How do I size a roof window for Part L daylight?

Part L requires that new dwellings provide reasonable daylight. The BRE recommends minimum average daylight factors of 1% for bedrooms, 1.5% for living rooms, and 2% for kitchens. Roof windows are approximately 2-3 times more effective than vertical windows at the same area. A common rule of thumb is that roof window glazing area should be 15-20% of floor area for good natural light.

Which flashing kit do I need?

Flashing kits are specific to your roof covering: slate flashings have a recessed profile to sit flush, tile flashings have a high profile to overlap interlocking tiles, flat roof flashings work on pitches from 0-15 degrees, and metal sheet flashings suit profiled roofing. Using the wrong kit risks leaks. Combination flashings are available for installing multiple roof windows side by side.

What depth lining kit do I need?

Lining kit depth must span from the roof window frame through the insulation and plasterboard to the room interior. Measure your rafter depth plus insulation thickness. Common depths are 200-250mm for 200mm insulation plus plasterboard. Some manufacturers offer telescopic lining kits that adjust from 180-350mm. The lining should be splayed (wider at the bottom) to maximise light distribution.

Do I need planning permission for roof windows?

Roof windows are generally permitted development provided they do not protrude more than 150mm above the roof plane and are not on a principal elevation facing a highway. In conservation areas and on listed buildings, additional restrictions apply. If the property has had its permitted development rights removed, planning permission will be required.

Can I install a roof window between rafters without trimming?

If the roof window outer frame width is less than the rafter spacing (typically 600mm centres, giving about 560mm clear), it can fit between rafters with no trimming. Larger windows require cutting one or more rafters and installing trimmer and trimming joists to maintain structural integrity. Any structural alterations should be checked by a structural engineer or comply with TRADA guidance.

TradeCalculator.co.uk

Roof Light Sizing Calculator

Select roof window size by room area, daylight requirements and roof covering type

Room floor area (m2)

Room type

Roof pitch (degrees)

Roof covering

Rafter spacing (mm centres)

Number of roof lights

How We Calculate This

This calculator uses a simplified BRE daylight factor method to determine required glazing area. It divides the room area by the sky factor (adjusted for roof pitch), maintenance factor, and frame factor to find the minimum glass area needed. It then matches this to standard UK roof window sizes, ensuring compatibility with rafter spacing. The flashing kit is selected based on roof covering type, and lining kit depth is calculated from insulation depth plus plasterboard and air gap.