Rooflight Kerbs and Upstands Explained

Kerbs come in a few forms, and the right one depends on the roof and the rooflight.

Think of the kerb as the foundation of the rooflight. A built-up timber kerb is the flexible choice. It can be made to any plan size and any height, which helps on a roof with an awkward fall or a rooflight of unusual proportions. The timber is fixed to the structure, wrapped in insulation and then covered by the roof membrane.

A prefabricated kerb arrives ready to fit, with insulation built in. It speeds the work and cuts down on joints, but it works best when the opening is a standard size. In a newer estate house with a flat-roofed extension, where the opening is often set out to a standard, this is a tidy route.

The maker's own base is worth asking about when you want a particular rooflight, because the frame is designed to fit it exactly. And a sound existing kerb can be kept, which is the subject of a later section.

A kerb has to rise above the finished roof surface by enough to keep water away from the frame and the sash. The exact height depends on the covering used, the thickness of insulation on the roof and any build-up around the kerb. We set it at the survey and confirm it on the quote.

The fall of the roof matters too. A flat roof is never perfectly flat, because it has a gentle slope to carry water to a gutter or outlet. If the kerb sits on the slow side of a pool, water can collect against it. When we plan a new kerb, we look at where water will run and place or build the kerb so it does not become a dam. Our page on flat rooflight fall goes into this.

One practical point: the finished height affects what you see from inside. A tall kerb creates a deep light shaft that narrows the view of the sky and can make the room feel boxed in. A low one gives a flatter ceiling opening. The compromise is chosen with the room in mind, and the splay of the internal lining can soften the effect. A sloping lining, angled outward from the glass to the ceiling, spreads the light over a wider area of ceiling and makes the shaft feel shallower than it is.

A kerb is a hollow box in a warm roof, and the sides can be cold if they are not insulated. A cold side of a kerb is one place where room moisture can gather, and the inner edge of the glass can show droplets when the kerb is the weak link.

For that reason, the kerb is insulated on the inside, outside or both, depending on the type. The insulation sits against the box and is covered by the membrane on the outside and the lining on the inside. The aim is to keep the surfaces around the glass close to room temperature, so the glass is not the only cold surface in the shaft.

For that reason a rooflight upgrade that swaps the glass but ignores the kerb can disappoint. We check both at the survey. If the rooflight is in a kitchen or a bathroom, the answer may be a better kerb, a vent setting on the sash, or both. The piece on flat rooflight condensation covers moisture on glass and what feeds it.

Insulation also has a regulatory side, and the kerb counts towards it. The same kerb that keeps rain out also has to keep warmth in. A replacement rooflight in an existing dwelling has to reach 2.2 W/m²K (Part L 2021, Table 4.2), and the kerb forms part of that heat-loss path.

The flat roof covering turns up the outside of the kerb and finishes under the rooflight's frame or a cover flashing. This is where a rooflight is either watertight or not, so the details below matter.

On a felt roof the covering is dressed up the kerb in layers, with the corners worked carefully. On a single-ply membrane, such as EPDM, the sheet is bonded to the upstand and the corners are formed with purpose-made pieces. On a GRP roof the laminate is taken up the kerb as part of the same finish. A good installer chooses the method to match the roof, which is why our page on a flat rooflight on a GRP roof treats that case separately.

Corners are the weak points of any roof covering, and a kerb in a Bedminster terrace extension is no different from one on a Keynsham semi, so a kerb with neat, square corners makes life easier. A kerb that is out of square or has rough joints forces extra folds and joins in the membrane. We build and check kerbs to be square, level and sound before the covering goes on.

One team does the whole job, and each evening the opening is closed against the weather. A half-built kerb never stays uncovered overnight, and floors and furniture below are protected during the work.

Replacing a flat rooflight does not always mean building a new kerb. A sound one can stay, and that saves time and keeps disturbance to the roof to a minimum.

At the survey we judge the old kerb on a few points. We look at whether the timber is firm and dry, whether the insulation is complete, whether the covering is still bonded at the upstand, and whether the kerb is the right size for the new unit. A rooflight from a different maker may have different outer dimensions, and the kerb needs to match the new frame.

1. Is the timber sound, with no soft spots or rot?
2. Is the kerb square and level?
3. Is it tall enough to clear the roof surface?
4. Is it insulated all round?
5. Does it match the plan size of the new unit?

If all five answers are yes, we can fit the new unit onto the existing kerb. If one is no, we rebuild that part. For the wider question of reusing what is already up there, read swapping a skylight and keeping the roof.

Swapping only a failed sealed glass unit in an existing frame is a different job and covered in our misted and failed rooflights section.

The size of the kerb sets the size of the rooflight, so the two decisions go together. A bigger kerb means more daylight, and also more cutting of the roof structure beneath.

The opening in the roof is framed with timber trimmers that carry the cut joists, and the kerb sits on top of them. A small opening fits between two joists. A larger one needs joists cut and trimmed, and the timbers around the gap must be supported, which is building work with its own rules. We show you the options on the quote.

For help on the sizing question itself, see what size flat rooflight. And if you are still weighing the sash, our guide to fixed or opening flat rooflights explains how a hinged unit changes what the kerb has to carry.

On the paperwork side, swapping the entire rooflight for a new one counts as notifiable building work, so we notify Building Control. Cutting a fresh opening is building work as well, with the structure trimmed to suit. Rooflights on a house in England usually fall within Class C permitted development, subject to the limits on height and projection, and our planning permission answer sets them out.

A short list of questions will tell you whether the kerb is being given proper attention.

• Will the kerb be new, or will the old one stay, and why?
• What is it made of, and how will it be insulated?
• How high will it stand above the finished roof?
• How will the covering be turned up it?
• Does the quote say what happens if the old kerb is found unsound once opened?

The kerb is one part of a wider flat roof job, and the flat roof rooflights hub lists the other decisions that sit round it.

We answer all of these on the written, itemised quote, after a free survey that commits you to nothing. Our costs page explains broad market ranges, and why a rebuilt kerb adds to the price, and the guarantee covers the workmanship for 10 years.