How high should a rooflight kerb be?
One hundred and fifty millimetres, measured from the finished roof surface to the top of the kerb. That is the figure quoted across the industry, written into most manufacturers’ fitting instructions, and used as the design default on every flat roof unless there is a specific reason to depart from it.
It is not a round number chosen for convenience. It is derived from two measurable things: the depth of water that can stand on a flat roof before it drains away, and the height that wind driven rain can be pushed up a vertical face. The construction of the kerb itself is described on what a rooflight upstand is.
Measured from the finished surface, not the deck
The commonest sizing mistake is measuring from the wrong datum. A hundred and fifty millimetres of timber sitting on a bare structural deck is not a hundred and fifty millimetre kerb once eighty millimetres of insulation and a covering have gone on around it. It is a seventy millimetre kerb, and it will behave like one.
The measurement runs from the top of everything that will be there when the job is finished, up to the point where the covering terminates and the frame lands. On a warm deck with thick insulation, the timber box being built may be two hundred and fifty millimetres tall to deliver a hundred and fifty millimetre upstand.
The two mechanisms the figure answers
The first is ponding. A flat roof laid to a nominal fall still holds shallow water in local depressions after heavy rain, and the depth of that water is a function of deflection and construction tolerance rather than design intent. Fifty to eighty millimetres is realistic on an older deck.
The second is capillary and wind driven rise. Rain arriving at an angle in a gale does not simply run down a vertical face. Pressure differences push a film of water up it, and the distance it climbs increases with wind speed and with the exposure of the site. The remaining margin between those two figures and the top of the kerb is the safety factor, and a hundred and fifty is where it lands with room to spare.
Where less than 150 can be defended
There are honest cases. A roof with a well built, verified fall of at least 1:80 in reality rather than on the drawing, draining cleanly to outlets that are accessible and kept clear, in a sheltered position. Some manufacturers will warrant units on kerbs down to around a hundred millimetres under exactly those conditions, and they state it in writing.
The requirement is that all of the conditions hold, not most of them. A sheltered roof with poor falls does not qualify, and neither does a well drained roof in an exposed position. The judgement is made at survey with the manufacturer’s own document in hand, not from memory.
Where more than 150 is the right call
Several situations argue for going higher, and the extra cost during construction is trivial.
- A roof with a history of standing water, where the fall cannot practically be corrected.
- An exposed position, which across this county means the Blackwater and Crouch estuary villages, the Dengie, and the reclaimed land on Canvey, where there is nothing between the roof and the North Sea.
- A parapet roof with a single internal outlet, where a blockage turns the roof into a shallow tank.
- A large single unit, where the kerb itself obstructs run-off across a wide front and water backs up on the high side.
Ballast, paving and a datum that moves
Inverted roofs put the insulation above the waterproof covering and hold it down with gravel ballast or paving slabs on pedestals. Green roofs add a growing medium and drainage layer. In every one of these the surface water can sit on is well above the membrane.
The kerb height is therefore measured from the top of the ballast, the paving or the substrate, which can add a hundred millimetres or more to the timber being built. Kerbs on planted roofs carry their own detailing requirements and are covered on rooflights on green roofs.
Height on the high side and the low side
On a roof laid to a fall, a kerb of uniform height is taller on the low side than on the high side relative to the water. The critical measurement is always the low one: the side where water arrives and accumulates against the face.
Where the unit itself is set to a fall, the kerb is tapered, and it is easy to build the tapered end down to a height that satisfies the eye and not the weather. The rule is to set the shortest point of the upstand at the design height and take everything else up from there, rather than averaging across the frame.
Building the height without a cold line
A tall timber kerb is a tall thermal bridge. Two hundred and fifty millimetres of softwood running from the warm side of the build-up to the cold air outside will sit noticeably below room temperature on its inner face all winter, and that is where surface condensation and eventually mould appear on the reveal.
The detail that prevents it is insulating the outer face of the kerb over its full height, butting the roof insulation tight to it, and carrying the vapour control layer up the inner face and sealing it to the frame. Preformed insulated kerbs solve this in the product. Site-built timber kerbs solve it with an applied layer, and it is not optional at a hundred and fifty millimetres, let alone at two hundred and fifty.
What the height costs you inside the room
The trade-off is real and worth stating. Every millimetre of upstand is a millimetre of reveal depth, and a deep reveal narrows the cone of light reaching the floor and can make the opening read as a shaft rather than as sky.
The answer is not a lower kerb. It is a splayed reveal, opening outwards towards the room, which spreads the light across the ceiling and roughly doubles the apparent size of the opening. It costs a little plastering and nothing in glass. Cutting kerb height for the sake of the internal look solves a visual problem by creating a water one, and only one of those is expensive to put right later. Sunspire builds the kerb as part of the installation and the workmanship carries a ten-year guarantee.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements