Flat roofing in Bath, built from the deck up

Underneath the covering of any flat roof sits a timber frame. Joists are the horizontal beams that span from wall to wall, usually a few hundred millimetres apart, and they carry everything above. Over the joists goes the deck, a layer of boards or sheet material fixed across them to give the roof a surface to build on. Modern decks are usually sheets of plywood or oriented strand board. On older roofs the deck may be tongue-and-groove boarding.

The word flat is a convenience. Every flat roof needs a fall, a gentle slope that sends water to an outlet or gutter. The fall can be built into the structure by laying the joists at a slope, or added on top with firring strips, which are tapering timber pieces fixed to the joists to create the slope. Where the fall is missing or has sunk, water sits in puddles, and puddles speed up the ageing of every membrane.

What goes wrong with a deck

The deck is where a lot of flat roof failures begin, because water that gets past the membrane is soaked up by the timber. Plywood and boards swell, delaminate and lose their grip on their fixings. Rot can then run along the joist tops and into the wall plate. In a roof that has been weeping for a long time, the deck can feel spongy underfoot, and the ceiling below will usually show a stain.

Joists can also be undersized or have sagged over the years, especially on old extensions or porches added without engineering input. A sagging joist makes a dip in the fall, so a puddle forms and stays. Sometimes we can sister a new timber beside the weak one. Sometimes the whole frame has to be rebuilt, which we do before any covering goes on.

Sheets of deck should have tight joints, staggered across the roof, and be fixed to every joist. Gaps between sheets let moisture move and flex the covering. Small roofs have the same needs on a smaller scale. A porch roof may be a single sheet on three or four joists, and a bay roof is cut to follow the angles of the bay, which means more joints between pieces of deck. Each of these shapes asks for its own set-out, so that the falls run to the gutter at every angle. When we strip a flat roof, we test every sheet, replace the ones that are soft or stained through, and check that the surface is flat and clean before the next layer goes down.

Our guide to flat roof design explains how joists, falls and decking are planned for an extension or porch. If the roof already has a dip, how much fall a flat roof needs will help you judge whether it is a surface problem or a structural one.

The membrane is the top waterproof layer of the roof, and three types cover most domestic flat roofs. EPDM is a synthetic rubber sheet, usually supplied in a single piece cut to the size of the roof and adhered to the deck or insulation with adhesive. GRP, glass-reinforced plastic or fibreglass, is laid wet: a resin is brushed over glass matting and sets hard into a single hard shell. Torch-on felt is a roll of bitumen-based felt, laid in two or three layers and adhered to the layer below by heating it with a gas torch.

Each has its strengths. EPDM is flexible, which suits roofs that move, and with a single sheet it has few seams to fail. GRP gives a rigid, joint-free surface that suits small roofs with awkward shapes, like porches and bay roofs, but it can crack on a deck that flexes. Felt is familiar, relatively quick to lay and easy to repair, though it has more laps and ages faster in direct sun. The right choice depends on the shape, the movement in the structure and how the roof is used. A roof that is walked on to reach a window or a plant needs a tougher surface than one nobody visits, and a roof under overhanging trees will collect leaf litter whichever skin it wears.

Where each one suits

On a large rear extension with a simple rectangle, EPDM is often the natural choice. On a small porch with curves or a bay roof with several angles, GRP can be shaped to fit. Felt remains common on garden buildings and shed roofs, where felt shingles or roll felt are laid in overlapping strips. Green roofs add another layer: a waterproof membrane, then a root barrier, drainage layer and planting medium. They are heavy, so the structure must be checked first.

Felt shed roofs sit at the simple end of the range. A garden building needs a level, dry board surface and a covering lapped from the eaves upwards so that each sheet sheds onto the one below. It is a small job, but the same rule applies as on a house: the laps decide how long it lasts. Whichever membrane goes on, workmanship at the seams and edges matters more than the brand. A membrane laid on a clean, dry, flat deck will last well. One laid in damp weather, over dust or onto a sheet with a lump under it will not. We pick the day and prepare the surface before the first sheet is unrolled.

A fixed price in writing for a flat roof names the membrane, the build-up beneath it and the trims at the edges, so you can compare like with like.

Figures: typical UK ranges, MyBuilder flat roof cost guide, 2026

Our guides to EPDM rubber roofs, GRP fibreglass roofs and torch-on felt roofs go through each covering. If you are weighing two of them, EPDM or GRP compares them directly.

The edge of a flat roof is the last thing to be fitted and the first thing to show wear. Its job is to let water leave the roof cleanly, without running back under the covering or down the face of the building. Most flat roofs use an edge trim, a shaped strip of metal or plastic fixed to the deck edge, over which the membrane is dressed. The trim has a drip, a small lip that kicks water clear of the fascia, so it falls into the gutter and not along the wall.

On a well-built roof the edge trim, the membrane and the gutter work together. The membrane overlaps the trim, the trim overlaps the gutter and the gutter is set at the right height to catch the flow. If the trim is too short, water spills behind the gutter and rots the fascia. If the membrane is not adhered under the trim, wind can lift it and peel it back from the edge.

The details that fail first

Edges fail in a few common ways. A trim that has been nailed through the top surface rather than fixed from the back will let in water at each nail hole. A drip that has bent or snapped no longer throws water clear. Sealant that was used in place of a trim shrinks and splits. And on older extensions, we find the fascia has rotted from years of water running behind it, so the board has to be replaced before the new trim can be fixed.

Verges on a flat roof, the sides that run along the slope rather than towards the gutter, need a different kind of trim. It must stop water running off the side across the wall. Where the roof meets a neighbour's wall, a cover flashing is fitted and agreed so that the joint stays dry without disturbing the other property.

Looks come into it as well, particularly on a house people see from the street or the lane. Colour and finish are options as well. Trims come in a range of finishes, and on a house in a conservation area, a dark, plain trim usually looks right. We match the trim to the existing fascia and guttering where the two meet.

The flat roof problems guide lists the common edge and seam failures, and our gutters, soffits and fascias service covers the boards and guttering that sit beside the trim. For a porch, porch roofs sets out the usual build.