Rates are UK national (non-London): central London and constrained urban sites typically run dearer — adjust the labour, plant and access lines in the build-up accordingly.
Warm-roof build-up — deck, VCL & insulation
per m² · supply & lay
£87 – £143 / m²
What is it
The supply and fixing of a warm-roof build-up — an 18 mm OSB3 structural deck laid over the roof structure, a self-adhesive vapour control layer lapped and bonded over it, and rigid PIR flat-roof insulation boards cut, bonded and fixed to a level braced substrate ready to receive the waterproofing covering — priced per m² of roof area. The warm-roof build-up is Building-Regs-driven on essentially every modern flat roof; it is the substrate that sits UNDER the single-ply / felt / GRP covering. This rate prices the BUILD-UP only — the covering over it, the roof structure below it and the rainwater goods are separate rates. The rate carries three operations:
Deck — 18 mm OSB3 boards loaded out, set out and cut around the perimeter, upstands and penetrations, and fixed down over the roof structure.
Vapour control layer — a self-adhesive VCL rolled out, lapped and bonded to the deck so warm moist air cannot reach the cold side of the insulation.
Insulation — rigid PIR flat-roof boards (120 mm default, to a modern Building-Regs U-value) cut, bonded / mechanically fixed to a level substrate ready for the covering.
Rate range by insulation spec
100 mm PIR£87 – £112 / m²
120 mm flat-roof PIR£96 – £124 / m²
150 mm PIR (enhanced)£111 – £143 / m²
Budget rate per m².
3 key cost drivers
Roof complexity & access
A simple open rectangular roof with good access lays fastest; many upstands, kerbs, penetrations, a tapered-to-falls scheme or restricted access add cutting and detailing time. Deck grade (18 mm OSB3 vs WBP structural plywood) flexes the build-up too. It is a key driver of the labour per m².
Insulation spec & thickness
Three genuinely different insulation thicknesses: 100 mm general-purpose, 120 mm flat-roof PIR (the default, to a modern Building-Regs U-value) and 150 mm for an enhanced U-value. A different thickness is a different BoQ item and U-value, so the control switches spec, not price grade.
Location
London and the South East typically add around 20% to labour.
Estimator
Budget rate generator
£96 – £124 / m²
Covers the 18 mm OSB3 deck, vapour control layer, PIR insulation, adhesive, fixings, the roofing gang and O&P · excl. the waterproofing covering, roof structure, rainwater goods, scaffold & VAT
Exclusions
Lifting & material distribution — telehandler / forklift, hoist and site-distribution labour (an attendance, in main-contractor prelims)
The waterproofing covering over the build-up (single-ply EPDM / felt / GRP — a separate rate) and the roof structure / joists / firrings below
Rainwater outlets & goods, leadwork / flashings, strip-off of an existing roof, and access / scaffold
Scaffold supply (usually in preliminaries)
Preliminaries — main-contractor site set-up, management & welfare