Most extension roofs in the Stroud valleys are not complicated shapes. They are a rectangle of deck, a membrane on top and a wall on one side. The trouble sits in the edges. Where the new roof meets a stone or brick wall that was built long before it, where a rooflight breaks the surface, and where the water finally leaves the roof, a small mistake in detailing lets in more rain than a poor covering ever will.
Here we walk through those junctions one at a time, with the national figures we rely on. A flashing is a lead or metal cover laid over a joint; an upstand is the short vertical turn-up of the membrane against a wall or kerb; a kerb is the raised frame a rooflight sits on. For the materials themselves, start at our flat roofing page. This guide stays with the extension and its details.
Where an extension roof leaks first
When we are asked to look at a leaking extension, the covering in the middle of the roof is rarely the first suspect. The field of the membrane is a single sheet or a few welded sheets, and it is laid over a deck that is flat and dry. The edges are different. Every edge is a place where the membrane stops and something else begins: brick, stone, glass, timber, a gutter. Each meeting needs its own detail, made by hand on site.
So the design starts at the perimeter and works inwards: what the roof touches on each side, how deep water can pile against it, and where it goes when it leaves. If you are weighing up an older roof that already leaks, our guide to the signs of a failing flat roof helps you tell an edge problem from a covering problem.
One roof, four edges
A typical rear extension has a wall edge against the house, two side edges (often a boundary wall or a neighbour's gable) and a front edge that drains. Each is a different job. The wall edge is a flashing. The side edges may be a parapet, a verge trim or an upstand. The front edge is a trim over a gutter. Treat them as four details, not one roof, and the survey becomes much easier to read.
On a hillside plot the access matters too. Carrying rolls and boards through a terraced house, or up a steep path from a lane, is priced into the quote beforehand.
Meeting the old wall: abutments in stone, brick and render
The abutment is the junction between the extension roof and the existing house wall. Two things have to work together. The membrane is turned up the wall as an upstand, and a flashing above it covers the top of that upstand and sheds water over it. The flashing is normally lead, dressed into a chase (a slot cut into the wall) or tucked into a mortar joint and wedged.
On brick this is straightforward, because the mortar joints are even and the chase can be cut along one. On Cotswold rubble or ashlar stone it needs more care. We cut into the mortar joint and never into the face of the stone, and then repoint with a lime mix that suits the wall. A chase ground through the stone face is a permanent scar, and on a listed or conservation area house it is one the planners will notice.
Lead codes for flashings
Lead sheet comes in numbered codes, and each code is a thickness. Leadworx calls Code 4, at 1.80 mm, the standard for general flashing, and Code 5, at 2.24 mm, is used where more strength is wanted. MyJobQuote notes that lead is typically fitted in lengths of around 1.5 metres. Checkatrade's 2026 repair guide puts lead flashing repairs at £50 to £70 per metre, excluding VAT, for Code 5.
If the old wall is damp at the junction, a flashing alone will not cure it. We check the pointing, the render and the ground level outside first.
Where the extension roof sits close under a window, upstand height becomes a design question too. Check it against the membrane maker's data sheet and ask for the height to be written on the quote.
Rooflights and kerbs: the hole in the middle
A rooflight lets daylight into a deep extension, which is why so many of them are fitted. It is also the one thing that interrupts an otherwise continuous membrane, so the detail around it deserves attention. A rooflight sits on a kerb. The membrane is turned up the kerb on all four sides, welded or stuck at the corners, and a cover flashing or the rooflight's own cill closes the top.
What Part L asks of a rooflight
Approved Document L, in the 2021 edition, sets a limit of 2.2 W/(m²·K) for a rooflight in a new or replacement element, measured on its outer developed surface area. A kerb supplied as a single rooflight-and-kerb assembly falls under the same limit, while a site-built upstand is held to a maximum U-value of 0.35 W/(m²·K). Those numbers are why we ask for the rooflight's data sheet at the quote stage. A cheap unit with a poor figure can be the weak point in an insulated roof.
Flat glass or a lantern
A flat rooflight keeps the roof line low, which matters where the extension must stay below a first floor window. A lantern stands proud, lets in light from the sides and costs more to detail. Either needs the opening framed with a trimmer (a short cross-beam) to carry the load.
The extension itself brings its own planning and Building Regulations picture. Our flat roof Building Regulations guide sets out the routes.
Edges, trims and the way out for the water
The front edge of an extension roof carries all the water the roof collects. The membrane runs over a drip trim, an aluminium or coated steel section that kicks the water clear of the fascia and into the gutter. Our guide to flat roof edge trims goes through the profiles. On an extension the trim must be fixed to a solid edge, not a soft fascia board.
