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Building Regulations Part L
- Saves energy by reducing heat loss.
- Requires the provision of daylight.
- Recognizes that in a well designed building the benefits from insulated rooflights can help save energy.
To satisfy Part L:
Rooflights with low U values of 2.2W/m2K or below, can be used on up to 20% of roof area.
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Part L: Rooflight Summary
Part L defines target U values for different elements of the building envelop.
It is possible in the current regulations to take advantage of the "trade-off" allowed.
The "trade-off" permits standard double skin rooflights to be used in smaller areas.
However in reducing rooflight area Part L requires that: "special care needs to be given to confirm that levels of daylight are adequate".
| Target U value for rooflights: |
2.2 W/m2K |
| U value: 2.2 or less: |
from 10-20% (Part L compliant) |
| U value: 3.0 or higher: |
from 10-14% (with trade-off) |
| Minimum rooflight area: |
10% |
| Requirement to prove adequate daylight: |
when rooflights are less than 20%* |
| Requirement to prove no solar overheating: |
when rooflights are over 12%* |
* An independent report from the Institute of Energy and Sustainable Development at De Montfort University shows the calculations for the required prove of overheating and daylight levels. In general solar overheating has proved not to be a problem up to 20% rooflights. 'Adequate daylight' is defined for a variety of internal functions.
Independent Report
The DMU report defines 'adequate daylight' in terms of light levels for different interior functions. This table shows what area of rooflights is needed to achieve the required level of daylight.
| Characteristics of activity / interior |
Illuminance required
(lux)* |
Recommended min. rooflight area
(%of floor area) |
Possibility for trade-off
(using double skin rooflight) |
Interiors used occasionally, with visual tasks confined to movement, limited perception of detail.
e.g. Corridors, bulk stores
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100 |
10%^ |
Yes |
Continuously occupied interiors, visual tasks not requiring perception of detail.
e.g. Loading bays, plant rooms
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200 |
10%^ |
Yes |
Moderately difficult visual tasks, colour judgement may be required.
e.g. Sports halls, packing, general offices, engine assembly
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300 to 500 |
13% - 15%^ |
Yes – up to 14% |
Difficult visual tasks, accurate colour judgement required.
e.g. Drawing offices, general inspection, electronic assembly, supermarkets
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750 to 1000 |
17% - 20%^ |
No |
^ rooflights with an average light transmission of 67% (some triple skin rooflights will have slightly lower light levels, in which case rooflight areas should be increased)
* in the horizontal plane
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