Can you fit a rooflight to a felt roof?

A rooflight does not sit flat on the covering. It sits on a raised timber frame, called a kerb or an upstand, that is built up from the roof deck. The rooflight is then fixed to the top of that kerb, so the glass sits a little proud of the surrounding roof. On a felt flat roof, the felt is carried up the sides of the kerb and turned over the top edge before the rooflight goes on, so the whole opening is wrapped in the same waterproof layer as the rest of the roof.

That height is the reason a felt roof takes a rooflight so well. Water on a flat roof does not run off cleanly the way it does on a pitch, so the covering has to be lifted clear of the wet part of the roof. A kerb of about 150mm above the finished felt is the usual figure, and it keeps the base of the glazing well above any water that pools after heavy rain. A rooflight bedded straight onto the deck with no kerb, or onto a kerb that is too shallow, is the single most common way a flat-roof unit ends up with water tracking under it.

Felt is a forgiving material to work into that detail. It is warmed and dressed by hand, so it moulds around the corners of the kerb and bonds to the timber. A well-formed felt upstand around a rooflight is a clean, continuous piece of work with no open joints at the corners, which are the spots to look at closely if you are checking someone else’s installation.

Felt roofs are not all the same, and the age of yours affects how the rooflight is installed. Older roofs tend to be built-up felt, laid in two or three layers bonded with hot bitumen, often with a mineral top layer. Newer ones are usually torch-on felt, where the layers are heat-bonded with a gas torch, or a modern high-performance felt with a polyester or glass-fibre core that lasts a good deal longer than the old organic felts did.

If your felt is sound, the new rooflight is dressed into it and the two become one covering. If the felt is at the end of its life, it makes sense to renew the covering across the whole roof at the same time as the rooflight goes in, rather than tie new work into a surface that is close to failing anyway. That is a judgement to make on the day, once the roof can be seen properly. A rooflight fitted into fresh felt, with the kerb and the covering formed together as one job, is the cleanest outcome and the one least likely to give you problems later.

Warm-roof and cold-roof build-ups also sit under all of this. A warm roof carries the insulation above the deck and below the felt, which suits a rooflight well because the kerb can be insulated as part of the same layer and there is less risk of cold spots and condensation around the opening. It is worth knowing which you have before the design is settled.

A flat roof is never truly flat. It is laid to a slight fall, usually around 1 in 40, so that water runs towards a gutter or outlet rather than sitting still. When a rooflight goes in, the fall has to keep working around it. The kerb interrupts the flow of water across the roof, so on the uphill side of the opening you do not want water backing up against the timber and sitting there.

This is where the design matters more than the felt. The rooflight is best positioned so water runs past it rather than into it, and on a larger opening a small tapered kerb or a diverter can be built so the flow splits and passes either side. Get that right and the felt around the rooflight stays dry between showers. Get it wrong and you have created a low point that holds water against the one part of the roof that has been cut open.

Most modern flat-roof rooflights are designed for exactly this setting. A fixed flat glass rooflight sheds water off the glass and onto the felt beyond the kerb, and a lantern or a pitched glass unit on a flat roof does the same more steeply. The felt does the waterproofing up to the kerb, the rooflight does it above the kerb, and the lap between the two is dressed so the rooflight’s own flashing sits over the felt, not under it. Water then always runs from the glass, onto the felt, and away.

Once the waterproofing is sorted, the question on a felt flat roof is heat, not rain. A flat roof points at the sky rather than at a compass direction, so it collects the high summer sun almost square on for most of the day, whichever way the house faces. In Southend the problem is sharper again, because the city faces south across the Thames Estuary with an open horizon and light coming back off the water, so a flat-roof extension here gathers more sun than the same room would inland.

That is why the glass specification matters as much as the felt detail. Clear double glazing has a G-value of around 0.6, meaning it lets roughly 60 per cent of the sun’s heat through. Solar-control glazing brings that down to around 0.3, so the room below a felt roof extension stays bright without turning into a greenhouse by mid-afternoon. On a rear kitchen extension that already makes its own heat from cooking, that difference is the one people notice most once the rooflight is in. We specify orientation first, then the glass, then the unit, and it is the heart of our energy-efficient skylight installation work.

None of this asks anything different of the felt. The waterproofing is the same whether the glass is clear or solar-control. It simply means that on a felt flat roof, where the sun hits hardest, choosing the glass is the decision that shapes how the finished room feels to live in.

If you have a sound felt flat roof, a rooflight is a normal, well-understood installation and you should treat the covering as an asset rather than an obstacle. Check the age and condition of the felt first, decide whether it is worth renewing the covering at the same time, and settle the position so the fall of the roof carries water past the opening rather than against it. New and enlarged rooflights are notifiable under Building Regulations, so Part L thermal performance and Part K safety glazing both apply, and that notification is part of the installation.

If the felt is failing across the whole roof, that is a roofing decision to take before any glazing goes in, and a roofer is the right first call there. Where the covering is sound, the rooflight and its felt upstand are formed together as one piece of work. You can read the fuller detail on our flat roof skylight installation page, or tell us the roof size and which way the house faces and we will come back with a glass specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the opening in it.