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Commercial flat roofing

Built-up felt · single ply · liquid · warm roof build-ups

Three systems cover almost every commercial flat roof in the UK: built-up felt at about £80 to £120 per m², single ply membranes at £90 to £140 per m² and liquid-applied membranes at £40 to £90 per m² as an overlay. All three are laid as warm roofs, with the insulation above the deck.

Which suits a building depends on its size and shape, how much foot traffic the roof takes, how long the owner plans to keep it and whether the old roof can stay in place.

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Choosing between the systems

  • Large, simple roofs

    Single ply goes down fastest on big open areas with few penetrations, which is why it dominates warehouses and retail sheds.

  • Busy roofs with plant and foot traffic

    Built-up felt with a mineral cap sheet is tough underfoot and tolerant of maintenance traffic around plant.

  • Complex detailing or no strip-out

    Liquid systems flow around pipes, outlets and awkward upstands, and can go over the old roof.

  • Green roofs and solar

    All three can carry them if specified for it; root resistance and ballast loads must be in the specification.

Systems side by side

ItemScopeTypical cost
Built-up feltTwo or three layers of modified bitumen. 15 to 30 years£80 to £120 per m²
Single plyOne membrane, welded seams. 25 to 35 years£90 to £140 per m²
Liquid-appliedReinforced, over the old roof. 20 to 30 years£40 to £90 per m²
New insulationExtra over any system£20 to £40 per m²

Budget figures read from national price guides and roofers' own published rates in November 2026, shown before VAT. Use them to plan; the contractor's written price is what counts. Work out a budget for your roof.

Notes on flat roof design

Warm roof build-up

Almost all new commercial flat roofs are warm roofs: a vapour control layer on the deck, rigid insulation above it and the waterproofing on top. The insulation keeps the deck warm, so condensation forms nowhere it can do damage. Cold roofs, with insulation below the deck and a ventilated void, are now rare on commercial buildings.

Falls and drainage

A "flat" roof should still fall to its outlets. The British Standard for flat roofs, BS 6229, was revised in December 2025: the finished fall on most flat roofs should be at least 1 in 80 with no backfalls, and the design fall is now worked out from the structure and a survey rather than a fixed 1 in 40. Tapered insulation is the usual way to create falls when a roof is renewed. Standing water is a common cause of early failure.

Fire and wind

Roof coverings must meet the external fire performance the building regulations require for their distance from the boundary, and be fixed to resist wind uplift calculated for the building's height and exposure. Both should be stated in the specification.

Upstands, edges and rooflights

Most leaks start at details, not in the field of the roof. Upstands are usually detailed at 150 mm or more above the finished roof, and every outlet, rooflight kerb and plant base needs its own detail. Ask for the detail drawings.

Where our information comes from

Prices are typical ranges from published UK contractor price guides and roofing firms' price pages, November 2026.

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How the job runs

How a new flat roof goes on.

  1. Survey

    A contractor walks the roof where it is safe to, or flies a drone where it is not, takes cores on flat roofs and photographs every defect. You get a written report with a recommendation.

  2. Specification and price

    A written scope naming the system, the details at edges, outlets and rooflights, the guarantee and the programme, with a fixed price excluding VAT.

  3. Strip or prepare

    Old covering removed or prepared for overlay, deck repaired, vapour control layer laid.

  4. Insulate and waterproof

    Tapered or flat insulation, then the membrane, details at every edge, outlet and upstand.

  5. Handover

    A final inspection, photographs, the guarantee certificate, building control completion where it applies, and a note of when the next maintenance visit is due.

Flat roofing questions

What is the best flat roof system for a commercial building?

There is no single best. Single ply is the usual choice for large, simple roofs; built-up felt for roofs with heavy foot traffic and lots of plant; liquid systems for complex details and overlays. A survey and the building's plans decide it.

How long does a commercial flat roof last?

Typically 15 to 30 years for built-up felt, 25 to 35 for single ply and 20 to 30 for reinforced liquid systems, with maintenance. Ponding, foot traffic and poor details shorten all of them.

How much does a commercial flat roof cost per m²?

Published rates: built-up felt £80 to £120 per m², single ply £90 to £140 per m² and liquid overlays £40 to £90 per m², excluding VAT. Add £20 to £40 per m² for new insulation.

What fall does a flat roof need?

A finished fall of at least 1 in 80 on most flat roofs, with no backfalls, under BS 6229:2025. Our library has a longer note on flat roof falls.

Can solar panels go on a flat roof?

Yes, on ballasted or fixed frames, if the roof can carry the load and has enough life left. Re-roofing under an array is expensive, so most owners renew an ageing roof first.

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