What makes a warm deck different
In a warm deck the rigid insulation sits on top of the structural deck and under the waterproof covering, so the joists and the deck stay on the warm side of the roof.
The build-up from the room upwards is usually: plasterboard ceiling, joists, a plywood or OSB deck, a vapour control layer, rigid insulation boards (PIR is common), then the membrane, whether that is EPDM, GRP, a single-ply sheet or torch-on felt. There is no ventilated void to worry about, which is why the warm deck has become the standard build-up for new single-storey extensions, from side returns on Romsey terraces to kitchen extensions on 1930s semis in Cherry Hinton.
For a rooflight, that layout is good news. The timber you cut and trim is warm and dry, and the insulation is a single, visible layer you can follow to the edge of the hole. On a cold deck the insulation sits down between the joists and is harder to join to the upstand neatly. On a warm deck the detail is right there at roof level, where the fitter can see it and check it.
Keeping the insulation line unbroken
The whole job comes down to one line drawn round the building on a section: the insulation, which has to stay continuous from the roof boards, up the kerb and into the frame of the rooflight.
Where a rooflight goes in, the insulation boards are cut back to the opening. If the upstand is plain timber with nothing against it, that timber now bridges from the warm room to the cold outside air. In winter the inside face of the kerb gets cold, warm moist air from a kitchen or bathroom meets it, and condensation forms at the top of the reveal. Over time that shows up as mould spots in the corners and staining on the plaster.
A sound detail avoids that in one of two ways:
- A factory insulated upstand, usually a PVC or GRP kerb filled with insulation, bedded on the deck with the roof boards butted tight against its sides.
- A built timber kerb wrapped in insulation, with PIR boards fixed to the outside face of the upstand at a thickness matched to the roof insulation, so the boards on the flat and the boards on the kerb meet at the corner.
The rooflight frame then sits on top, and its own thermal break continues the line. Part L of the Building Regulations sets a limiting U-value of 2.2 W/m²K for a rooflight, and a new or renewed flat roof in an extension is expected to reach around 0.15 W/m²K. The glass alone does not decide whether the room stays free of condensation. The kerb does.
How the insulation depth sets the upstand
A warm deck is thick, and that thickness is added to the upstand, which changes both the roof outline and the reveal you see from below.
To reach current standards on a timber deck, rigid insulation of roughly 120mm to 150mm is typical, and a tapered scheme can be deeper at its high end. The rooflight still needs to stand at least 150mm above the finished roof surface, as our answer on upstand height explains. Measured from the deck, the kerb can therefore be 300mm or more before the frame goes on.
From outside, on a roof behind a parapet or seen from an upstairs window, that rarely matters. Inside, it means a deeper light well between the ceiling and the glass, which brings us to where the light lands.
The deeper reveal and the light in the room
A tall kerb acts like a short tube. The deeper it is, the narrower the cone of light that reaches the floor, and the more the sides of the reveal matter.
With straight, square reveals, a skylight on a deep warm deck throws a sharper, smaller sun patch that moves across the floor through the day. That can be exactly what you want over a dining table, or awkward over a worktop where you prepare food at midday. Lining the reveal in plasterboard and painting it a light, matt colour bounces more of the daylight down. Splaying the reveal, wider at the ceiling than at the glass, opens the cone and spreads the light further across a long room, which helps most in the deep back half of a terrace extension.
These choices are drawn on the written light plan that comes with every quote, along with glass and shading, because they are hard to change once the ceiling is boarded. Our page on how we plan the light shows the method, and the glazing guide covers solar control glass for a roof that takes strong afternoon sun.
New warm deck or an existing one
The detail is simplest when the rooflight goes in while the roof is being built, and perfectly workable when it is added to a warm deck that is already finished.
On a new extension the kerb goes down on the deck before the insulation, the boards are laid up to it, and the membrane is dressed up the upstand in one sequence. Adding a skylight to an existing warm deck means cutting through the membrane and the boards, trimming the joists, fitting and insulating the kerb, then weathering the membrane back up it. The covering has to be sound enough to take that, and the insulation has to be dry when it is opened up. Wet boards tell you something about the roof, and that gets dealt with before the rooflight is fitted. Our answer on fitting to an existing roof covers the retrofit checks, and rooflights and roof coverings explains how each membrane is turned up the kerb.
A new rooflight is normally notifiable building work, and we handle the Building Control notification where it is. Full details of the service sit on our flat roof rooflight installation page.
Questions people also ask
Is a warm deck better than a cold deck for a skylight?
For the insulation detail, generally yes. The boards sit at deck level, so they can be joined to an insulated upstand in plain sight. A cold deck can take a skylight too, but the insulation between the joists and the ventilation path need more care round the opening.
Does the rooflight provide the fall on a warm deck?
The roof has its own fall, often from tapered insulation, but the glass needs its own pitch of around 5 degrees to shed water. That comes from an angled kerb or a unit with built-in pitch. See minimum pitch for a flat rooflight.
Will cutting the opening weaken a warm deck roof?
The insulation carries no load, so the question is the joists below it. A modest opening is trimmed with doubled joists either side; a large one may need an engineer. Our answer on strengthening a flat roof goes through it.
What about an inverted warm roof?
In an inverted roof the insulation sits above the membrane, held down by ballast or paving. A rooflight can still go in, but the upstand is measured from the top of the ballast, so it stands taller again. If you are not sure which build-up you have, book a light survey and we will check before anything is ordered.
