If you are having a flat roof installed or replaced in Necton, here is exactly what the job involves from the initial inspection through to the final tidy-up.
Before any work is priced or planned, the roof needs to be assessed properly. A roofer should physically get up onto the surface - not estimate the job from the ground or from photos alone. On the roof, the key things to check are the condition of the existing membrane, the state of the deck beneath it, how well the current falls drain, and the detailing at every edge, wall, parapet and pipe penetration.
The deck inspection is particularly important. Pressing down on the surface reveals whether the timber beneath is firm and dry or has become soft and spongy through water ingress. A deck that sounds hollow or gives underfoot has usually been wet for long enough to begin rotting, and those sections need to come out before any new membrane is laid. Missing this at the inspection stage means the new roof will fail in the same spots.
The inspector should also check the falls. A flat roof should drain at a minimum gradient of 1:80 - roughly 12mm of drop per metre of roof width. If the falls are inadequate, simply laying a new membrane over the old surface will not prevent ponding. Tapered insulation or a structural adjustment may be needed to correct this, and that affects both the cost and the programme of work.
After inspecting the roof, a roofer should be able to tell you clearly whether a repair is sufficient or whether a full replacement is the better course of action, and explain why.
On a full replacement, the first physical task on site is stripping back the existing covering. On a felt roof, this means pulling up the layers of membrane and the timber battens or bonding beneath them. On a single-ply system, the old membrane is cut away. The stripping generates waste material that needs to be removed from site - this should be part of what is included in the price, not an extra.
Once the old membrane is off, the deck is exposed and can be assessed properly for the first time. Even if the deck looked reasonable from the surface, stripping sometimes reveals isolated patches of softness or rot that were not visible from above. Any compromised sections of decking board are cut out and replaced at this stage. Using tongue-and-groove oriented strand board (OSB) or marine-grade plywood is standard practice for a new flat roof deck. These materials are dimensionally stable and bond well with membrane adhesives.
The deck must be swept clean and allowed to dry before the next stage begins. Laying insulation or membrane over a damp or dusty deck is one of the main causes of early adhesion failure, blistering, and delamination in the years after installation.
For a warm roof construction - which is now the standard best-practice approach and is required by building regulations for a full replacement - rigid insulation boards are fixed to the top of the deck before the membrane goes on. This places the insulation above the structural deck, keeping the timber warm and above the temperature at which condensation forms within the structure.
The most commonly used boards are PIR (polyisocyanurate), which offer good thermal performance at a manageable thickness. The depth of board required depends on the target U-value, but for current building regulations compliance the insulation typically needs to achieve a U-value of 0.18 W/m2K or better. The boards are cut to fit precisely around any penetrations, upstands and perimeter details, because gaps in the insulation layer create thermal bridges and potential points of weakness in the membrane above.
Where the existing roof has inadequate falls, tapered insulation boards can introduce the necessary slope while also providing the thermal layer. This approach is more expensive than flat boards but avoids the need for structural alterations to create drainage falls.
The membrane stage is where the system type makes the most difference to the installation process. The two main systems used on domestic flat roofs today are EPDM rubber and torch-on felt.
For an EPDM installation, a primer is applied to the insulation surface and left to cure to the correct tack. The rubber sheet is then unrolled, positioned with an even overlap at every edge, and bonded down with contact adhesive. Larger roofs may use two or more sheets joined with a seam tape system. Every seam and every edge must be pressed out carefully to avoid air pockets. The membrane should turn up at least 150mm at every upstand and wall - this is non-negotiable for a watertight finish.
For a torch-on felt installation, the layers of bitumen felt are progressively heat-bonded using a gas torch. The number of layers varies depending on the specification - a high-quality installation typically uses three layers, with the final capsheet carrying the mineral-granule finish. The torch work requires care and controlled heat to bond properly without damaging the substrate.
In either case, the edges of the roof are finished with a metal drip trim or a matching fascia trim to carry water clear of the fascia board beneath. This detail prevents the fascia from rotting and gives the roof a neat, finished edge.
The upstand and flashing details are where flat roofs most often fail - and where the quality of the workmanship is most visible to a trained eye. Wherever the flat roof meets a vertical surface - a house wall, a parapet, a chimney stack, a soil pipe or a roof light curb - the membrane must be taken up the vertical face and terminated securely.
On an EPDM roof, the rubber is bonded up the upstand face using compatible adhesive and terminated with a mechanically fixed bar and capping strip, or dressed into a chase cut into the mortar joint of a brick wall. On a felt roof, the upstand is usually finished with a lead flashing dressed into a raked-out mortar joint and pointed back in. Lead should be in short lengths, lapped correctly, and fixed to allow thermal movement - long, rigid runs of lead will crack over time.
Any pipe penetrations through the membrane - soil vent pipes, extraction ducts, drainage outlets - should be dressed with purpose-made rubber pipe boots or lead soakers, not simply sealed with mastic. Mastic-only seals around pipe penetrations are a known failure point on older roofs.
Rainwater needs a clear path off the roof. On most domestic flat roofs, this means either a flat roof outlet through the deck (which connects to a downpipe below) or a flush weir outlet at the edge that spills into a gutter. Both types need to be set at the lowest point of the fall, and there should be at least one outlet per drainage area.
Outlets are set before the final membrane is laid and are bonded into the system as part of the waterproofing layer. A gutter that discharges onto the flat roof surface, or an outlet that sits proud of the membrane rather than flush with it, is an installation defect that will cause ponding.
Once the membrane, trims, flashings and outlets are complete, the roofer should carry out a final walkover to check the whole surface. This includes checking every edge is fully adhered, every upstand is properly terminated, every outlet is clear, and there are no punctures or unintended holes in the membrane surface.
All waste materials - stripped membrane, old timber, packaging - should be removed from site. The gutters and any surrounding areas affected by the work should be cleared of debris before the job is signed off.
If you are having a flat roof installed or replaced in Necton, call Dereham Roofers to arrange an inspection and get a clear account of what the job will involve for your specific roof.
No obligation · We usually respond within 2 hours
Storm damage, active leak, damaged ridge - we aim to be on-site within 24 hours.
