Why Roofers Still Favour Bitumen Felt for Flat Roofs
Modern reinforced bitumen membrane, usually called torch-on felt, remains a trusted covering.
Unlike basic shed felt, quality systems normally use two or three reinforced layers, creating a durable, repairable surface.
How is it installed?
The deck must be sound, dry, stable and formed to suitable falls. On a warm roof, an air and vapour control layer, insulation and underlayer are installed before the mineral-finished cap sheet. The rolls are measured and cut with a roofing knife into manageable lengths, positioned and allowed to settle. Depending on the system, layers are bonded by controlled torching, self-adhesion, hot air or another approved method.
Joints are staggered between layers; side and end laps are fully bonded and sealed to the required width. Finished laps should lie flat without wrinkles, voids or open edges. A small, even bitumen bead is normally visible along torch-welded laps. Torch application requires fire-risk assessment and Safe2Torch precautions.
Why is felt useful around rooflights?
Felt can be cut into accurately sized strips and separate corner pieces. This allows it to be dressed around rooflight kerbs, outlets, pipes, parapets and other awkward shapes. At rooflights, the material is taken from the main covering up the kerb, normally to a minimum 150mm upstand where the design permits, with reinforced corners and a protected upper edge. This flexibility suits altered and irregular roofs. However, excessive folding can create thick ridges and concealed voids. Corners should be formed using the manufacturer’s recognised detailing method rather than simply bunching the material.
Lifespan and why our roof inspection matters
In practice, many domestic bitumen-felt roofs provide a service life of approximately 10–20 years.
Higher-quality professionally installed multi-layer systems can last 25–35 years or longer, but these figures should not be assumed for every roof.
The actual lifespan depends heavily on the original specification, quality of installation, roof exposure and ongoing maintenance. Repeated wetting and drying, frost, summer heat, ultraviolet exposure, thermal movement, foot traffic and standing water can gradually weaken the covering. Poor falls, blocked outlets, inadequate bonding, open laps and badly formed details around rooflights, parapets and other upstands can result in much earlier failure.
During our RICS Surveys, we do more than simply identify the covering as bitumen felt. We assess the visible surface condition, falls and drainage, side and end laps, folds, corners, upstands, flashings, outlets, perimeter trims and detailing around rooflights, pipes, chimneys and parapets. We look for open laps, “fishmouths”, blistering, splitting, surface cracking, mineral loss, exposed reinforcement, ponding water and repeated patch repairs. Accessible internal areas are also checked for visible staining or other evidence of possible water penetration.
For RICS Level 3 Building Surveys only, and where it is safe, practical and permitted by the conditions on the day, we may use a drone to assist with viewing roof areas that cannot be adequately seen from ground level or nearby windows.
Our report explains whether the covering appears serviceable, requires maintenance or repair, or shows signs that renewal may need to be budgeted for. Where concerns remain, we recommend appropriate further investigation by a competent roofing contractor before purchase, helping buyers understand the likely risks and costs before committing.
What are the alternatives?
No single material is best for every flat roof. The correct choice depends on the roof shape, supporting structure, intended use, budget and quality of the available installer.
EPDM rubber is flexible and commonly supplied in large sheets, reducing the number of joints. It can accommodate movement and may last 30–50 years or longer. However, it can be punctured by sharp objects, and its performance depends heavily on the adhesive seams, perimeter edges, outlets and upstand details.
PVC or FPO single-ply membranes are lightweight, quick to install and joined using heat-welded seams. They are suitable for larger or more complicated roofs and can provide around 30–40 years of service. However, the relatively thin membrane requires puncture protection, while seams, fixings and junctions must be completed by a trained specialist.
GRP fibreglass creates a hard, seamless and visually neat covering that is particularly suitable for smaller, simply shaped roofs. It commonly lasts around 20–30 years. Its main disadvantage is rigidity: the deck must be stable, with movement properly accommodated. Poor weather during installation, incorrect resin mixing or inadequate preparation can result in cracking or early failure.
Liquid-applied systems create a seamless membrane and are especially useful around rooflights, pipes, outlets and other complex details. They can also be suitable for overlaying an existing covering. Their success depends on thorough preparation, compatible primers, correct reinforcement, accurate mixing and the specified application thickness.
Zinc roofing provides a traditional, high-quality appearance and, when correctly designed and installed, may last 60–100 years. It is more expensive and requires a specialist metal roofer. The roof must have suitable falls, support, ventilation and allowance for thermal movement. Standing seams, folded joints, welts, corners and junctions must be formed with considerable precision. Even a small defect in a fold or seam can create a route for water or restrict movement, potentially causing distortion, splitting and leakage.
A genuinely local firm
At Tollington Surveyors, we are a genuinely local firm — based here, working here and familiar with the buildings, roads and communities that make this part of London so distinctive. We cover Highgate, Hampstead, Finsbury Park, Crouch End, Tottenham, Muswell Hill, Islington, Clissold Park, Haringey, Camden and the surrounding neighbourhoods. Our local knowledge means our guidance is relevant, practical and tailored to the homes we see every day.
Contact us today to arrange a RICS HomeBuyer or Building Survey — our process is straightforward, our advice is independent, and you will always deal directly with a qualified local surveyor.
Tollington Surveyors is a small, locally-focused surveying company based in Stroud Green, and dedicated to serving the Haringey area. Learn more >
