Roof Light Sizing Calculator

No. Approved Document L (Conservation of fuel and power) governs energy and U-values — it sets no minimum daylight factor and no minimum glazing area for natural light. Daylight is a design and planning matter, covered by BS EN 17037:2018 (which superseded the now-withdrawn BS 8206-2:2008). The familiar average-daylight-factor working-room minima — bedroom 1%, living room 1.5%, kitchen 2% — come from BS 8206-2 and are still widely used in UK planning; the 0.5% figure often quoted for circulation and landing spaces is a non-working-area value drawn from combined BS 8206-2 and BRE daylight practice (BR 209) rather than a stated BS 8206-2 minimum. Where Part L does bite on rooflights is the thermal side: if total glazing (windows + rooflights + glazed doors) exceeds 25% of floor area, you must meet a higher fabric performance, and large rooflight areas should also be checked against Approved Document O (overheating). As a sizing rule of thumb, VELUX guidance suggests pitched roof-window glazing of about 15-20% of floor area (10-15% for flat roofs), with roof windows giving roughly 2-3 times the daylight of an equal area of vertical window.

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.

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.

Roof windows, rooflights and skylights are usually permitted development under the GPDO 2015 Schedule 2 Part 1 Class C. Two Class C conditions must both be met: the window must not protrude more than 150mm beyond the plane of the existing roof slope, and it must not result in the highest part of the alteration being higher than the highest part of the existing roof. Unlike dormers and roof enlargements (which fall under Class B and cannot be on a principal elevation facing a highway), Class C places no restriction on street-facing or principal elevations — a rooflight can be fitted on a front-facing roof slope. Windows in a side elevation must be obscure-glazed and either non-opening or have any openable part at least 1.7m above the floor. In conservation areas and on listed buildings, on flats, and where permitted development rights have been removed (for example by an Article 4 direction or a planning condition), planning permission may still be required — always check with your local planning authority.

Only if the roof window outer frame width is less than the clear gap between rafters. At 600mm centres you have roughly 560mm clear, so only the narrowest VELUX-pattern sizes (the 550mm-wide CK frames) fit without cutting; a 660mm FK or anything wider does not and needs the rafter trimming. This calculator flags whether the recommended size fits your stated spacing. Any window wider than the clear gap requires cutting one or more rafters and forming a structural opening with a double trimmer across the head and foot and trimming (jack) rafters either side, typically in C16 or C24 timber. Cutting structural rafters changes the load path, so the opening should be sized by a structural engineer.