Know your new rooflight regs

Rob Hamblen
Rob Hamblen

By Rob Hamblen, commercial director, Whitesales.

You might think you’ve heard everything there is to say about Part L, but did you know that new rules apply to rooflights as well? If you’re contemplating adding rooflights to your offering in 2023, here’s a quick guide to what has changed.

The new Part L is the first step on the road towards the 2025 Future Homes standard. While Future Homes will initially only apply to new homes as part of the government’s drive towards net zero carbon, the 2022 Building Reg changes already apply to both new build and refurbs and to houses and commercial buildings.

There are still ongoing consultations with the industry on what the 2025 standard will actually look like and we wait to see what further implications there will be for rooflights but, for now, the updated Approved Doc L looks like this.

The new documents

There are no longer two documents split into four parts (Part L1A for new build dwellings and Part L1B for existing dwellings and Part L2A for new non dwellings and Part L2B for existing non-dwellings). Instead, things have been simplified into Part L (volume 1) for dwellings and Part L (volume 2) for buildings other than dwellings. Both volumes use the U-Value method to demonstrate compliance with the new thermal efficiency limits for buildings, measured in W/m2K and calculated in accordance with BS EN 10077-1.

Is it a rooflight or a roof window?

Different U-Value limits have been set for windows, doors and rooflights, so the first thing to establish to ensure compliance is whether what you are fabricating or installing is defined as a roof window or a rooflight. Essentially, a rooflight is an out-of-plane glazed unit that projects from the surface of the roof on a kerb or upstand, and a roof window is a glazed unit installed in plane with the surrounding roof and in the same orientation.

There is a sliding scale for the limiting U-Values that apply to in-plane roof windows, with adjustments of between 0.1 W/m2K and 0.4 W/m2K applied from the limits for windows as they move from an incline of more than 70° to an incline of more than 30° (these should be calculated as being vertical). Anything with an incline of less than 30° is treated as a rooflight and should be calculated as being horizontal.

What are the limiting U- values?

The inevitable solar gain that comes from being horizontal means the limiting U-Values for rooflights themselves are higher than they are for windows. In both volume 1 and volume 2 of the new Part L (new, existing and non dwellings), the maximum limiting U-Value is 2.2.

The rules on upstands and kerbs

The U-Value calculation should be done using the outer surface area of the rooflight or rooflight and kerb assembly rather than the area of the roof opening. An important point to make though is that the 2.2 Ud-value can only apply to a rooflight and kerb assembly if it is sourced from the same supplier. If the kerb or upstand is built on site, then a maximum U-Value of 0.35 W/m2K applies, which is very challenging to achieve without making the kerb so deep that it reduces the amount of light coming through the rooflight.

Whitesales range of em.glaze rooflights easily comply with the 2.2 limiting value and the good news is that we have also developed a new proprietary timber sloping kerb for the em.glaze range which makes compliance quick and easy.

All the details are at: