How-To Guide · Updated June 2026 · UK Extensions

How to Plan a Roof Lantern Installation in 2026

A roof lantern is one of the best ways to flood a flat-roof extension with overhead daylight, but a good result starts with planning. You need a roof that can take it, the right size for the room, the correct frame and glazing, and a properly built, weatherproofed kerb. This 2026 guide walks through the installation step by step, from checking the roof to booking a vetted installer, so the lantern goes in cleanly and stays watertight.

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Step By Step · 2026

The 6 Steps to a Clean Installation

Follow these steps in order. The first four are planning and design, the last two are the build itself. Getting the kerb and weatherproofing right is what keeps a lantern leak-free for the long term.

StepWhat to do
1. Check the roofConfirm it is flat or low pitch and sound
2. Size itAround 15-20% of the floor area below
3. Specify glazingLow U-value, solar-control glass
4. Structure and planningTrim the opening, check PD rights
5. Build the kerbInsulated, flashed and weatherproofed
6. Fit and sealSet the lantern, seal and finish inside

On a new extension, steps 4 to 6 are built into the roofing stage, which is the cleanest and cheapest time to install a lantern.

Get It Right

Do This, Avoid That

✓ Do

  • Size the lantern to 15-20% of the room
  • Use solar-control glass on sunny aspects
  • Build the kerb to the maker dimensions
  • Get the opening trimmed by an engineer if large
  • Fit it during the roofing stage on a new build

✗ Avoid

  • An oversized lantern that overheats the room
  • Skimping on flashing and weatherproofing
  • Cutting joists with no structural advice
  • Plain glass on a south-facing roof
  • Fitting onto a steep pitch - use a roof window
The Critical Detail

The Kerb and Weatherproofing

Nearly every roof lantern problem traces back to the kerb, the raised timber upstand the lantern sits on. It must be built to the manufacturer dimensions, properly insulated to avoid cold bridging and condensation, and flashed and weatherproofed into the surrounding roof covering before the lantern is set down.

Get this right and the lantern is watertight for decades. Rush it and you risk leaks and cold spots that are expensive to fix later. This is the single biggest reason to use an experienced installer rather than a general handyman.

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Talk to Roof Lantern Installers

A vetted installer will confirm the roof is suitable, size and specify the lantern, build the kerb and fit it watertight. BestBuilders matches you with up to 3 UK specialists.

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FAQ · 2026

Roof Lantern Installation FAQs · 2026

Start by confirming your roof is flat or low pitch and structurally able to take a lantern, then size the opening, choose the frame and glazing, check Permitted Development, and book a vetted installer. Most lanterns are fitted in 1 to 3 days once the timber kerb is built and the opening is formed.
As a rule of thumb the lantern should be around 15 to 20 percent of the floor area of the room below for a bright, balanced result. On a typical kitchen-diner that means a unit of about 1.5 by 1.0 metre up to 2.5 by 1.5 metre. Your installer will confirm what the roof structure can carry.
Often yes, where the opening is large or you are cutting into an existing roof. A structural engineer specifies the trimming timbers or steel needed to support the opening. On a new extension the lantern kerb is usually designed in from the start.
Fitting the lantern itself takes around 1 day once the timber kerb is built and the opening is formed and weatherproofed. Allow 2 to 3 days in total for the kerb, flashing and internal finishing. On a new extension it is fitted as part of the roofing stage.
Specify double or triple glazing with a low whole-unit U-value of around 1.2 to 1.5 W/m2K and solar-control glass to limit summer overheating while keeping the daylight. A self-cleaning coating is worth it on a high or hard-to-reach lantern.
Usually no. Fitting a lantern to a flat-roof extension is generally Permitted Development, provided it projects no more than 150mm above the roof plane and is not on the principal elevation. Conservation areas and listed buildings can need consent, and Building Regulations always apply.
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