Approved Document L2B
Approved Document L2B covers all work (other than new build) for non-domestic buildings, including extensions, refurbishment, and replacement of rooflights.

U-Values
Rooflights should generally achieve a U-value of 2.2W/m²K which will require triple skin rooflights.

This applies wherever rooflights are replaced, or a roof is refurbished, and to extensions.

For extensions, the area weighted U-value of rooflights must not exceed 2.2W/m²K, whilst the U-value for an individual rooflight must not exceed 3.3W/m²K.

This means that if all rooflights across a roof are the same, they must have a U-value of 2.2W/m²K or better ? this will require all rooflights to be triple skin.

If a double skin rooflight (typical U-value 3.0W/m²K) was used, then the area weighted U-value of rooflights must still meet the limit of 2.2W/m²K, so other rooflights on the roof would have to be significantly better insulated to keep the average constant. This is therefore not applicable where all rooflights on a roof are the same.

In general, the requirement is easily met by specifying triple skin rooflights throughout.

Rooflight area
In general, rooflight area should not generally exceed 20% of roof area ? although if an existing building has over 20% rooflights, then any extension (or refurbishment or replacements) can have the same rooflight area as the existing.

Full guidance is given in NARM document ?Designing with Rooflights: Supporting the Guidance in AD L2A & L2B (2006)?

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