Warm Roof Construction for Flat Roofs

A warm roof is a flat roof where the insulation sits on top of the deck, between a vapour control layer and the waterproof membrane. The deck and joists stay on the warm side, so the roof needs no ventilated void. We build warm roofs on extensions, new builds and existing roofs being upgraded.

We have built warm roofs on houses, terraces and dormers, and on a hospital pharmacy block, a school extension, a hotel and a racecourse bar. This page explains how the build-up works, when it is the right choice, and shows the jobs behind it.

The difference is where the insulation goes. As general information, the main points are:

Not every roof on a job has to be warm. At Newbury we built a 90m² warm roof on the extension and waterproofed the 25m² terrace beside it as a cold roof, with the membrane adhered straight to the ply.

Warm roof construction is a sequence of layers, each with its own job. On our jobs the build-up runs:

• Deck. Usually plywood, laid to fall. At Salisbury Racecourse the ply deck was built with a 1:40 fall and a box gutter.
• Vapour control layer (VCL). Laid on the room side of the insulation to stop warm, moist air getting into the build-up, where it could condense. At Lymington that was a polythene VCL; on other roofs we have used self-adhered and bitumen VCLs.
• Insulation. Rigid boards, either bonded or mechanically fixed. Tapered boards can create the fall, as on the main roof at Newbury, where the boards ran from 150mm down to 120mm.
• Membrane. The waterproof layer, fully adhered to the insulation. On most of our warm roofs that is single ply, usually Alwitra Evalon VSK, with laps heat welded. On some we use felt.
• Edges and outlets. Trims, drips, capping and outlets finish the roof and carry the water away.

The U-value a roof needs sets the insulation thickness. At Arvensis Perennials 200mm tissue-faced boards gave a 0.11 U-value. At Springfield Avenue, 120mm of insulation gave 0.18 W/m²K on the flat roofs and dormers. At Westbourne, door thresholds left no room for a deep board, so we used Deck VQ, a compressed 70mm board with the same U-value as a 120mm board.

• Extensions. A room you will live in needs an insulated roof. At Bournemouth we built a warm roof on a new extension with 120mm boards, designed around a skylight so condensation would not build up.
• New builds. At Seven Acres in Surrey we built over 400m² of warm roofs on three properties on a development site.
• Upgrading an existing roof. An existing roof can often be insulated from above without stripping it. At Leydene Road we took core samples to work out how much insulation each roof needed, then bonded insulation to the existing felt and finished it with Alwitra Evalon VSK.
• Renewing a tired covering. If the roof needs a new covering anyway, it is a good time to add insulation. See flat roof replacement for how we strip, re-deck and rebuild.

For homes (more on our domestic flat roofing page):

• Anthonys Avenue, Poole. Full warm roof systems on the main roofs and terraces of two identical houses, in Alwitra Evalon VSK with matching aluminium parapet capping.
• Norton Drive, Fareham. A warm roof over a new pool and hydrotherapy extension, starting with a Stratavap VCL suited to high-humidity areas, then 140mm PIR and Alwitra VSK.
• Chilbolton Avenue, Winchester. More than 500m² of warm roofs over terraces and garages, with a tapered insulation scheme and, on some areas, a wildflower roof on top.
• Westbourne, Bournemouth. Terrace warm roofs around the whole perimeter of an apartment building, with 60 balustrade posts weathered into the membrane.

For more on work for businesses, schools and public bodies, see commercial flat roofing.

• Bournemouth Hospital. The new pharmacy block: plywood deck, VCL, 140mm PIR tissue-faced insulation and fully adhered Alwitra Evalon VSK.
• Broadstone First School. A felt warm roof on a new extension block, with 150mm insulation under a Strata-Cap SBS system, finished to a tight timetable in the summer holiday.
• Moonfleet Manor Hotel, Weymouth. A warm roof upgrade laid over the existing main roof: self-adhered VCL, 150mm PIR and Alwitra Evalon VSK.
• Care home, Salisbury. A 200m² main warm roof insulated to a 0.11 U-value and three dormers to 0.15, all in Alwitra 1.2mm VSK.

As general information, a new extension roof has to meet the Building Regulations, and so can work that renews a large part of an existing roof. The energy-efficiency guidance is in Approved Document L on GOV.UK. Your architect or building control body will confirm what applies to your roof, and we build to the U-value they specify, as we did on the Lymington extension.