Harrow at a glance
- Postcode districts
- HA3, HA1, HA2, HA7, HA5, HA8, HA6
- Council area
- London Borough of Harrow
- Building approval
- London Borough of Harrow building control, or a registered building control approver
- Guide price region
- London (+20% on West Midlands rates)
Why upgrade to a warm roof in Harrow?
Many flat-roofed extensions and dormers in Harrow were built with little or no insulation, or with a cold roof that was never properly ventilated. Those rooms are often hard to heat in winter and can suffer condensation, mould and, in time, a rotten deck.
Around Harrow, flat roofs are common on mansard and dormer loft conversions and on shops with flat-roofed stores below flats above. The Thames basin is one of the drier parts of Britain at roughly 730 mm of rain a year, but summer heat is the bigger test here: dark membranes on south-facing roofs swing a long way in temperature.
How much does a warm roof upgrade cost in Harrow?
Guide price in Harrow: £3,350–£5,850 for a 20 m² extension roof in EPDM with a new deck and insulation, including VAT at London rates. The fixed price comes after a free survey for new and replacement roofs.
| Job | Guide price inc VAT |
|---|---|
| Extension (20 m²), EPDM, new deck + insulation | £3,350–£5,850 |
| Extension (20 m²), EPDM with sedum green roof | £5,300–£9,700 |
Will insulating the roof raise its height, and what about doors and upstands?
Yes. A warm roof makes the roof thicker, often by more than 100 mm depending on the insulation and the U-value target. That extra height is the main thing we plan around.
Waterproofing at a wall usually needs to rise about 150 mm above the finished roof surface, so if the roof gets higher, the upstand has to go higher too. That can be a problem where the roof meets a door threshold, a low window sill, a cavity tray or a rooflight kerb. At the edges, trims and fascias are reset to the new level, and gutters may need to move.
Where there isn't enough height, the options include using thinner, higher-performance insulation, lowering the deck where the structure allows, raising the sill or threshold, or, where it is genuinely not feasible to reach the full target, insulating as far as is reasonable and recording why with building control. We work this out at the survey so there are no surprises on the day.
What's included?
- Survey of the deck, joists and existing insulation
- Rigid insulation boards sized to the U-value target
- Vapour control layer on the warm side of the insulation
- New waterproofing, with a written workmanship guarantee
Building regulations in England
Replacing more than half of a flat roof over a heated room usually counts as renovating a thermal element, so the roof is insulated to around 0.16 W/m²K where that's technically and economically feasible, and the work is notified to building control: the council or a registered building control approver, unless a registered competent person self-certifies it. A roof on a new extension is built to 0.15 W/m²K. Patch repairs, and most unheated detached garages, don't trigger the upgrade. Structural changes, or a new covering that adds 15% or more to the roof's weight, also need approval. More on the rules.
Which nearby areas do we also cover?
Warm roof insulation upgrades in Harrow: questions
Do you cover Harrow?
Yes. Harrow is part of our London coverage, including postcode districts HA3, HA1, HA2, HA7, HA5, HA8 and 1 more. Surveys and roofs are carried out by vetted roofing teams working under S&K Property Investments and Improvement Ltd.
Who checks building regulations for a flat roof in Harrow?
Where the work needs approval it goes to London Borough of Harrow building control, or a registered building control approver. We tell you at the survey whether your job needs it and deal with the paperwork as part of the quote.
How much does a warm roof upgrade cost in Harrow?
£3,350–£5,850 for a 20 m² extension roof in EPDM with a new deck and insulation, at London guide rates including VAT. The fixed price comes after a free survey for new and replacement roofs.
Is there VAT relief on flat roof insulation?
Don't count on it. Insulation fitted as part of a roof replacement is usually treated as part of a single standard-rated job. Check your own case with HMRC guidance or an adviser.
How thick does flat roof insulation need to be?
It depends on the insulation product and the U-value target. With common PIR boards, meeting a renovation target usually means well over 100 mm. Thinner, higher-performance boards are available where height is tight.