Roof Window Calculator

This calculator estimates roof window costs (supply, flashing and installation) and gives an indicative average daylight factor for the room.

The cost formula

Total = (Unit Price × Opening Multiplier × Glazing Multiplier × Qty) + Flashing + Installation

• Electric operation adds ~60% to unit cost
• Triple glazing adds ~35% to unit cost
• Flashing kit ~£75 per window
• The default £250 fitting figure is a baseline for an easy, good-access swap — increase “Install per Window” (£300-600+) for new openings, scaffolding or roof trimming, which is why a full market supply-and-fit is typically £900-1,500

Daylight factor

Uses the recognised one-fifth rule floor-area shorthand for the average daylight factor: DF_av ≈ (1/5) × (glazed area ÷ floor area) × 100 — i.e. multiply the window-to-floor area ratio by 0.2. This rule of thumb (Littlefair, “Designing with Innovative Daylighting”, BRE 1996; CIB “Daylight Design Rules of Thumb”) estimates the average daylight factor from just window area and room floor area, which are the only daylighting inputs this tool collects; the 0.2 coefficient already embeds typical glazing transmittance and room surface reflectance.

It is a quick guide for a typical light-coloured, unobstructed room, not a compliance figure. The full BRE average daylight factor is DF = (A_g × τ × θ) ÷ (A × (1 − R²)), where A is the room’s total internal surface area (not floor area), τ the glazing transmittance (≈ 0.68 double / 0.60 triple), θ the visible-sky angle and R the mean surface reflectance — that needs full room dimensions, and external obstructions are assessed separately under the BRE BR 209 site-layout guidance, so a detailed assessment also accounts for obstructions and glazing-bar losses. The familiar adequacy targets (about 2% for kitchens, 1.5% for living rooms and 1% for bedrooms) came from BS 8206-2, which has since been withdrawn and superseded by BS EN 17037:2018 (the current standard assesses daylight via median/minimum daylight factor or climate-based target illuminance rather than the average daylight factor).