On a flat roof, the glass gets the attention and the kerb does the work. A flat rooflight kerb is the short box of timber, or a moulded upstand, that lifts the rooflight clear of the roof surface. Everything the roof sheds, from a summer cloudburst to a week of standing water in November, runs past it. This guide explains what the kerb is for, how it is built, how it ties into the roof covering and why a skylight upstand is chosen to suit the roof it sits on.
What a flat rooflight kerb actually is
A flat roof is not truly flat. It falls gently towards a gutter or an outlet, and water crosses it on the way. A rooflight set directly on that surface would sit in the path of the water. The kerb raises the glass above it.
The words kerb, upstand and curb get used for the same thing. Some makers sell the kerb as a finished part, in GRP or insulated plastic, bolted to the roof. Others expect a carpenter to build it on site from treated timber. Either way the job is the same: a rigid, level ring around the opening that the waterproof covering can climb, and that the rooflight frame can sit on.
Fixed rooflights, opening ones and walk-on glass all need one. The height, the material and the way the corners are finished change, but the principle does not.
Why the glass cannot sit flat on the roof
There are three reasons, and they work together.
Water has to pass. After heavy rain a flat roof holds a shallow film of water, and where the fall is slight it holds more. A frame lying on the surface would sit in that water, and every joint in it would be tested by standing water rather than by rain.
The covering needs somewhere to stop. Felt, rubber and GRP are all waterproof sheets. At the edge of an opening they have to turn upwards and finish at a height the water will never reach. The kerb gives them a face to turn up against.
The glass needs a level seat. A sealed glass unit in a rigid frame does not like twisting. Set on a roof that falls, the frame would rack and strain. The kerb is cut and packed so that its top edge is level, whatever the roof below is doing.
How a kerb is built
When a new opening is cut, the framing comes first. The joists crossing the hole are trimmed and carried by new timbers, as Part A of the Building Regulations expects for any structural change. The kerb is fixed on that framed edge so the weight of the rooflight travels into the structure, not just into the deck.
A timber kerb is made from treated softwood or exterior plywood, screwed down through the deck into the framing. Its inside faces are lined with insulation, because bare timber at the opening is a cold edge, and condensation collects on cold edges. Its outer faces are left clean and square, ready to take the covering.
A factory kerb arrives as a one-piece insulated unit. It goes down quicker and has no corner joints to make on site. On a small, standard rooflight it can be the neater choice. On a large or odd-shaped opening, a built kerb is easier to cut to size, and that is the route we put forward.
Garages and older kitchen extensions often have a roof deck that has seen some weather. Before a kerb of either kind is fixed, we press the deck round the opening for softness and replace any board that gives. A kerb is only as firm as what it is screwed to.
Whichever is chosen, we check the top edge with a spirit level in both directions before anything else is fixed to it.
How tall should a skylight upstand be
The height is not a matter of taste. It is set by the rooflight maker's instructions and by how thick the roof build-up is where the kerb stands. The covering has to climb the upstand, and the rooflight has to sit above the top of that climb with a clear gap. Roofs with thick insulation need a taller upstand than thin ones.
Fall matters too. On a roof that drops noticeably across the opening, the upstand has to clear the water on the uphill side, where it gathers against the kerb, not just the downhill side, where it runs off. We measure the roof at that edge during the survey and not just at the middle of the hole.
There is also an upper limit. A tall skylight upstand adds to the height of the finished rooflight above the roof plane, and on a house, as opposed to a flat, planning rules limit how far a rooflight may stand proud. Those limits are laid out in our planning and Building Regulations guide, and at survey we tell you which side of them a proposed height falls.
Kerb and covering: the joint that decides everything
The kerb itself is rarely where a flat rooflight fails. The failure is at the joint between the kerb and the roof covering, because that is where two materials are expected to behave as one. The three common coverings finish at an upstand in different ways.
| Covering | How it meets the kerb | Where it needs care |
|---|---|---|
| Felt | Layers are bonded up the outer face and folded over the top edge | Corners, where the sheets must be cut and lapped without gaps |
| EPDM rubber | A single sheet is bonded up the face, with moulded corners for the angles | The adhesive bond, which needs a dry, clean face |
| GRP | Resin and glass mat run up the upstand as one jointless skin | The angle at the foot, which gets a reinforced fillet |
The detail we look at first is the corner. Four corners mean four places where a sheet is folded, and a fold is where water finds its way in if it is going to. For that reason, EPDM kerbs use moulded corner pieces, and GRP builders round the corner into a fillet before laying the mat.
