What does Part L say about rooflights?

Part L is the section of the Building Regulations that deals with the conservation of fuel and power, which in plain terms means stopping heat leaking out of the building. For rooflights it does that by capping the U-value, the rate at which heat passes through the whole unit on a cold night. The lower the figure, the less you lose. A single-glazed patent light from decades ago might sit above 5 W/m²K. A modern double-glazed rooflight built to the current standard has to come in at or below the limit.

For work in an existing dwelling the limiting value is 2.2 W/m²K, and that applies whether you are cutting in a brand new rooflight or swapping a tired one for a fresh unit. A replacement is held to the same thermal standard as a first-time installation, so pulling out an old lantern and dropping in a like-for-like antique specification is not allowed. New build is tighter still, because there the rooflight is one line in a whole-dwelling energy calculation rather than a stand-alone element, and the target for the house as a whole pulls the glazing down with it.

Two practical things sit inside that single number. The glass itself carries a centre-pane U-value, and the frame and spacer bar around it are almost always worse than the glass. The whole-unit figure that Part L cares about averages the two together, which is why a good pane in a poor frame can still fail. When you are comparing units, the whole-unit U-value is the one to read.

Putting a new or enlarged rooflight into a roof is notifiable under the Building Regulations, and that catches a lot of people out. It is not the same as planning permission, which is a separate question about whether you are allowed to alter the roof at all. Building Regulations are about how the work is built, and notification is the step that gets it inspected and recorded against the property. Skip it and the job has no certificate, which surfaces later as a query on a solicitor’s list when the house is sold.

There are two routes to compliance. A registered installer can self-certify certain glazing work under a competent-person scheme, or the work is notified to the local authority Building Control body, which inspects and issues the certificate. On our installations we make the Building Control notification to Southend-on-Sea City Council, or to whichever local authority covers the property, so the paperwork is handled as part of the job rather than left as a loose end for you to chase.

Part L rarely acts alone here. Part K governs safety glazing and guarding, so a rooflight within reach of a floor or in a position where someone could fall against it has to use the right toughened or laminated glass. A compliant rooflight satisfies both at once, but they are separate parts of the Regulations and both get looked at.

A rooflight U-value can be quoted two ways, and the difference is large enough to change whether a unit passes. Glass loses heat faster when it lies flat and faces the cold night sky than when it stands upright, so the same product tests worse horizontally than it does vertically. To keep the comparison fair, Part L assesses rooflight U-values in the vertical plane, the same convention used for an ordinary window.

That means a unit sold with a flattering low figure measured in its actual sloping or horizontal position is not being measured the way the Regulations require. When you ask for the U-value, ask which plane it was assessed in. A vertical-plane figure at or below 2.2 is what you are checking against. A horizontal-plane figure has to be read differently, and holding one supplier’s horizontal number against another’s vertical number will point you at the wrong unit entirely. It is the rooflight equivalent of comparing centre-pane against whole-unit: two honest numbers that do not describe the same thing.

Here is the gap that matters most on a south-facing roof. Part L puts a ceiling on the U-value, which is a winter measure about keeping heat in. It sets no equivalent ceiling on the G-value, the solar factor, which governs how much of the sun’s heat gets through the glass in summer. A rooflight can pass Part L comfortably and still turn the room beneath it into a greenhouse by mid-afternoon in July.

That is a live problem in this city. Southend-on-Sea faces south across the Thames Estuary with an open southern horizon and reflected light off the water, so a south-facing rear extension or loft collects sun from mid-morning until the sun swings west. A great many of the 1930s semis across Southend, Westcliff and Thorpe Bay have their gardens, and their rear extensions, facing south. Meet Part L on the U-value alone and the room can still be unusable in the heat of the day. The fix is to specify the G-value deliberately, asking for solar-control glazing around 0.3 or below where the roof faces the sun, and that orientation-first thinking is the whole basis of our energy-efficient skylight installation work. Clear double glazing sits near 0.6; solar-control double drops to around 0.3, and on a hot south-facing room that difference is the difference between bright and unbearable.

If you are planning a rooflight, treat Part L as the floor rather than the finish. Ask any installer for the whole-unit U-value, confirm it is a vertical-plane figure at or below 2.2, and check who is making the Building Control notification so the work ends up certified against the property. Then, separately, ask for the G-value, because that is the number Part L leaves silent and the one that decides how the room feels in summer.

If you would rather hand the whole question over, tell us the orientation and the rough opening size and we will come back with a glazing specification that clears Part L and is right for the way the roof faces. Ring 01702 898232 or request a quote, and if you want the fuller cost picture our skylight installation costs page sets out where the numbers land.