find around Worthing

The pattern in this area is fairly predictable by age.

Rear extensions from the 1960s to the 1980s, of which there are a great many across Worthing, Lancing and Goring, are almost all cold decks with a bitumen covering. Extensions from the 1990s and 2000s are mixed, with GRP appearing and warm decks becoming common. Anything from the last fifteen years is very likely a warm deck.

Many of the older ones have been recovered at least once. A recover that simply replaced the covering left the cold deck in place; a recover done properly converted it. Which of those happened is visible once the covering is lifted at the edge, and it is one of the things we look for at survey.

What we establish first on a flat roof

We establish which build-up the roof is before quoting, by looking at the vents, the roof thickness at the edge and, where we need to be certain, by lifting a small area at the perimeter. We detail the kerb for the build-up that is actually there, and where the roof is being recovered we say plainly whether converting to a warm deck is worth doing while everything is open.

A rooflight is a hole in a system. Knowing which system it is comes first.