Building the Kerb for a Flat Roof Rooflight

A flat roof rooflight kerb is a braced, insulated timber box fixed down into the trimmers around the structural opening, built to at least 150mm above the finished roof surface measured at the lowest point of the fall, faced with rigid insulation continuous with the roof insulation, and squared to within 3mm on the diagonals before anything else happens to it. Get those four things right and the rooflight above it has almost nothing left to fail at. Get any one of them wrong and no amount of glass quality, sealant or goodwill will save the detail.

This page is the construction specification. It is written for anyone who is about to build one, specify one, or judge whether the one they already have is worth keeping. The wider job it sits inside is covered on our flat roof skylight installation page.

What the kerb is actually doing

Three jobs, and they pull in different directions.

The first is height. The kerb lifts the glazing above the plane where water sits and where wind-driven rain runs, and it gives the roof covering a vertical face to turn up against and a top edge to wrap over. Without that vertical face there is no termination, and without a termination the covering has to be stopped somewhere and sealed, which is a five-year detail at best.

The second is structure. A rooflight frame is a rectangle that has to stay a rectangle. Flush glass units are bonded assemblies with tight tolerances, and they are set onto the kerb head and mechanically fixed through it. If the kerb twists, bows or racks, the frame is asked to take that movement and the seal line between frame and glass is the thing that gives. On a 2m span this is not theoretical. Unbraced 47mm timber will bow visibly under its own restraint once the covering is bonded to it and the sun heats one face.

The third is thermal. The kerb is a continuous ring of relatively conductive material running from the warm room to the cold roof. Left bare it is a cold bridge around the entire perimeter, and in January it condenses. That is why the insulation is not an upgrade item on our jobs. It is part of the kerb.

Timber sizing and the spec table

We build kerbs in treated softwood, C16 or better, to the sizes below. These are the sizes we work to across Ipswich, Suffolk and north Essex, and they are deliberately a step above the minimum you can get away with, because the cost difference between 47mm and 38mm stock across a domestic opening is a few pounds and the difference in how it behaves over twenty winters is not.

A note on the 150mm figure, because it gets quoted loosely. It is not the depth of the timber. It is the finished height of the kerb head above the finished roof surface, after the insulation, the deck build-up and the covering are all on, measured at the lowest point of the fall. On a roof falling 40mm across a 1.5m opening, a kerb built from 150mm stock sitting on the deck will finish somewhere near 130mm at the low corner once 20mm of covering build-up is around it. That is why we set the kerb head level and let the height vary at the base, packing the high side down onto the deck rather than building a kerb that follows the fall.

Setting out and squareness

The kerb is built to the rooflight, not the other way round. Every manufacturer publishes a kerb size for a given unit, usually as an external kerb dimension the frame sits over, occasionally as an internal daylight opening. Those two numbers are not interchangeable and confusing them is the single most expensive mistake available on this job, because a flush glass unit is made to order and there is no adjustment in it.

Our sequence is fixed. Confirm the unit and get the manufacturer’s kerb detail in writing. Set out the structural opening from that, allowing for the kerb timber thickness. Fix the trimmers. Build the kerb on the deck. Then check the diagonals before a single fixing goes into the top face.

Squareness is checked three ways, and all three have to pass:

• Diagonals. Measure corner to corner both ways across the kerb head. The difference tells you whether the rectangle is a rectangle. Tolerance is in the table above.
• Level of the kerb head. Not level with the roof. Actually level, or laid to the shallow fall the manufacturer specifies for a flush unit, typically 2 to 5 degrees on units designed to self-drain. A kerb head that is out of level in two axes puts the frame in wind, and a frame in wind loads the corner of the glazing seal.
• Plumb of the faces. A kerb face leaning outwards sheds the covering away from the head. A face leaning inwards traps water at the base of the vertical. Both are avoidable with a level held against the face at four points per side.

Fixing the kerb down

The kerb transfers wind uplift from the glass into the roof structure. On a 2.0 x 1.0m unit on an exposed Suffolk roof that is a real load, and it is a load that acts upwards, which timber connections handle far worse than they handle downward load.

The kerb sole plate is fixed directly into the trimmers and trimming joists, never into the deck alone and never onto the insulation. Deck board is 18mm OSB or ply, and a screw into 18mm of board with nothing behind it has almost no withdrawal resistance. We fix at 300mm centres maximum along each side, with a fixing no more than 100mm from every corner, using structural timber screws long enough to gain at least 60mm of penetration into the trimmer below. On coastal work those fixings are stainless.

Where the kerb sits over the deck rather than on a trimmer at one point, which happens on non-standard openings, a timber packer is let in and fixed through to structure below rather than relying on the board.

Insulation and the ply facing

This is the part most commonly left out, and it is the part that decides whether the finished rooflight condenses.

The outer face of the kerb is clad in rigid insulation, PIR in most cases, of a thickness that brings the insulation line continuous with the roof insulation. If the roof deck carries 100mm of PIR above it, the kerb face wants insulation that meets that layer and continues up to the kerb head without a gap. A 20mm shortfall at the top of the kerb face is a 20mm cold ring around the whole rooflight, and warm moist air from a kitchen will find it in the first cold snap.

The joint between the roof insulation and the kerb insulation is taped or foamed and taped. Not left open and covered by the membrane. The membrane is a water barrier, not an air barrier at that junction, and air movement through an open joint carries moisture to the cold face where it condenses out inside the construction rather than on the plasterboard where you would see it.

Over the insulation goes an 18mm WBP ply facing, screwed back through the insulation into the kerb timber. The ply does three things. It gives the covering a continuous, rigid, bondable substrate, because no membrane bonds properly to PIR foil facing and no membrane should be asked to. It protects the insulation from mechanical damage during the rest of the build. And it lets the covering be mechanically terminated at the head with a bar or with fixings that have something to bite into.

