Is polycarbonate or glass better for a conservatory roof?

Polycarbonate is lighter and cheaper but yellows over time and offers poorer thermal performance (U-value 1.1-1.6 W/m2K). Glass roofs are heavier, require stronger framing, but provide better insulation (U-value 0.8-1.2 W/m2K), superior light transmission, and longer lifespan. Self-cleaning coatings on glass reduce maintenance. Solid insulated roofs (U-value 0.18) make the room usable year-round.

What U-values does Part L require for conservatory glazing?

If the conservatory is separated from the house with external-grade doors and walls, it is exempt from Part L. If open to the house or replacing an existing room, roof glazing must achieve U-value of 1.6 W/m2K or better, and wall glazing 1.4 W/m2K or better. From 2025 updates, lower values are encouraged.

How do I calculate conservatory roof panel sizes?

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.

What are valley and hip bars in a conservatory?

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.

Do I need planning permission for a conservatory?

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 30m2 and connected to the house services.

TradeCalculator

Conservatory Glazing Calculator

Calculate roof panels, wall glazing, ridge, eaves beam and structural bar requirements

Width (m)

Projection / depth (m)

Eaves height (m)

Roof pitch (degrees)

Conservatory shape

Roof material

Wall glazing type

Dwarf wall height (m)

How We Calculate This

This calculator determines conservatory glazing quantities based on the plan dimensions, roof pitch, and shape. Roof area is calculated from width and slope length (projection/cos pitch), adjusted by a shape multiplier for Victorian facets or combination designs. Rafter bar count is derived from width divided by bar spacing. Wall glazing area accounts for dwarf wall height. Heat loss is estimated using U-values for selected materials at a 20C temperature differential per BS EN ISO 6946.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.