Roof windows and rooflights: which U-value to use, and the BR 443 pitch adjustment

A roof window and a rooflight can look similar on a drawing, but the building regulations treat their U-values differently. A roof window's U-value also depends on the angle it's installed at. Get the basis wrong and the figures in your calculation won't compare like with like.

Roof windows and rooflights

• A roof window is fitted in the plane of a sloping roof, like a standard pitched-roof window.
• A rooflight usually sits proud of the roof on an upstand or kerb, typically on a flat roof. BR 443 calls these "out-of-plane" rooflights (clause 11.5.1).

They are tested differently. A roof window's U-value is usually measured, or modelled, with the unit in the vertical position, so that products can be compared whatever pitch they end up installed at (BR 443, clause 11.4). A rooflight's U-value is determined in the horizontal position, and is based on the outer developed surface area of the product, which is often larger than the opening in the roof (BR 443, clause 11.5.1; Scotland's Table 6.2 note 5; England's Table 4.2 note 11).

Why the pitch matters

Heat escapes faster through a glazed unit laid towards the horizontal than through the same unit standing upright. So a roof window loses more heat installed in a roof than its vertical-position U-value suggests. BR 443 notes that a roof window's U-value in the inclined position will be higher than in the vertical position (clause 11.4).

The BR 443 adjustment

Where a roof window's U-value is only available for the vertical position, BR 443 gives indicative adjustments to convert it to the installed pitch (clause 11.4, Table 2). Add the figure for your roof pitch to the vertical U-value:

Figures in W/m²K, from BR 443 Conventions for U-value calculations (2019 edition), Table 2, published by BRE.

For example, a double-glazed roof window rated at 1.4 W/m²K in the vertical position, installed in a 35° roof, becomes 1.4 + 0.4 = 1.8 W/m²K. A triple-glazed roof window rated at 1.0 W/m²K, in a 45° roof, becomes 1.0 + 0.2 = 1.2 W/m²K.

BR 443 attaches some conditions to these figures:

• Energy calculations only. They are for energy calculations such as SAP, and adjusted values are "not suitable for the assessment of compliance criteria" (Table 2, notes 1 and 2).
• Only when you don't have the real figure. Use them only when you have measured or calculated data for the vertical position, not the inclined one (note 3).
• Roof windows only. They apply to roof windows fitted in the plane of the roof, not to rooflights on upstands or kerbs (note 4).
• Whole windows. They apply to the whole window, frame included, not just the glass (note 5).

Checking a roof window against the limits

When you're checking whether a product meets the maximum U-value, use the plane the guidance specifies, not the adjusted figure:

• Scotland. The limit for a roof window is assessed and reported on the vertical plane. Rooflights are already calculated on the horizontal plane (Technical Handbook, Table 6.2, notes 3 and 4).
• England and Wales. Windows and roof windows are assessed in the vertical position and rooflights in the horizontal position. If your data is in the wrong plane, adjust it using BR 443 (Approved Document L Volume 1, paragraph 4.5, in both nations).
• Northern Ireland. The U-values in Technical Booklet F1 for both roof windows and rooflights assume the unit was assessed in the vertical position. Where a unit has been assessed in another plane, the standards themselves are adjusted, following BR 443 (paragraphs 2.45 and 3.47).

In a heat loss calculation

For calculations of heat loss and energy use, the installed pitch matters. Scotland says that for a SAP calculation, a roof window's U-value should be calculated for the pitch of the roof it's installed in, adjusting the vertical-plane figure using BR 443 where that information isn't available (Table 6.2, note 3). England and Wales likewise note that SAP calculations use each element's installed plane (paragraph 4.5).

Scotland's compensatory approach follows the same logic. In the handbook's attic conversion example, the roof windows are at a 35° pitch, so the maximum U-value of 1.4 W/m²K is adjusted to 1.8 W/m²K using BR 443, and the adjusted figure is applied to both the notional and the proposed attic (Technical Handbook, Annex 6.A.1).

England, Wales and Northern Ireland take the same approach. Their guidance says U-values should be assessed using the methods and conventions in BR 443 (Approved Document L Volume 1, paragraph 4.1, in both England and Wales; Technical Booklet F1, paragraphs 2.42 and 3.46). BR 443 in turn says that for heat loss calculations, a roof window's U-value should relate to its pitch as installed (clause 11.4). So in the area-weighted comparison, use pitch-adjusted U-values for roof windows, and adjust the benchmark's roof windows in the same way, as Scotland's example does.

Sources

This guide refers to the editions in force in September 2026:

• BRE: BR 443, Conventions for U-value calculations (2019 edition), clauses 11.4 and 11.5.1, and Table 2.
• Scotland: Building Standards Technical Handbook, Domestic (April 2026), clause 6.2.1 (Table 6.2, notes 3 to 5) and Annex 6.A.1.
• England: Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), paragraphs 4.1 and 4.5, and Table 4.2. A 2026 edition applies from 24 March 2027.
• Wales: Approved Document L, Volume 1: Dwellings (2022 edition), paragraphs 4.1 and 4.5. A 2026 edition applies from 4 March 2027.
• Northern Ireland: Technical Booklet F1 (2022), paragraphs 2.42, 2.45, 3.46 and 3.47.

The Scottish example is from the Building Standards Technical Handbook (Domestic), © Crown copyright 2026, and contains public sector information licensed under the Open Government Licence v3.0.