Do extension rooflights need to open?

They do if your extension has no other openable windows, because Part F purge ventilation requires openable area equal to a twentieth of the floor area. Fixed rooflights satisfy the daylight need but not ventilation on their own, so a deep-plan room relying on rooflights alone will fail building control.

What is the maximum U-value for a roof window in an extension?

Pitched roof windows in extensions are assessed as windows under Part L 2021, with a maximum whole-unit U-value set in the approved document. Most modern double-glazed Velux and Fakro units meet it comfortably, and triple-glazed options exceed it.

What is the difference between a standard roof window and a bespoke rooflight?

Standard Velux and Fakro pivot windows are stock products sized to fit between rafters and installed by your roofer in a day. Bespoke aluminium rooflights are made-to-order architectural glazing spanning structural openings of several metres, installed by a specialist, at a far higher price and longer lead time.

Why do roof windows leak around the edges?

The failure is almost never the window itself. It comes from the surrounding details: the wrong grade of breathable membrane, lead flashing formed by a roofer rather than a lead specialist, or a flashing kit made for the wrong tile profile.

Does the roofer fit the lead flashing around a roof window?

Not always. Tiling and lead forming are different trades with different skills. Some roofers do both, many do not, so ask for evidence of recent lead work and bring in a separate lead specialist if there is any doubt.

How long do bespoke aluminium rooflights take to arrive?

Made-to-order aluminium rooflights run a long lead time from a specialist supplier, far beyond a stock Velux. You must lock the rooflight specification before the roof structure is designed, because the trimmer opening is built to the confirmed unit dimensions.

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Phase 4 · Structure · Task 05 of 09

Roof Windows and Rooflights

Specify, order, and install roof windows correctly for your extension. Covers Part L U-values (max 1.4 W/m²K), Part F ventilation, membrane grade, lead flashing, and the pre-scaffold inspection that prevents expensive re-work.

11 min readUpdated 2026Premium

Process infographic showing the five-stage installation sequence for a pitched roof window: structural trimmed opening, window frame installation, breathable membrane with underfelt collar, tiling with flashing kit, and lead flashing with patination oil

Prerequisites

Complete this first

Roof Structure

The roof goes on. The tiles look right. Scaffold comes down. Then it rains.

Leaks at the window junction are one of the most common post-construction defects on extensions with pitched roof windows. They're also one of the most avoidable. The failure almost never comes from a faulty window. It comes from what surrounds the window: the wrong grade of breathable membrane, a roofer who doesn't do lead work, or a flashing kit specified for the wrong tile profile. By the time you discover the problem from inside the extension, scaffold has been gone for months.

What follows covers how to specify, order, and install roof windows correctly. And the one inspection you must do before scaffold strikes to catch problems while they're still accessible.

Do this first

The roof structure must be complete with correctly sized trimmer rafters forming the window opening before your windows can be delivered and installed. Confirm the structural opening dimensions from the manufacturer's actual unit spec (not your design drawings, which may have been drawn before the window was finalised) and make sure your structural engineer has signed off the trimmer arrangement before the carpenter starts cutting.

What this guide covers

01Why Rooflights, and When You Need Them
02Fixed vs Opening: Which Do You Need?
03Standard Roof Windows vs Bespoke Rooflights: Two Different Products
04BCO Glazing Requirements
05The Membrane: Where Most Roof Window Failures Start
06Lead Flashing Is a Separate Trade
07Order Before Structure Starts
08Installation Sequence
09Inspect Before the Scaffold Comes Down

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