Falls: design for one figure, finish at another
A flat roof is not flat. BS 6229 guidance, as set out by IKO and Rockwool, recommends designing for a fall of 1:40 to leave a finished 1:80 after tolerances and sag. At 1:80 the roof drops 12.5 mm over a metre. A roof built dead level, or one that sags between joists, holds a pool of water that the membrane must resist every day.
Extension roofs often get their fall from tapered insulation boards or from firring strips (tapered timber laid on top of the joists). Either works if it is set out on the drawing. Our page on flat roof minimum fall covers the numbers in more detail.
Gutters and downpipes
Manufacturers recommend a gentle fall to the gutter outlet, somewhere between about 1:600 and 1:350 depending on the system. If the extension shares a downpipe with the house, check the pipe can take the extra roof area. More on this in our note on flat roof drainage.
Warm roofs, insulation and the paperwork
Most new extension roofs are built as warm roofs, with the insulation above the deck and the membrane over the top. The deck stays on the warm side of the insulation, so condensation has nowhere to form inside the structure. A cold roof puts the insulation between the joists and leaves a ventilated void above it. BS 5250, as summarised by Marley, is the reference for those ventilation openings, and a cold roof with blocked ventilation is a common reason for damp timbers. The pros and cons of each are in our guide to warm and cold flat roofs.
What the regulations say about the roof element
The new-element limit in Approved Document L Table 4.2 for a roof in an existing dwelling is 0.15 W/(m²·K). An extension has its own route through Part L, and your building control body or designer will confirm which calculation applies. What matters on site is that the insulation thickness on the drawing, the roof build-up and the rooflight data all agree with one another.
An extension is notifiable, and a registered building control approver or the council's service (in Stroud District, the Gloucestershire Building Control Partnership) inspects it. We take care of the Building Regulations notification for our part of the work.
VAT on the insulation line
The insulation part of the job is zero-rated for VAT until 31 March 2027, while the rest of the roof is charged at the standard rate, and the quote says which applies.
Fire performance of the covering also matters near a boundary; raise it at the survey if a neighbour's wall is close.
Planning and conservation settings in Stroud District
An extension is a planning question before it is a roofing one. Whether yours needs planning permission depends on the design and the location, and that is a matter for the planning team at Stroud District Council. Our job is to make sure the roof detail matches what was approved.
Stroud District has 41 conservation areas, and Article 4 Directions cover five of them, among them Bisley and the two Stroud town areas. Nothing published by the council settles whether those directions reach roofs, so raise it with the planners at Ebley Mill before treating a flat roof there as free of conditions. More than half of the district sits within the Cotswolds National Landscape, which draws attention to materials on new work. On a listed house the whole building is protected, so work touching the wall an extension abuts normally needs listed building consent.
Ask the planning team early whether the covering colour, the edge trim finish and the rooflight type were conditions of the permission. A change at quote stage is cheap; after the scaffold is up it is not.
Detailing that sits well beside old stone
A flat roof is usually hidden from the lane, but not always from the upper windows next door. A dark membrane, a stone-coloured trim and a low-profile rooflight keep the new roof quiet. Where a parapet is allowed, a stone coping or a lead cap on the wall hides the roof edge entirely. Our guide to a flat roof in a conservation area covers the decisions in more detail.
If you want a roof that people can use as a terrace, the detailing changes substantially. See our guide to balconies and roof terraces over living space.
Costs, the survey and the decision to repair or replace
Published national ranges give a guide to the covering. MyBuilder's 2026 flat roof guide puts bitumen felt at £70 to £100 per m², EPDM at £90 to £130, GRP at £95 to £150, PVC single-ply at £95 to £140 and lead at £150 to £200 and above. A small extension at those figures often lands in the low thousands, and the abutment, rooflight and trim details add to it. Scaffold adds to the cost; Checkatrade puts it at around £700 to £1,300 for a re-roof.
Once we have surveyed, the written quote arrives alongside pictures of the roof you own, covering the strip, deck repair, insulation, membrane, each abutment, the rooflight, trims, scaffold and waste.
A leaking extension is not always a new roof
If the extension is older, the details are often where the money should go. A failed flashing or a split at an upstand can be repaired for a modest sum, whereas a failed deck needs renewing. Our repair or replace guide sets out how we decide between the two. Whatever route, the work carries a 10-year workmanship guarantee in writing.
To start, send photographs of the wall junction, the rooflight and the front edge to 01453 933613 by WhatsApp, or use the enquiry form.