If you are choosing between coverings for a roof with a rooflight in it, our page on which flat roof covering suits a rooflight goes into the trade-offs. If a leak is what sent you here, do flat roof rooflights leak covers where water actually gets in.
Flashing the top edge and finishing the cap
Once the covering has climbed the upstand, its top edge needs protection. If left exposed, wind-driven rain can get behind the folded edge and track down the back of it. A capping, usually powder-coated aluminium or another metal trim, finishes the covering at the top of the upstand and sheds water outward.
The rooflight frame then sits on this cap with a continuous compressible seal, and is held down with fixings that do not pierce the waterproof layer where they can be avoided. Where a fixing has to go through, it is sealed. The point of the arrangement is that the covering, the cap and the frame each have a job, and none of them depends on a single bead of mastic.
For homeowners this is the practical test of a quote: does it name the cap, the seal and the way the frame is held down? If those lines are missing, ask why.
Keeping the kerb warm inside
A kerb is a small piece of structure, but it is also a thermal break in the roof. The roof insulation stops at the edge of the opening. If the kerb is bare, the cold travels in through the timber and reveals itself as a dark line of condensation or mould round the inside of the opening, on cold mornings first.
Insulating the inner faces of the kerb closes that gap. The reveal inside the room is then boarded and finished warm. Under Part L of the Building Regulations (2021), the rooflight itself has to meet 2.2 W/m²K for an existing home, and a well-insulated kerb stops the heat that the rooflight keeps in from leaking out round its base.
A deep kerb on a thick roof can leave a long internal reveal. We put forward angling it, splayed outward at the bottom, so light spreads further into the room. The splay is cut and boarded to suit the room, not left as a square-sided tunnel.
What this means for the quote and the paperwork
Our written quote lists the kerb as its own item: whether it is built or supplied, its height, the way it is insulated, the covering it will be sealed into and the cap that finishes it. That list is part of the life plan, the document that names each part of your rooflight, what can wear and how it would be changed later. For the kerb, the plan records the height and the sealing method, so a future roofer knows what is under the covering before they open it.
Cutting a new opening is notifiable work, and we see to the Building Control notification. What lands with you at the close is a completion certificate. Keep it with the deeds, because a solicitor acting on a sale will ask to see it.
The 10-year guarantee we give on our workmanship reaches the part of the job this page is about: the kerb we build and the sealed joint where the covering meets it. Glass, seals and any opening mechanism are listed separately in the life plan, with a checkpoint for each, so you know when to ask us to look at them.
If you are weighing whether to reuse an existing kerb, build a new one or lay a new roof around it, read new flat roof or just a new rooflight. If you are deciding whether the new unit should open, see fixed or opening flat rooflight. The overview of flat roof rooflights ties the pieces together.
Kerb and upstand queries, answered
Can a new rooflight go on the old kerb?
Sometimes. We look at the old kerb at survey: is it level, is the timber sound, is it the right size for the new unit, and is the covering around it in good order? A kerb that fails any of those is replaced as part of the job.
Is a taller kerb always better?
No. The kerb needs to be tall enough to clear the water and take the covering, and no taller. Extra height adds to how far the rooflight stands above the roof, which matters for planning and for appearance from the garden.
Why does my existing rooflight have condensation round the edge?
Often the kerb is uninsulated or its insulation has slumped, so the edge of the opening is cold. Wet air from the room meets that cold surface and condenses. A properly insulated kerb on a replacement rooflight deals with it.
Does a skylight upstand change how the room looks inside?
It sets the depth of the reveal. A deep kerb on a thick roof makes a deeper reveal. We can splay the sides to spread light into the room and board and finish the reveal so it looks like part of the ceiling.
Will the whole flat roof need re-covering when the kerb goes in?
Not if the existing covering is sound. The new kerb is sealed into it locally. If the survey shows a covering too worn to take a good joint, the quote says so and sets out the choices.