Can a Flat Roof Skylight Be Fitted on a Warm Roof?

Most single-storey extensions built in the last three decades, and every one built to current standards, use a warm roof: a deck of timber or engineered board, a vapour control layer laid over it, then a continuous layer of rigid insulation, with the waterproof covering, felt, EPDM or GRP, laid over the top of that. A flat rooflight sits inside that same stack rather than beside it, so the question is not whether a warm roof will take a rooflight but how the kerb is detailed to pass through the insulation without leaving a gap.

A cold roof, where the insulation is fitted between the joists below the deck instead, is the older method and is less common on anything built recently. It still turns up on garage conversions and on some flat-roofed additions from the 1970s and 80s across Bretton, Orton, Werrington and Hampton, where the roof has never been upgraded. A rooflight fits a cold roof too, but the kerb detail and the height it finishes at are different, because there is no insulation layer above the deck for it to clear.

On a warm roof, the kerb has to rise from the deck, past the vapour control layer, past the full thickness of the insulation, and only then meet the waterproof covering that the glazing unit sits on. Skip that and the insulation stops short at the opening, leaving a cold ring of deck and structure around the rooflight that the rest of the roof does not have.

That ring matters more than its size suggests. It is a direct path for heat to escape and for cold to reach the underside of the reveal, and on a still winter morning it is often the first place condensation forms, sometimes as a line of moisture around the frame rather than on the glass itself. Our answer on whether flat roof skylights get condensation goes through the wider causes, but a poorly detailed kerb on a warm roof is one of the few that is purely a build-up problem rather than anything to do with the glass.

Getting it right usually means one of two things: insulation cut and packed tightly against every face of the kerb as it is built, or a kerb that already carries its own insulation core, so there is no site-cut joint to miss in the first place.

Both kerb types can be detailed correctly on a warm roof, and the choice usually comes down to the shape of the opening and how far ahead the roof plan is fixed, rather than the insulation itself.

• A factory-made insulated upstand arrives with continuous insulation already formed into its walls, so the height needed to clear the deck's insulation layer is built in rather than measured and packed on site.
• A site-built timber kerb can match the same height, but the insulation around it has to be fitted by hand, board by board, with no gaps left at the corners or where the kerb meets the deck.
• An irregular opening, a step in the roof, or a late change to the layout usually favours a site-built kerb, since a factory unit is ordered to a fixed size before it reaches the roof.

Our guide to kerbs and upstands for flat rooflights sets out both routes in full, including how each meets felt, EPDM and GRP coverings once the kerb itself is built. Flat rooflights on EPDM, felt and GRP roofs covers that final layer in more depth.

A kerb built up through a warm roof's insulation nearly always finishes taller than the equivalent kerb on an older cold roof of the same span, simply because it has more depth to clear before the waterproofing starts. That extra height carries straight down into the room as reveal depth: the lined shaft between the ceiling and the underside of the glass.

A deeper reveal is not a problem in itself, but it changes what a splayed lining does for the room. Widening the reveal towards the ceiling opens more sky into the sightline from a chair or a bed than a square-cut reveal of the same depth would, which matters more as the kerb, and the reveal beneath it, gets taller. Internal reveal finishes under a flat rooflight goes through the square and splayed options in detail, and it is worth reading alongside a warm roof kerb decision rather than after the glass has already gone in.

Everything above assumes a roof being built from scratch, where the insulation depth and kerb height are agreed on paper before anyone is on the roof. An existing extension is a different starting point: the build-up is already there, and the survey has to establish what it actually is before a kerb height can be fixed.

Some older flat roofs turn out to be cold roofs with thin, patchy insulation that would not meet current standards on its own, and Building Control treats a new rooflight opening as a chance to bring at least that section of roof up to today's thermal performance, whatever the original build-up. Our answer on adding a rooflight to an existing flat roof covers how that survey works, and what Building Control asks of a flat rooflight sets out the thermal element rules behind it. Where a roof is due to be re-covered anyway, that is usually the simplest moment to correct an old cold roof to a warm one and fit the rooflight in the same visit, which our page on adding a rooflight when a flat roof is re-covered explains.

A warm roof does not rule anything out for a flat rooflight; it is simply the build-up the kerb has to be measured against. What decides whether the finished job stays warm, dry and clear at the frame edge is whether the insulation is carried continuously past the opening, not stopped short of it, and whether the kerb height that follows from that has been allowed for in the reveal below.

Our flat roof rooflight installation service surveys the existing or planned build-up first, so the kerb, the fall and the reveal are all agreed before the roof is opened, and every quote comes with a sightline sketch showing what the finished opening will look like from where you actually sit.

Does a warm roof always need a taller kerb than a cold roof?

Usually, yes, because the kerb has to clear the full depth of insulation sitting above the deck rather than stopping at the deck itself. The exact height depends on how thick that insulation layer is, which varies from roof to roof.

Can an old cold roof be corrected to a warm roof when a rooflight is added?

Yes, and it is often the practical moment to do it, since the roof is already open around the new kerb. The insulation across the rest of the roof can be upgraded at the same time if the covering is also due for renewal.

Does a warm roof rule out a site-built timber kerb?

No. A timber kerb works on a warm roof provided the insulation is packed tightly around every face as it is built, with no gap left at the corners for cold to bridge through.

Will a taller kerb on a warm roof make the room feel darker?

Not if the reveal below it is lined to suit. A splayed reveal opens the sky back out as the kerb gets taller, which is why the reveal finish is worth agreeing alongside the kerb rather than as an afterthought.