If you need a new or replacement flat roof in Harleston, choosing the right system for your property and budget is the most important decision you will make before work starts.
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Diss Roofers install and repair flat roofs for homeowners in Harleston and the surrounding villages. Extensions, garages, outbuildings and bay window roofs all fall within the scope of flat roofing work, and each one benefits from choosing the right system from the outset. This page sets out the materials available for domestic flat roofs, what each one involves in practice, and the honest pros and cons of each - so you can have a genuinely informed conversation with any roofer you speak to.
Flat roofs have improved considerably in the last two decades. The choice is no longer simply between one type of felt and another. Understanding the options means you are less likely to end up with a system that underperforms or that needs replacing again in ten years.
For most domestic flat roofs in the UK today, the practical choice comes down to two systems: built-up felt and EPDM rubber membrane. Each has its place, and neither is universally right for every job. What matters is matching the system to the specific roof, the budget and the expected lifespan.
Built-up felt has been used on flat roofs for well over a century. The system uses layers of bitumen-impregnated felt, typically two or three, torched or bonded together to form a waterproof membrane. The top layer - the cap sheet - has a mineral finish that protects the bitumen from UV degradation and gives the surface its characteristic texture.
Modern torch-on felt (sometimes called high-performance felt or SBS-modified felt) is a significant improvement on the older cold-applied felt systems used on many properties built in the 1970s and 1980s. The modified bitumen is more flexible and more durable. If you have an older property with an existing felt roof, the material under there may be nothing like the specification available today.
Built-up felt is not a poor choice. It is a well-established system that, properly installed with good detailing, gives reliable performance. The honest limitation is longevity: if you are replacing a flat roof on an extension you intend to keep for 30 years, you should understand that a felt roof will likely need replacing again within that period.
EPDM stands for ethylene propylene diene monomer - a synthetic rubber that has been used in flat roofing for several decades. In domestic roofing it is usually supplied and installed as a single large sheet, bonded to the deck with a contact adhesive. Because the main field of the roof is a single piece with no mid-field seams, many of the most common failure points of traditional felt systems are eliminated at source.
EPDM remains flexible across a very wide temperature range - from around minus 40 to plus 130 degrees Celsius - which means it moves with the structure rather than cracking under thermal stress. This is a significant practical advantage in the UK, where roof surface temperatures swing considerably between summer and winter.
There is no single correct answer. For a straightforward garage roof or simple extension where the budget is tight and you want a reliable system, built-up felt is a sensible choice - provided the installation specification is good and the detailing at edges and outlets is done properly. For a roof you want to last as long as possible without revisiting, and where the upfront cost is manageable, EPDM is the stronger long-term investment.
A useful way to think about it: a good felt roof on a 20-square-metre garage extension will cost roughly £1,200 to £1,800 installed. EPDM on the same roof would cost roughly £1,600 to £2,400. The difference in upfront cost is real, but so is the difference in expected service life.
The membrane is only part of the story. The deck it sits on - usually plywood or oriented strand board (OSB) on modern roofs, or timber boarding on older ones - must be sound, correctly designed and properly prepared before any membrane goes down. No membrane will perform well on a soft, rotted or deflecting deck.
When replacing a flat roof, any soft or damaged deck boards must be replaced before the new membrane is laid. On older roofs, it is common to find at least some boards that need attention, especially around outlets where water has been sitting. Ask the roofer to confirm the condition of the deck once the old membrane is stripped, before committing to the membrane installation cost.
A flat roof is not truly flat - it has a designed slope (fall) to encourage water to run to the outlet. Building Regulations and good practice both require a minimum fall of 1:80, and in practice a fall of 1:40 or more gives much better drainage and reduces the risk of ponding. If the fall is insufficient, water sits and puts constant pressure on any weakness in the membrane.
Many flat roof failures on older properties trace back to insufficient fall built into the original construction, not to the membrane itself. When replacing a flat roof, it is worth asking whether the fall can be corrected - either by adjusting the deck with firring pieces (tapered timber strips that create a slope) or by repositioning the outlet to the genuine low point of the roof. This is not always possible, but where it is, it significantly improves long-term performance regardless of which membrane is used.
Older flat roofs are commonly built as cold roofs - the insulation sits between the joists below the deck, and the deck itself gets cold. This increases the risk of interstitial condensation within the structure, which over time can cause the deck to rot from beneath without any visible signs from above.
A warm roof places the insulation above the deck, keeping the deck at room temperature and largely eliminating the condensation risk. When replacing a flat roof entirely, upgrading to a warm roof construction is widely considered the better approach for long-term performance. The insulation boards add cost - typically £15 to £30 per square metre depending on thickness - but they also improve the thermal performance of the room or space below, which may be relevant for a habitable extension.
If you are replacing a flat roof on a room that is lived in or regularly heated, the warm roof approach is particularly worth discussing with the roofer. Building Regulations may also require insulation to meet current standards when the roof is being replaced.
Many flat roofs include rooflights, and some have pipes, vents or flues passing through them. Every penetration is a potential weak point - water finds its way around any detail that has not been correctly sealed. When replacing a flat roof, any existing rooflights should be assessed: are they in good condition and worth retaining, or would replacement make sense while the membrane is already off? Sealing around a rooflight is a significant part of the installation cost, so combining rooflight replacement with a new membrane makes practical sense if the lights are old or leaking.
Before agreeing to any flat roofing work, it is worth being clear on the following:
Getting clear answers to these questions before work begins means you know what you are paying for and can compare quotes on a like-for-like basis.
Not every flat roof problem requires full replacement. If the membrane is fundamentally sound but has a localised failure - a split at an upstand, cracked edge trim, or a section damaged by impact - a targeted repair with compatible materials can extend the roof's life at a fraction of replacement cost. A split or open lap that is caught early is a much simpler job than water that has been tracking into the deck for two years.
Where a repair is the right answer, the work should use materials compatible with the existing membrane. Patching felt with EPDM tape, or sealing an EPDM roof with bitumen products, rarely holds well long-term. Any repair should also address why the failure happened - if an upstand split because the membrane had lost flexibility, the same problem will reappear nearby within a few years unless the whole detail is addressed properly.
If you are weighing up repair versus replacement, or trying to decide between felt and EPDM, call us for a flat roof survey and we will give you a straight assessment of what the roof needs and a clear price for the work. We cover Harleston and the villages around it, and we will tell you honestly which system makes sense for your particular roof rather than simply quoting the cheaper option.
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Call any time. Calls are answered day and night and passed straight to our roofers.
