Flat roof stripped to the joists with new insulation boards being laid on a UK extension

Flat roof replacement: do it once, do it right

Full strip, warm roof upgrade and a new covering that lasts decades, at fair 2026 prices, with free quotes from vetted local roofers.

Quick answer: flat roof replacement costs £80 to £150 per m² in 2026 depending on material. A typical single garage runs £1,400 to £3,000 and a 25m² extension roof £2,300 to £4,000. Vetted local roofers quote free through Expert Roofers.

Flat roof replacement is the job you should only ever pay for once a generation, and how it is done matters more than which material goes on top. A proper replacement strips to the deck, fixes what the old covering was hiding, brings the insulation up to standard and details the new covering so it outlives its guarantee.

Full strip vs overlay

The tempting shortcut is to lay new felt or a new membrane straight over the old covering. Do not take it. Overlaying a failed covering traps moisture in the old layers, and that trapped moisture rots the timber deck underneath, so the new roof fails from below within a few years, taking your money with it. A proper replacement strips back to the deck, replaces any rotten boards at an agreed per-m² rate, and starts from a sound, dry base. If your existing roof only has one isolated fault, a flat roof repair may be all you need; this page is for coverings that have failed as a whole.

When replacement is the right call

Blistering, crazing or cracking spread across the surface, not just one spot.
Repeated leaks in different places, each patch chasing the last.
A spongy or bouncy deck underfoot, which points to rot below the covering.
Persistent ponding that the falls can no longer shed.
A covering at the end of its lifespan: felt in particular rarely makes it past 20 years.

The warm roof upgrade

Replacement is the one moment you can fix a flat roof's insulation properly. A warm roof build-up puts rigid insulation above the deck, under the new covering, keeping the structure warm and eliminating the condensation that plagues cold roofs. It adds £40 to £60 per m², and in most wholesale replacements it is not optional: building regulations treat the covering as a thermal element, so replacing it usually triggers the requirement to bring insulation up to current standards. The warm vs cold roof guide explains both build-ups in detail.

Red flag: a quote that undercuts everyone else usually means the insulation upgrade has been quietly left out, and a roof priced without it may not pass building regs sign-off. Ask every quote to state the insulation spec in writing before comparing prices.

How the job is done

Survey and written spec: material, insulation, deck repair rate and details, all itemised.
Strip to the joists or deck and dispose of the old covering.
Deck repairs: rotten boards replaced at the agreed rate.
Vapour barrier and insulation laid as a warm roof build-up where regulations require.
New covering: EPDM bonded in one sheet, GRP laid wet, or 3-layer torch-on felt, with trims, upstands and outlets detailed properly.
Sign-off and guarantee: building regs notification through a competent person scheme, plus a written guarantee.

What flat roof replacement costs

Job	Felt	EPDM	GRP
Single garage (approx 18m²)	£1,400 – £2,000	£1,600 – £2,600	£1,900 – £3,000
Extension roof (approx 25m²)	–	£2,300 – £3,500	£2,800 – £4,000
Small dormer	£600 – £1,200

The full flat roof cost guide covers per-m² rates, insulation and decking extras, and how to choose between the three systems. Not sure which material to put back? Our flat roofing page matches each system to the roofs it suits.

Why use a vetted roofer

Overlays, skipped insulation and missing deck repairs are invisible on the day of handover and expensive three winters later. Every roofer in the Expert Roofers network is vetted for insurance, trading history and real customer feedback, and quotes must specify the strip, the insulation and the full build-up in writing.

Replace your flat roof properly

Up to three itemised quotes from vetted local roofers, checked against the fair rates on this page. Free, no obligation.

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Flat roof replacement FAQs

Flat roof replacement questions, answered

Flat roof replacement costs £80 to £150 per square metre in 2026 depending on material. A typical single garage of around 18 square metres runs £1,400 to £3,000, a 25 square metre extension roof £2,300 to £4,000, and a small dormer £600 to £1,200. A warm roof insulation upgrade adds £40 to £60 per square metre. See the flat roof cost guide.

Stripped, in almost every case. Overlaying a failed covering traps moisture in the old layers, and that trapped moisture rots the timber deck underneath, so the new roof fails from below within a few years. A proper replacement strips back to a sound, dry deck, replaces any rotten boards and builds up from there.

A warm roof puts rigid insulation above the deck, under the new covering, keeping the structure warm and eliminating the condensation problems cold roofs suffer. When a covering is replaced wholesale, building regulations usually require the insulation to be brought up to current standards, so most full replacements become warm roof jobs. It adds £40 to £60 per square metre. See warm vs cold roofs.

Usually yes. Replacing more than a limited area of the covering counts as renovation of a thermal element, which triggers the requirement to upgrade the insulation to current standards. A reputable roofer prices this in and handles the notification through a competent person scheme; a quote without an insulation spec is a warning sign.

Most domestic flat roof replacements are measured in days, not weeks: a garage or small extension is typically stripped, insulated and re-covered within a few days of dry weather. Rotten deck repairs and complex details extend that, and rain pauses work because no covering bonds to a wet deck.