Kingston upon Thames at a glance
- Postcode districts
- KT1, KT2, KT5, KT6, KT3, KT9
- Council area
- Royal Borough of Kingston upon Thames
- Building approval
- Royal Borough of Kingston upon Thames building control, or a registered building control approver
- Guide price region
- London (+20% on West Midlands rates)
What makes a good extension roof in Kingston upon Thames?
Condensation drives some replacements in Kingston upon Thames. Older extension roofs were often built cold, with insulation between the joists and no reliable vapour control. Warm air from the kitchen reaches the cold deck, moisture builds up and the timber suffers. A warm roof deals with the cause, not just the stain.
In and around Kingston upon Thames, flat roofs are typically found on shops with flat-roofed stores below flats above, and on mansard and dormer loft conversions. 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 new extension roof cost in Kingston upon Thames?
Guide price in Kingston upon Thames: £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), GRP, new deck + insulation | £3,600–£6,450 |
| Extension (20 m²), EPDM, insulation + 1 rooflight upstand | £3,600–£6,300 |
| Single garage (18 m²), EPDM rubber | £1,900–£3,350 |
Why does water pond or leak where the extension meets the house?
Usually because the fall runs the wrong way, the abutment detail has failed, or the roof has sagged close to the wall. The junction with the house is the most common leak point on an extension roof.
If the roof falls towards the house, or the deck dips near the wall, water sits against the upstand and finds any weakness in the flashing above. Older roofs often have the felt simply turned up the brickwork with no cover flashing, or a flashing that has pulled out of its joint. A patch will not change a fall. We re-firr or use tapered insulation so water runs away from the wall, then form a new upstand, usually 150 mm above the finished roof, with a flashing chased into the mortar joint.
Rooflights and lanterns are the other weak point. A lantern needs a properly built kerb with the membrane dressed up and under its frame. We rebuild rooflight upstands within the roof job; we do not supply the lantern itself. Our guide to ponding water explains the causes in more depth.
What's included?
- Free survey of the roof, abutment and room below
- New deck, vapour control layer and warm-roof insulation
- New abutment upstand and flashing, plus rebuilt rooflight kerbs
- Falls set away from the house wall
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?
- Richmond upon Thames · 3 mi
- Merton · 5 mi
- Wandsworth · 5 mi
- Hounslow · 5 mi
- Sutton · 6 mi
- Hammersmith and Fulham · 7 mi
Extension and kitchen roofs in Kingston upon Thames: questions
Do you cover Kingston upon Thames?
Yes. Kingston upon Thames is part of our London coverage, including postcode districts KT1, KT2, KT5, KT6, KT3, KT9. 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 Kingston upon Thames?
Where the work needs approval it goes to Royal Borough of Kingston upon Thames 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 new extension roof cost in Kingston upon Thames?
£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.
Why does my extension ceiling get damp in winter if the roof isn't leaking?
That is often condensation in a cold roof, where warm, moist air reaches the cold underside of the deck. A warm-roof replacement with a vapour control layer usually solves it.
Can a sedum roof go on my extension?
Possibly, if the structure can carry the extra weight and the roof has suitable falls and drainage. We check the joists at survey before recommending it.