If you are planning a flat roof replacement or repair in Bury St Edmunds, knowing what a properly run job looks like from start to finish helps you judge whether the work is being done right.
Call any time. Calls are answered day and night and passed straight to our roofers.
Flat roofing is specialist work - the sequence of the job, the deck preparation and the edge detailing all matter as much as the membrane material itself.
This page walks through what a flat roofing job should involve at every stage, so you know what to expect and what to ask about before any work begins.
A proper flat roof survey is not a two-minute look from ground level. The roofer needs to get up onto the roof surface and inspect it methodically. On an existing roof, that means checking the membrane condition across the full field, at every upstand, around any roof lights or penetrations, and at the gutter or outlet edge.
The deck below the membrane matters just as much as the surface. Any soft or springy areas underfoot suggest the plywood or OSB decking has absorbed water and begun to deteriorate. A thorough inspection will identify whether the deck is sound enough to re-membrane over, or whether it needs to come off entirely. Skipping this step is how roofers end up laying a new membrane over a deck that fails within two years.
The fall of the existing roof should also be assessed. The minimum acceptable fall for a flat roof is 1 in 80, which is enough to move rainwater reliably to a gutter or outlet. A roof that ponds water persistently needs the fall to be addressed, not just a new membrane.
On a replacement job, the old membrane comes off first. On a felt roof, this typically means stripping back to the deck boards using scrapers and hand tools. The deck boards are then inspected individually. A board that has delaminated, that shows dark staining from prolonged wetting, or that feels soft underfoot needs to come out and be replaced with a new exterior-grade plywood board of the correct thickness - at least 18mm for most domestic flat roofs.
All the old edge trims, upstand flashings and any lead details at the junction with the adjoining wall are removed at this stage too. Reusing old lead or edge trims under a new membrane is poor practice. The deck should be clean and dry before anything new goes down.
Once the deck is stripped, any replacement boards are fixed. The fall is checked using a spirit level or a long straightedge. If the fall is insufficient, it can be created using tapered insulation boards laid over the deck. These boards are supplied with a built-in slope and allow a new fall to be introduced without rebuilding the roof structure.
Any penetrations through the deck - for soil pipes, extraction vents or roof lights - are checked and any damaged trimmers around them are replaced. Roof lights that are cracked or failing are best replaced at this stage rather than worked around, since the detail of dressing the membrane up to a roof light frame is one of the more technically demanding parts of the job.
On a new installation or a full replacement, insulation should be considered seriously. Building regulations require a minimum level of thermal performance for replacement flat roofs on heated spaces. The warm roof build-up - rigid insulation boards laid on top of the deck, below the membrane - is the most reliable approach. It keeps the deck warm, above the dew point, which prevents condensation forming on the underside of the boards and rotting them from below.
Insulation board thickness depends on the U-value target, but 100mm to 150mm of rigid PIR board is typical on a domestic flat roof. The insulation boards are laid in two offset layers where possible to eliminate thermal bridging at the joints.
For an EPDM rubber installation, the membrane sheet is cut to size, folded back and the deck surface is primed and adhesive-coated in sections. The rubber is then carefully laid into the adhesive, rolling out any air bubbles as it goes. The field of the roof needs to be flat and fully bonded - any unbonded areas will lift at the edges and allow water underneath over time.
For a torch-on felt system, a base layer is fixed to the deck first - either by mechanical fixings or by torching - and then the cap sheet is torched on top, with laps of at least 75mm overlapped and fully fused. Each lap needs to be checked for full bond; a lap that is stuck at the edge but not fully bonded across its width will let in water as soon as the felt moves in cold weather.
The edges of a flat roof are where most leaks start. On both EPDM and felt roofs, the membrane must be dressed tightly over a drip-edge trim at the gutter and bonded securely at every upstand where the roof meets a wall or parapet. On an EPDM roof, upstands are finished with bonded strip and cover tape. On a felt roof, the upstand is torched in place and then covered with a separate lead flashing that is cut into a chase in the masonry.
Lead flashing at upstands should not be fixed with mastic alone. The lead needs to be wedged or clipped into the chase and then pointed with a suitable mortar or lead sealant. A mastic-only fix is a temporary measure and will split within a few years as the lead moves with temperature changes.
The outlet or gutter must be at the lowest point of the finished roof surface. On a re-roofing job where insulation is being added, the finished surface level rises, and the outlet or gutter height must be adjusted to match. If the gutter ends up sitting below the finished surface, water will pond at the edge. This is a detail that is worth discussing with your roofer at the survey stage so there are no surprises during the job.
Flat roof outlets - the through-deck drains used on roofs that drain internally rather than to a perimeter gutter - need to be fitted with compatible collars that bond into the membrane. A loose or poorly bonded outlet collar is a reliable source of leaks.
A good roofer will walk the finished roof before packing up. On an EPDM installation this means checking every bonded edge, every seam and every upstand detail by hand - pressing along the bond line to feel for unbonded sections. On a felt roof it means checking every lap for full fusion and looking along the surface for any bridging at board joints.
All stripped material - old felt, off-cuts of board, packaging - should leave the site with the roofer. Gutters should be cleared of any debris from the job. If lead has been cut in at upstands, the pointing should be checked and any mess on the wall face cleaned off.
Before any roofer starts work on your property, confirm how they plan to handle the deck inspection, what their approach is to fall correction, how edge and upstand details will be finished and what drainage adjustments will be made if insulation is being added. A roofer who can answer those questions clearly is one who understands the job properly.
Call Mildenhall Roofers to arrange a flat roof survey and a clear description of what the work will involve on your specific roof.
No obligation · We'll call you to arrange a free visit
Call any time. Calls are answered day and night and passed straight to our roofers.
