Conservatory Glazing Calculator

Polycarbonate is lighter and cheaper but offers poorer thermal performance: multiwall sheets run roughly U-value 2.4 W/m²K (16mm triple-wall) down to about 1.2 W/m²K (35mm 5/7-wall) per the Brett Martin Marlon ST datasheet. Glass roofs are heavier and need stronger framing, but provide better insulation (sealed glass units typically 0.8–1.2 W/m²K), superior light transmission, and a longer lifespan. Self-cleaning coatings on glass reduce maintenance. Solid insulated (tiled) roofs (U-value around 0.18) make the room usable year-round but are no longer "glazed" for conservatory-exemption purposes.

A conservatory is exempt from Part L only if it is thermally separated from the house by external-quality walls, windows and doors, sits at ground level, is under 30 m² internal floor area, keeps at least 75% of the roof and 50% of the walls translucent, and the dwelling heating is not extended into it. If it does not meet the exemption (open to the house, or a replacement room), the current Approved Document L 2021 (2023 amendments) Table 4.2 limiting U-values apply: windows and roof windows 1.4 W/m²K (or window energy rating band B), rooflights/lanterns assessed in the horizontal plane 2.2 W/m²K, and doors 1.4 W/m²K. The old 1.6 W/m²K roof-glazing figure came from the superseded 2010/2013 Part L1B and should no longer be specified.

Roof panels fit between rafter bars, typically spaced at 600mm centres. Panel width is bar spacing minus bar width (usually 30-50mm). Panel length runs from eaves beam to ridge (or wall plate for lean-to), calculated as projection divided by cos(roof pitch). Standard polycarbonate sheets come in lengths up to 4m; glass panels are made to measure.

Valley bars create an internal angle where two roof sections meet (common in P-shape and T-shape designs). Hip bars create external angles at the corners of Victorian-style faceted fronts. Both require specialist flashings and careful sealing. Victorian conservatories typically need 4-6 hip bars; P-shapes add 2-4 valley bars.

Most conservatories are permitted development, but you must stay within limits: no more than 50% of garden covered, no higher than the existing roof, no forward of the principal elevation. Single-storey rear extensions can project up to 4m (detached) or 3m (attached) under PD rights. Listed buildings and conservation areas have additional restrictions. Building Regulations apply if the conservatory is over 30 m² internal floor area or not thermally separated from the house.