Flat Roof Rooflight Installation
The default question on a rear extension, and the one where the roof matters more than the glass. Falls, upstand and vapour control decided before anything is ordered.
A flat roof rooflight is a made to size glazed unit sitting on a timber kerb built up above the surface of a flat or near flat roof. It is the most common rooflight in Colchester by a wide margin, because the rear extension is the standard improvement across the post-war and inter-war suburbs and a flat roof is what most of them have.
It is also the rooflight that leaks most often, and almost never because of the unit.
Flat roofs are not flat
Every flat roof is laid to a fall so water leaves it. Building Regulations expect a finished fall of at least 1:80, and because timber deflects, most roofs are designed to 1:40 so the finished fall still clears 1:80 once the structure has settled.
That matters for a rooflight because the unit sits proud of the roof on a kerb, and the kerb interrupts the flow of water across the surface. Water arriving at the upslope face of the kerb has to be able to get around it. On a wide unit with a shallow fall, water ponds along the upslope edge, sits against the upstand, and finds any weakness in the termination. That is the failure, and it is designed in before anyone lifts a tool.
The fix is not a better unit. It is positioning the rooflight so water reaches it already moving, and detailing the kerb so the covering is dressed up its full height and terminated under the flange.
The upstand
The kerb, or upstand, raises the frame above the water line. On a warm roof build up it is formed in timber and insulated on its outer face so the perimeter is not a cold bridge. On a cold roof it is built differently and ventilated differently, and getting the two confused is how you end up with condensation running down the inside of the frame in January and a homeowner convinced the unit leaks.
Height matters. Too low and the covering has nowhere to terminate. Too high and the unit sits up like a chimney and looks wrong from the garden. The working range is usually 150mm above the finished roof surface at the lowest point, and it is measured from the finished surface rather than the deck.
Vapour control
The layer that stops warm moist air from the room getting into the roof build up has to be continuous, and a rooflight puts a hole in it. Sealing the vapour control layer to the kerb, and the kerb to the frame, is the detail that decides whether the perimeter stays dry.
It is invisible once the plasterboard is on, which is exactly why it gets skipped. Every misted perimeter and every damp patch at the corner of a reveal traces back to it.
Flush glazed against domed
Flush glazed units are flat glass in a slim frame. They look contemporary, they suit the modern extension, and they are what most people picture now.
Domed polycarbonate units are cheaper, lighter, and shed water and debris by shape rather than by fall, which makes them genuinely useful on an outbuilding or a garage. They yellow, they scratch, and they look their age quickly. We do not fit them on a house, and that is a positioning choice rather than a technical one.
How many, and how big
The instinct is one large unit. Two smaller units usually give a better result in a long room, because light falls away about a metre past the edge of any rooflight and two pools of light cover more of a kitchen-diner than one.
Two smaller units are also easier to support, easier to lift, and cheaper to replace if one ever fails. On a standard rear extension, two units at around 1000 by 1500 will out-light one at 1500 by 2500 in everything except the photograph.
Glass
Toughened outer and laminated inner is standard overhead and is a safety requirement, not an upgrade: laminated glass holds together if it breaks rather than coming down into the room. Beyond that, solar control on anything facing south, and a low emissivity coating and argon fill on everything.
Self cleaning coatings work by letting rain sheet off the surface, so they need a fall to work with. On a very shallow roof they are worth less than the sales sheet suggests.
What the installation involves
- Survey. Roof build up, existing fall, deck condition, access and the position of anything in the ceiling below.
- Fixed written price, itemised, including Building Control where notifiable.
- Manufacture. Made to size units are typically two to four weeks.
- Fit. Usually one day for a single unit once the kerb is ready. The roof is weathertight at the end of every day.
- Reveal. Splayed where depth allows, because a splayed reveal throws visibly more light into the room.
What it costs, and what moves it
We give an indicative range by email and a fixed written price after survey. On a flat roof we will not quote a fixed figure over the phone, because the kerb decides this job and a price given without seeing it is a guess with a number attached.
Whether the opening exists. Fitting into a sound existing kerb is the cheapest job in this trade. Forming a new opening in a finished roof is several times the work.
Stock size or made to measure. On a new extension, designing the kerb around a stock size saves real money and weeks of lead time. Manufacturers hold common sizes from about 600 by 600 up to around 2000 by 3000, and they are cheaper and quicker because there is no bespoke fabrication.
The covering. Single ply membrane welds cleanly and a new joint is as good as the original. Torch-on felt bonds well if the existing material is sound. GRP is harder to make a durable new joint into, because the existing resin has fully cured, and on an older GRP roof the honest answer is sometimes that it wants recovering rather than cutting.
Structure. Trimming between joists is routine. A wide opening needs a steel designed by an engineer.
Access. Single storey from a tower costs nothing extra. Higher, or over a fragile roof, needs scaffold.