The ply is dressed over the top of the kerb head as well, so the head presents as a continuous flat timber surface the covering can be wrapped over and the frame can be bedded onto. The arris at the top outer corner is eased, not left sharp. A sharp arris under EPDM or single-ply is a stress line, and under GRP it is a crack.

Internal corners, fillets and what the covering needs from you

The kerb builder decides how easy or impossible the roofer’s job is. The internal angle where the kerb face meets the deck gets a timber or foam angle fillet on GRP and on felt. GRP laminated into a sharp 90 degree internal corner cracks at that line, reliably, usually in the first hard frost. Felt turned into a sharp angle stacks its layers in the corner. EPDM and single-ply do not need a fillet in the same way, but they do need a clean, well-supported angle to bond into.

Then leave room to work. A kerb built 300mm from a parapet leaves nobody able to dress a covering up the kerb face and the parapet at the same time. We hold to 500mm clear all round wherever the roof allows it. How each membrane is then dressed, terminated and warranted at that kerb is covered in full on our rooflight flashing and membrane page.

Bedding and fixing the rooflight frame

The frame sits on top of the dressed covering, not beside it. That is the whole principle. Water arriving at the frame has to climb the kerb face, cross the wrapped head, and pass under a compressed bedding line and a mechanical fixing before it reaches anything it can damage.

The bedding compound is a low-modulus neutral-cure silicone or a butyl bedding tape, applied as a continuous bead or strip set back from both edges of the kerb head so it squeezes into a band rather than out over the covering. It is a bedding compound. It levels the frame onto the covering and excludes air from the joint. It is not the waterproofing layer, and any installation where it is doing that job is a failure waiting for a date.

Mechanical fixings then go through the frame flange into the kerb head, at the manufacturer’s centres, which are typically 300 to 400mm with a fixing within 150mm of every corner. Two rules we hold to regardless of what the instructions say:

• Fix through the covering, and seal the penetration. Each fixing passes through the wrapped membrane at the kerb head, which is above the water line and under the frame, and each one gets a sealed washer or a dab of compatible sealant in the pilot hole. A fixing driven dry through EPDM leaves an open path down the shank into the timber.
• Do not overdrive. An overdriven fixing dishes the frame flange and lifts the flange either side of it, which breaks the bedding line in exactly the place you were trying to compress it. We set a torque and stick to it.

On coastal jobs at Felixstowe, Harwich, Frinton and along the open-sea belt those fixings are 316L stainless throughout, with isolation washers where the fixing passes through aluminium, because aluminium in direct contact with steel in a chloride environment corrodes galvanically. Estuarine locations such as Woodbridge, Wivenhoe and Shotley carry moderate salt loading rather than open-sea exposure, so upgraded fixings are sensible there without the full marine package.

Rebuilding an existing kerb

On a dome replacement the kerb underneath is usually 30 or 40 years old, bare uninsulated timber, and it has been damp for a long time. We survey on the assumption that it may need rebuilding, price the rebuild as a provisional item at £350 to £900 depending on size and how much timber has gone, and photograph what we find once the old unit is off so you can see the reason before we go further.

What decides it, in order:

1. Top edge condition. Probe the kerb head. Soft timber at the head means the old bedding line has been letting water in for years, and that timber will not hold a fixing.
2. Corner condition. Corners rot first because that is where the old covering was folded and mastic-filled.
3. Height. A 1980s kerb is frequently 75 to 100mm, and the roof has usually been recovered at least once since, taking it lower still.
4. Insulation. Almost never present on a kerb of that age. If the room below has ever had condensation on the reveals, this is why.
5. Squareness. An old kerb built for a dome was built to a loose tolerance, because a dome frame does not care. A flush glass unit does.

In practice, a sound, tall enough, square enough old kerb is worth keeping and we keep it. On a like-for-like dome swap into a genuinely sound kerb the job runs at £1,150 to £1,650 for units up to 900 x 900mm. Anything else and we rebuild it insulated to 150mm, or 200mm on the coast, because the alternative is putting a twenty-five-year unit on a five-year base. The full picture on pricing is on our flat roof rooflight installation cost page.

Judging a kerb someone else built

From inside the room, or from a window, or from a photograph. Not from the roof. Nobody should be walking a flat roof to check a kerb, particularly a wet or frosty one, and least of all a roof with an ageing rooflight in it.

The tells that a kerb is the problem rather than the unit: condensation forming evenly on all four internal reveals in cold weather points at an uninsulated kerb, not a leak. Staining at one corner only points at a failed covering termination at the kerb head. A unit that visibly sits low in the roof, with the covering close to the underside of the frame, has a kerb that has been overtaken by later roof build-up. Any of those and the answer is construction, not sealant. Our flat rooflight kerb leak page walks through the diagnosis, and cold bridging at flat rooflights covers the condensation side in detail.

If you want the short numbered build sequence rather than this level of specification, for example because you are auditing a kerb that already exists or checking a builder’s work as it goes, that is on how to build a rooflight timber kerb. For why a kerb detail leaks after a storm specifically, see flat rooflight leaking after a storm, and for the regulations that apply to the opening below it, flat roof rooflight building regulations.

Get a fixed price

Every kerb we build is treated timber, corner-braced, insulated continuous with the roof, ply-faced and squared to the tolerances in the table above, and it is included in the quoted price rather than itemised as an upgrade. Send us the opening size, whether it is new or a replacement, what the existing covering is if you know it, and roughly where the property sits. Request a fixed quote and we will survey it, check the fall on all four faces, and give you one written price with a 10-year workmanship guarantee behind it.