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| Minimum & Maximum
Rooflight Area |
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| The recent Building Regulations
also, for the first time, recommend a minimum rooflight
area for industrial and commercial buildings, recognising
the significant benefits of natural light and the effectiveness
of rooflights in allowing daylight into a building.The
Regulations recommend a minimum rooflight area of 10%
to achieve a 'daylit' classification and a nominal area
in heated buildings of 20%. If the rooflights achieve
a U value below 2.2W/m²K, a larger rooflight area
could be incorporated.The new standards are easily achieved
using multi-skin FAIRs in GRP or polycarbonate, with little or no increased cost or complexity. |
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| Building
Regulation Rooflight Summary |
| Target U
value for the roof |
0.25 W/m²K
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| Target U
value for rooflights |
2.2 W/m² K |
| Maximum permitted
rooflight area |
20% of roof area |
| Minimum recommended
rooflight area |
10% |
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| Recommended minimum rooflight
area for desired illuminance level |
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Characteristics
of activity/interior |
Level
of illuminance required (lux) |
Recommended
minimum rooflight area (% of floor area) |
| Interiors used occasionally, with visual tasks confined
to movement and limited perception of detail e.g. bulk
stores. |
100 |
10% |
| Continuously occupied interiors, with visual tasks not
requiring perception of detail e.g. loading bays, plant
rooms. |
200 |
10% |
| Moderately difficult visual tasks, colour judgement
may be required e.g. sports and assembly halls, packing,
general offices, engine assembly, retail shops. |
300 to 500
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13% to15% |
| Difficult visual tasks, accurate colour judgement required
e.g. drawing offices, inspection, electronic assembly. |
750 to 1000 |
17% to 20% |
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*Table based on research by Institute of Energy and
Sustainable Development, De Montfort University for illuminance
in the horizontal plane - rooflight area should be greater
where illumination is needed in the vertical plane eg.
where vertical racking is used.
**Figures for the level of illuminance required are taken from CIBSE GUIDE A (table 1.12). |
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| Solar Control |
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Whilst Part L2 requirements on thermal insulation are based
on rooflight areas of 20%, Part L2 Regulations also require
that measures are taken to demonstrate that solar overheating
will not occur. They deem that one method of achieving this
is to restrict rooflight area to 12%; an alternative is to demonstrate
by calculation that solar overheating will not occur. Recent
independent research by De Montfort University has demonstrated
that this 12% figure is based on high internal heat gains within
the building, and that in large span buildings where internal
gains are typically 5 to 10W/m, rooflight areas of 17 to 20%
can be fitted without causing risk of solar overheating. The
research by De Montfort University has concluded that, in general,
the minimum rooflight areas needed to provide adequate illuminance
are significantly lower than the maximum limits on rooflight
area which ensure solar overheating will not occur, giving the designer
freedom to choose an appropriate rooflight area to meet both
criteria.
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