Put a seating area on a flat roof and the roof gets a second job. Underneath it is a bedroom or a kitchen; on top, a table, chairs, planters and a steady tread of feet. Everything that keeps rain out of a quiet flat roof still applies, but now the covering is under load, the finish has to be level enough to use, and a leak lands in a room that people live in.
That is why a terrace is a designed build-up and not a covering with paving thrown on top. This guide follows the layers from the deck upwards: insulation, membrane, protection, supports and finish. It then covers the edges that fail first (upstands, door thresholds and outlets) and the planning and Building Regulations questions to settle before work starts. Occasional access for maintenance is a different matter; see our guide to a walk-on flat roof. For the wider service, see flat roofing in Stroud.
Foot traffic changes every layer of the roof
On a plain flat roof, the covering sits in the open and is looked at twice a year. On a terrace it lives under pavers, decking or tiles, it is loaded all the time, and nobody sees it again unless something goes wrong. That reverses the usual priorities. A terrace covering has to be chosen for how it behaves under weight, how it will be protected, and how easily it can be lifted for repair.
Four things the design must settle
Before a membrane is chosen, four questions need an answer on the drawing. What will the finished surface be? How is the load carried down to the structure? Where does the water go, both on the membrane and on the surface above it? How will the junction at the door be dealt with? A design that leaves any of these to the day is a design that will cost more later.
The structure underneath
A deck built for a flat roof is not automatically strong enough to be a terrace. Pavers, pedestals and people are heavier than a membrane alone. Where the structure cannot sustain the extra loading, Approved Document A calls for strengthening, so the joists and deck should be checked by your structural engineer before work is priced. Sag under load shows up later as ponding.
If you are weighing the whole project against other options, our guide to flat roof materials compares the coverings.
The build-up: deck, insulation, membrane and protection
Most terraces over a heated room are warm roofs: insulation above the deck, membrane above that, paving system on top, with the deck kept warm so condensation stays out of the structure. Part L applies too. Table 4.2 of Approved Document L sets 0.15 W/(m²·K) for a new roof, and a fresh membrane on a flat roof is renovation of a thermal element.
Insulation that can take the load
The insulation under a terrace carries weight, so the board must be one designed for compressive loads, and the maker's data sheet should show it. Soft insulation under a heavily loaded paver will compress and let the surface settle. We check the product against the intended use before the quote is written.
Membrane choice
Several coverings suit a terrace: GRP fibreglass, EPDM rubber, single-ply PVC, built-up felt and lead. MyBuilder's 2026 flat roof guide gives national ranges per m² of £70 to £100 for bitumen felt, £90 to £130 for EPDM, £95 to £150 for GRP and £150 to £200 and above for lead. Lead lasts longest, with Less quoting 50 to 100 years for lead flat roofs, but it costs more and needs careful detailing for expansion. GRP is jointless and tough underfoot. EPDM is flexible and quick to lay in a single sheet. The right answer depends on the area, the shape and how the surface will be finished.
A protection layer
Between the membrane and whatever sits on it, a protection layer takes the point loads and stops the pedestals, slabs or timbers from wearing through. It is a cheap layer that saves an expensive repair. Our page on EPDM versus GRP compares the two most common terrace coverings in more depth.
Paving supports, decking and the finished surface
The finished surface is what you see and use, and its job is to spread the load and keep feet off the membrane. There are three usual routes.
Paving on pedestals
Slabs rest on adjustable plastic pedestals, one at each corner, with a gap between slabs so water falls through to the membrane and runs away to the outlet. Because the pedestals are adjustable, they can level the surface even though the membrane below has a fall. That is the main reason this system is popular on terraces: the fall stays in the waterproofing where it belongs, and the walking surface stays flat.
Timber or composite decking on bearers
Decking boards fix to bearers (timber or plastic battens) which sit on pads. Gaps between boards allow water through. Decking is lighter than slabs and easier to lift, but timber needs treating and composite needs the right bearers. In either case the bearers must not puncture the membrane, so they sit on pads over the protection layer rather than being screwed through.
Fixed finishes
Some terraces carry a fixed finish, such as a grit-surfaced coating on GRP or tiles on a screed over a suitable base. These suit small balconies where the surface is part of the waterproofing, but they need a deck that does not move.
Whichever surface you choose, ask how a section can be lifted to look at the membrane. A terrace that can only be repaired by breaking up the finish will cost more to maintain every time.
Stop pavers short of the parapet or gutter so water and leaves can clear.
Upstands, door thresholds and the junctions that fail first
On a terrace the places where water enters are almost always the junctions: the door, the walls and the outlets. A membrane that is perfectly sound across the field can still leak at an upstand that is too low.