Warm roof and cold roof
Flat roofs come in two arrangements and a rooflight kerb is detailed differently in each. Confusing them is a reliable way to produce condensation that gets diagnosed as a leak.
Warm roof
Insulation above the structural deck, vapour control below it, waterproofing above. The deck and the structure stay warm, on the room side of the insulation. This is the modern default and what almost every new extension uses.
Here the kerb interrupts the insulation layer. Unless its outer face is insulated it is a cold bridge running the entire perimeter of the rooflight, losing more heat than the glass and forming condensation on the inside of the frame in winter. Insulating it is cheap while it is being built and impossible afterwards without opening the roof.
Cold roof
Insulation between the joists, below the deck, so the deck itself is cold and a ventilated void above the insulation is essential. Common on older extensions and generally not used for new work now.
Here a kerb built tight down between the joists blocks the cross ventilation that keeps the deck dry, and the damp appears downstream of the rooflight rather than at it, which makes it hard to attribute. The kerb sits on top of the deck instead, with the ventilation path preserved around the opening.
On an existing roof it is not always obvious from below which you have. Establishing it is part of the survey, because the detailing follows from it.
| Element | What it has to be | What happens when it is not |
|---|---|---|
| Height above the finished surface | About 150mm at the lowest point of the fall | Standing water reaches the termination and sits against it |
| Squareness | Square, within a few millimetres across the diagonals | A made to size unit will not sit down on it, and cannot be packed square |
| Top edge | Level all the way round, even though the roof beneath falls | The frame is twisted, and the seal is loaded unevenly |
| Insulation | Insulated on the outer face, continuous with the roof insulation | A cold bridge right at the frame, and condensation on the inside of it |
| Vapour control layer | Carried up the kerb and sealed to the frame | Warm moist air enters the build-up and condenses. It looks exactly like a leak |
| When it is measured | Before the unit is ordered, never after | The unit is made to a size the opening turns out not to be |
How the job runs
- Indicative range by email, free, from a description and photographs.
- Survey, free: roof build up, existing fall, deck condition, access, and what sits in the ceiling below.
- Fixed written price, itemised, including Building Control where notifiable.
- Kerb built or checked, then measured. The unit is ordered from a measured kerb rather than a drawing.
- Manufacture, typically two to four weeks for made to size, days for a stock size.
- Fit, usually one day for a single unit once the kerb is ready.
- Making good, reveal splayed where the depth allows.
Replacing an existing flat roof rooflight
A large part of what we do, and the kerb decides whether it is a one day job or a three day one.
A sound kerb, square and level along the top with adequate height and no rot, takes a new unit directly. A kerb rotted at the corners, out of square, or too low for the covering to have terminated properly, which is usually why the old one leaked, has to be rebuilt. Rebuilding means cutting back the covering around the opening, which is a different job with a different price.
If the kerb is being touched at all, insulating its outer face and sealing the vapour control layer to the new frame are cheap while it is open and impossible afterwards. On an old installation neither was likely done, and that is frequently why the room was cold and the perimeter misted.
What goes wrong
Almost never the unit. A kerb too low for the covering to terminate. A rooflight positioned where water ponds against it. A vapour control layer never sealed to the frame, so the perimeter is wet from the inside rather than the outside.
The distinguishing test for the third is simple: does it happen in cold dry weather? Rain leaks follow rain. Condensation follows cold.
From quote to daylight, with no surprises
Fixed quote
A clear, fixed written price, online or by phone.
Survey
A specialist confirms the right glazing spec. Flexible times.
Install
Clean, weather-tight fitting that protects your property.
Guarantee
We tidy up and back the work with a written guarantee.
Frequently asked questions
Do flat rooflights need to be sloped?
The roof does, and the unit follows it. A rooflight sitting dead level on a roof with no fall will pond water along its upslope edge and eventually find a way in. Where a roof genuinely has no fall, the answer is to build the fall into the kerb rather than to fit the unit level and hope.
Why do flat rooflights leak?
Almost never the unit. The three causes we see are a kerb too low for the covering to terminate properly, a rooflight positioned where water ponds against it, and a vapour control layer that was never sealed to the kerb so the perimeter is wet from the inside rather than the outside.
Do flat rooflights need building regulations approval?
New openings are notifiable, and so is a replacement where the opening changes size. Like for like replacement usually is not. Where it is notifiable we handle the notification as part of the job and the certificate comes to you.
How many rooflights does a flat roof extension need?
Usually two rather than one. Light falls away about a metre past the edge of a rooflight, so two pools cover a long kitchen-diner better than one large one in the middle, and two smaller units are easier to support and cheaper to replace.
Can a flat rooflight be opened?
Yes, manually or electrically. On a flat roof the unit is above reach by definition, so electric with a rain sensor is the version people actually use. It is worth deciding before the kerb is built, because the opening version is heavier.
Helpful reading on this
Costs, comparisons and answers to the questions we’re asked most.
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