The short answer
Often, yes. If the kerb or upstand is sound, level, correctly sized and well insulated, a new rooflight can be fitted onto it, with the weathering renewed around it. If the kerb is damaged, out of square, too low or poorly insulated, it is rebuilt first. We inspect it at the free survey and say which case you have in the written quote.
Replacing a rooflight on the same kerb
When someone asks whether they can replace a rooflight on the same kerb, they are usually hoping to avoid structural work. That is sometimes possible, and it is the neatest outcome when it is. The kerb, sometimes loosely called an upstand, is the raised frame of timber or other material that the rooflight sits on above a flat roof. It carries the unit, lifts it above the water on the roof and gives something to waterproof against. If it is in good shape, it can serve a new unit. Whether it is in good shape is something only an inspection can tell. Our skylight and rooflight service across Coventry starts with exactly that check.
What makes a kerb reusable
We look at five things before saying yes.
- Soundness: no rot, no softness in the timber and no cracking in the surround.
- Level and square: the new frame needs a true surface to seal against.
- Size: the new unit's footprint must match the kerb, or be made to suit it.
- Height: the kerb must stand high enough above the finished roof surface for the waterproofing to be dressed up it properly.
- Insulation: the kerb should be insulated so that the opening does not become a cold bridge.
If every item passes, the existing kerb can take the new unit. If one fails, we correct it. If several fail, rebuilding the kerb is the sensible option, and it saves the new unit from being sealed to something that will not last.
The problems that rule it out
The most common reason a kerb cannot be kept is a dropped or damaged upstand where water has been getting in around the old unit. Timber that has been wet for a long time softens and will not hold fixings. A kerb that was built too low leaves too little height for the roof membrane to be turned up it, and a new rooflight sealed onto a low kerb is only as watertight as the thinnest part of the junction. A kerb that has moved, so that it is no longer square to the roof, makes a new frame sit unevenly and strains the seals. In each of these cases we would rather rebuild than fit onto a weakness.
Timber, GRP and other kerbs
Kerbs come in different forms. Many are timber boxes built on site and covered by the roof membrane. Others are factory-made upstands, formed in insulated composite or GRP, that come with the rooflight. A site-built timber kerb deserves the closer look, because it is the type that rots when the old seal has failed. A factory upstand is usually kept only if the new unit is made for it, so we match the new unit to what is there or replace the pair together.
When only the glass has failed
If the frame and kerb are both sound and the sealed glazed unit alone has failed, the simplest route is new glass in the existing frame. That is not a controlled fitting, so it needs no notification. We tell you at the survey whether that suits, or whether the frame itself is too tired to keep. Whichever it is, you receive a fixed written quote that names the kerb decision, so nothing about the base of the rooflight is left to be discovered once the old unit is lifted off.
Size and sightline
Replacing on the same kerb means keeping the same opening size, or something very close to it. A new unit built to the old outer dimensions is the straightforward route. If you want to enlarge the rooflight, the roof structure changes, and that moves from a replacement to a new opening with trimming. Some modern frames are slimmer than the units they replace, which gives a slightly larger glazed area within the same outer size. We work out the figures for the exact kerb at the survey, and the unit is ordered to match, so there is no guesswork when it arrives.
Weathering and insulation
Even when the kerb stays, the waterproofing around it is renewed. The roof membrane is dressed up the kerb and under the new frame, with the junction sealed according to the manufacturer's details. The insulation around the kerb is checked and topped up where it has slumped, since the kerb is a cold bridge if left bare. This is also why replacing the whole rooflight is notifiable building work: the new unit must satisfy Part L, which is 2.2 W/m²K for flat-roof rooflights, and the weathering and insulation are part of what the inspector wants to see. The full position is in our answer on Building Regulations for a rooflight.
Finishing inside
With the old unit out, the lining inside the opening is reviewed too. Roof depth permitting, we splay the reveal, so light fans into the room, then plaster and make good. The result is a rooflight that looks as if it was always part of the ceiling.