Upstands
An upstand is the membrane turned up a wall or parapet. On a terrace the surface above the membrane is raised by the paving build-up, so the upstand must start from the membrane and stay clear of the finished surface by a margin that the membrane maker's data sheet sets. Ask for the upstand height to be shown on the drawing and in the quote. A lead flashing covers the top of the upstand, and Leadworx names Code 4 as the usual weight for general flashing. Our guide to flat roofs on extensions shows how the wall junction is built.
The door threshold
The door is the tricky part. A level threshold from a living room to a terrace looks neat, but it removes the step that normally keeps water out. The usual approach is a drainage channel at the door and a raised cill, with the membrane turned up behind the door frame. Settle it with your designer or building control body before the order is placed, because the door and the roof must be set out together.
Rooflights, pipes and balustrades
Any post fixed through the membrane for a balustrade is a hole. Where possible, fix guarding to the side of the structure or to a base plate on a sealed upstand, not through the waterproofing. Pipes and vents need proper collars, and a rooflight below the terrace needs its kerb detailed with the same care as the rest.
Falls and drainage on a roof you walk on
A terrace needs the same fall as any flat roof. BS 6229 guidance, as set out by IKO and Rockwool, designs for a fall of 1:40 so the finished roof achieves 1:80 after tolerances and deflection. Under pavers, water runs through the joints and along the membrane to the outlets, so the membrane needs its own fall even if the surface looks level. Our guide to flat roof drainage deals with outlets in more detail.
Outlets that stay clear
Leaves, grit and plant debris collect at outlets, and a terrace gives them more to collect. Outlets should be reachable from above without lifting the whole surface. A removable paver or a covered access box over each outlet makes cleaning a five-minute job. An overflow set slightly above the normal outlet is a sensible safeguard when the terrace sits over a living room, because a blocked outlet then spills over the edge in a controlled way.
Gutters and downpipes
If the terrace drains to a gutter, makers advise a slight fall towards the outlet, from about 1:600 to 1:350 depending on the system. A gutter that is full of leaves at the base of a terrace wall is a common source of damp on the wall below. Check that the downpipe is sized for the area and has a clear route to a drain.
Planning, conservation and Building Regulations
Turning a roof into somewhere people sit can raise planning questions. Whether a terrace needs permission depends on the house, its position and what overlooks what. Check with Stroud District Council's planning team, or the council for your town, before you design. Privacy for neighbours is the usual issue, and it can lead to conditions about screens or the size of the terrace.
Stroud District has 41 conservation areas and Article 4 Directions in five of them, and more than half of the district lies in the Cotswolds National Landscape. If your house sits in one of those places, or is listed, say so at the outset. A listed building is protected inside and out, and work on a roof can need listed building consent from the council.
Building Regulations
A new terrace over a living room is not a repair. The structure, the insulation, the guarding and the drainage all fall under Building Regulations, so a registered building control approver or the council's building control service checks the work. In Stroud District that service is the Gloucestershire Building Control Partnership. We explain what the application needs and look after the notification for our part. Our guide to flat roof Building Regulations sets out the routes.
Insulation and VAT
The insulation element of a terrace roof fitted to a home is zero-rated until 31 March 2027, while the other works are standard-rated. Your quote shows how VAT is treated.
Guarding height and the strength of balustrades are set by Building Regulations. Your designer or building control body confirms the figures for your site; we do not guess them.
What a terrace costs and how we price it
A terrace costs more than a plain roof of the same size, because it has more layers and more detail. National ranges give a starting point for the membrane: the MyBuilder figures above span £70 to £200 and over per m², depending on the covering. The paving system, the insulation, the upstands and the door detail come on top, and so does scaffold. Checkatrade gives roughly £700 to £1,300 for scaffold on a re-roof, and an upper-floor terrace needs it too. Our flat roof cost guide sets out the covering ranges by material.
The written quote
After a survey, with photographs of the roof you have, we write a quote that covers the strip, deck repairs, insulation, membrane, protection layer, paving or decking, upstands, door detail, outlets, scaffold and waste. If you want to compare the terrace against a simple roof, the survey shows where the extra comes from.
Hillside and access
Many Stroud valley houses have steep paths and narrow lanes. Carrying heavy slabs up to a first floor terrace is a job in itself, and we price it in before the day, not after.
Maintenance after
Check a terrace once a year: lift an access paver, clear the outlets and look at the door sealant. The work is covered by our 10-year workmanship guarantee, issued in writing. To begin, WhatsApp photographs of the roof, the door and the edges to 01453 933613, or use the enquiry form.