Can you fit a flat rooflight on a warm roof?

The two build-ups put the insulation in different places, and that single difference decides how a rooflight has to be fitted.

A cold roof carries its insulation down between the joists, with the structural deck and the waterproof covering sitting on top and a ventilated void underneath. A warm roof puts the insulation on top of the deck, in an unbroken layer, with the waterproofing over that. The deck and the joists stay at room temperature, which is where the name comes from. Almost every flat roof built or re-covered to a modern specification is a warm roof, because it keeps the structure warm, removes the need for a ventilated void and gives condensation far less opportunity to form inside the build-up.

To tell them apart from below, look at the total thickness at the edge and whether there is any ventilation gap. A warm roof reads as a thick sandwich with no air path through it. If you are unsure, the covering and the insulation position get confirmed the moment the opening is formed, and the fitting method follows from there.

A flat rooflight does not sit flat on the roof surface. It sits on a raised upstand, also called a kerb, that lifts the glass clear of standing water and gives the waterproofing something to turn up against. On a warm roof, that upstand is the whole job.

The insulation layer has to continue up the sides of the upstand and meet the underside of the rooflight frame, so the thermal envelope stays unbroken all the way to the glass. Skip that, build a bare timber kerb and drop the unit straight on, and you have created a cold bridge: a ring of uninsulated structure running right round the opening. Warm, moist air from the room below finds that cold ring, and the result is a band of condensation and dark staining around the frame that looks exactly like a leak but is not one. A purpose-made insulated upstand, or a timber kerb wrapped in insulation to match the roof deck, keeps the line continuous and keeps that ring warm.

Two other details matter on the upstand. The waterproof covering must turn up the outside of the kerb and be dressed to a proper height, so wind-driven rain cannot track back under the frame. And the height above the finished roof surface needs to be enough to stay clear of ponding, which on a flat roof is never truly flat and always drains slowly. Get the upstand right and the unit sitting on it is straightforward. Get it wrong and no amount of quality in the glass above will save the detail.

A pitched rooflight faces a compass direction, so its exposure to the sun depends on which way the house is turned. A flat rooflight does not have that luxury. It points straight up at the sky and sees the high summer sun almost square on from mid-morning to late afternoon, whichever way the building faces. For solar gain, every flat rooflight behaves like a south-facing one.

That is where the glass specification earns its place. The G-value is the share of the sun’s energy that passes through the glass and becomes heat in the room. Clear double glazing sits around 0.6, which lets roughly sixty per cent of that energy through. Solar-control double glazing brings it down to about 0.3. On a flat roof over a kitchen or a deep rear extension, that difference is the gap between a room you can use on a July afternoon and one that behaves like a greenhouse over the one space in the house that already makes its own heat from cooking.

Southend faces south across the Thames Estuary, with an open horizon and light bouncing off the water, so flat roofs here collect more sun than the same roof would inland with houses and trees in the way. On a warm roof over a hot room, specify the upstand for the physics of water and heat loss, and specify the glass for the physics of solar gain. Our solar-control glazing is built around exactly that reasoning. Ask for the glass specification sheet and read the G-value, the light transmittance and the U-value together, rather than accepting the phrase “solar control” as if it were a number.

A new or enlarged rooflight is notifiable under Building Regulations, warm roof or not. Part L sets a ceiling on the U-value, which is how fast heat escapes on a cold night, and a warm roof helps you meet it because the continuous insulation reduces losses around the opening. The frame and glass still have to be specified to hit the figure, but the build-up is working in your favour rather than against you.

Part K covers safety glazing and guarding. A rooflight within reach, or one a person could fall against, needs the appropriate safety glass, and a walk-on or step-out unit is a different specification again. On a house, a flat rooflight is generally permitted development where it projects no more than 150mm above the roof plane and sits below the ridge, but flats, maisonettes, listed buildings, conservation areas and Article 4 zones are the exceptions, and the Leigh conservation areas near here carry an Article 4 Direction that removes those rights for roofing and window changes. Treat that as general guidance and confirm your own position with the local authority before you commit. On our installations the Building Control notification is made on your behalf.

Start by confirming the build-up. If your flat roof is a modern warm roof, or is about to be re-covered as one, a flat rooflight fits it well and the insulated upstand is the detail to get specified and priced properly. If you have an older cold roof, a rooflight still fits, but the upstand detailing changes and it is worth deciding whether the whole covering is due to come up anyway, because forming the opening as part of a warm roof upgrade is tidier than working around a cold one.

Then think about the glass before anyone quotes you, because a flat rooflight always sees the summer sun. Note the roughly square opening size you are planning and the room it will light, and if that room already runs hot, ask for a lower G-value.

If you would like us to look at it, tell us the roof build-up and the rough opening size and we will come back with an upstand detail, a glass specification and a price. You can read how we handle the whole job on our flat roof skylight installation page, see where the numbers land on our skylight installation costs page, or ring 01702 898232 and request a quote. We will look at the whole roof, not just the hole in it.