Roof lanterns are a great way to flood a room with daylight and lift the ceiling, and are a popular choice in modern homes. How it fits to the roof, ensuring it is watertight, comes down to a part of the construction which nobody sees: the builder's kerb the roof lantern sits on.
It is important to get three key elements right when installing a roof lantern: ensuring you order the lantern to the finished external kerb dimension, build the kerb completely square, and set the upstand 150mm above the finished roof.
This guide walks a competent builder through forming the opening, sizing and squaring the kerb, getting the height and waterproofing right, and seating the lantern.
At Rooflights and Skylights, we have 43 years' experience in daylight solutions. If you'd like us to sense-check your dimensions before you order, give the team a call on 01276 450626
What is a builders' kerb?
A builder's kerb, also referred to as the upstand, is the raised frame, built up from the roof structure, to which the roof lantern is fixed.
The builder's kerb has three jobs: it lifts the glazing clear of any standing water, it gives the lantern a flat, square and level surface to fix onto, and it ensures that the roof insulation and waterproofing can be carried up to meet the lantern.
There are two ways to create this kerb -
1) A builder's kerb is constructed on site, most often using treated timber, by the builder or roofer when installing the lantern;
2) A PVC kerb is pre-made as part of the unit. This is more typical where a flat roof light or dome is being installed.
Detailed below are step-by-step instructions explaining how to create the builder's kerb and install the roof lantern.
Stage 1: Form the opening and set out the kerb
The structural opening in the roof is not the kerb
Three dimensions get confused on site, and confusing them is the number-one cause of a lantern that "won't fit". These are explained below -
- The structural opening is the hole cut through the roof deck/joists.
- The clear (daylight) aperture is the internal opening you see light through from the room below.
- The finished external kerb size is the outside face of the completed, waterproofed kerb.
The lantern is manufactured to sit over the outside of the kerb, so the finished external kerb size is the dimension you order to, not the clear opening beneath it.
The required kerb varies depending on which lantern system you choose, so it's important to work out the external size you need before the kerb is built.
R&S Insight "In four decades of supplying roof lanterns, the single most common sizing mistake we see is ordering to the internal opening instead of the finished external kerb. The lantern sits over the kerb, so it's the outside of the kerb that's build to - every time. Measure that, and measure it twice."
Kerb width is a construction decision
The kerb needs enough width across its top face to give the lantern frame a solid, level bearing and room for the fixings, and enough depth in its walls to carry the roof insulation up the upstand. In practice that's typically around 75–150mm, depending on the lantern system and how much insulation the roof build-up needs. A wider kerb takes more insulation and more fixing surface but reduces the clear opening (and the daylight) below, so it's a balance.
(For more information on picking the right roof lantern, see the .)
Square, level, and true - check the diagonals
A kerb can be exactly the right length and width and still not be square. A square lantern will never sit flush on a kerb if it is not true, so it's important to undertake a couple of checks before you go any further.
- Diagonals: measure corner-to-corner both ways. If the two diagonals aren't equal, the kerb isn't square. Adjust this before fixing the lantern.
- Level and flat: the top face must be level all the way round and flat enough to give the frame continuous bearing. If you have any bumps or unevenness on the kerb, the lantern won't sit evenly and may even rock. This means that you won't achieve a good seal.
R&S Insight "A kerb that's right on length and width can still be out of square, and you can't see it by eye. Check both diagonals before you order, it takes just two minutes with a tape saves a fortnight waiting on a remake."
Stage 2: Get the upstand height right
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Building Regulations (Approved Document C, resisting moisture) point to a minimum upstand of 150mm, but the detail that catches people out is where you measure it from.
It's 150mm above the finished roof, not the structural deck. The upstand has to clear the entire completed roof build-up, which includes the insulation, boards and waterproofing. This ensures that water will run across the finished roof without reaching the top of the kerb or the lantern seal. Set the kerb height from the finished level; if you set it from the deck, the build-up eats into your 150mm, and you can lose that protection before the lantern is even on.
Cold roof vs warm roof. Where the insulation sits changes how the kerb is built:
- In a warm roof, the insulation runs continuously over the deck and up around the kerb, keeping the structure and the kerb on the warm side — the preferred detail for minimising cold-bridging and condensation at the upstand.
- In a cold roof, insulation sits between the joists below the deck; the kerb is built off the deck and insulated within its walls, with care taken at the junction to avoid a cold bridge.
Either way, insulate the kerb itself so the upstand doesn't become a cold spot that draws condensation.
Stage 3: Waterproof the upstand
With the kerb built, square and to height, it then needs to be weatherproofed as part of the roof covering.
Two things to consider -
- Don't compromise the fixing surface. The waterproofing must be dressed neatly so the top of the kerb stays flat and level. Any lumps or doubled-up membrane under the frame will lift the lantern off its seat.
- The membrane changes the finished size. Once it's dressed up and over, the finished external kerb is a few millimetres bigger than the bare timber. Falls and drainage around the opening should also be set so water sheds away from the kerb, not toward it.
R&S Insight "Take your ordering dimensions from the waterproofed kerb, not the bare carcass. The felt or membrane has a thickness, and once it's wrapped over the top the finished external size grows by a few millimetres. This can be enough to matter on a precision-made lantern."
Stage 4: Seat and fix the lantern
When the opening, external kerb size, height, squareness and waterproofing have all been completed, the lantern drops cleanly onto the kerb. Bed and fix it per the manufacturer's instructions. Every lantern we supply comes with drawings and fixing guidance, so be sure to follow those carefully. Finish by sealing the frame-to-kerb junction, and the result is the clean, symmetrical lantern.
Common mistakes (and how to avoid them)
- Ordering to the internal opening instead of the finished external kerb → lantern too small/large. Order to the outside of the kerb.
- Out-of-square kerb → lantern won't sit flush. Check both diagonals before ordering.
- Upstand measured from the deck → too low once the build-up is on. Measure 150mm from the finished roof.
- Membrane thickness ignored → finished size out by a few mm. Measure the waterproofed kerb.
- Uneven/doubled membrane under the frame → lantern rocks or won't seal. Keep the top face flat.
- No falls/water shedding toward the opening → standing water at the kerb. Set falls away from the lantern.
