Old bitumen felt and modern breathable underlay are not the same thing. Here is what the difference means for your Winchester roof and why it matters.
If your Winchester property was built before 1990 and still has its original roof, it almost certainly has bitumen-based felt underlay — the black, tar-impregnated membrane that was standard for decades. Any re-roof carried out today uses breathable underlay instead. Here is what the difference means in practice.
Bitumen felt is impermeable — water vapour cannot pass through it. In a roof space above a heated house, warm moist air rises from the living spaces and hits the cold underside of the felt. It condenses. Over time, this condensation saturates insulation, rots timber and eventually leads to structural damage.
The traditional solution was ventilation: leaving a gap at the eaves and ridge to allow air circulation under the felt and carry the moisture away. This works, but it requires correctly sized ventilation paths throughout the roof space — something that gets blocked by insulation, disrupted by loft conversions and frequently absent on Winchester's older housing stock.
Breathable roofing membrane is vapour-permeable: water vapour (gas) can pass through the membrane from inside to outside, while liquid water (rain) cannot pass through in the other direction. This means condensation in the roof space dissipates through the underlay naturally without needing a ventilated airspace beneath it.
The practical benefit: no eaves ventilators needed, no ridge ventilation calculation, no risk of the ventilation path being blocked by insulation. The roof space remains dry through the management of vapour rather than airflow.
Breathable underlay makes loft conversions significantly more straightforward. With bitumen felt, loft insulation laid against the underlay blocks the ventilation path and causes condensation. Breathable underlay allows insulation to be laid directly against the membrane without creating a condensation problem.
Breathable underlays have been available since the late 1980s and became the standard specification on new buildings from around 1995 onwards. Any Winchester property re-roofed since 2000 should have breathable underlay. Any property re-roofed before 1990 almost certainly has bitumen felt, now 35+ years old and approaching or past its design life.
Bitumen felt has a design life of 30–40 years. After this, it becomes brittle, cracks at the laps and can decompose entirely in the zones of greatest thermal movement — typically at the ridge, eaves and around penetrations. On Winchester's post-war estate properties in Stanmore, Winnall and Harestock, the bitumen felt installed in the 1960s–70s is now 50–70 years old and has typically decomposed entirely. When we strip these roofs for re-roofing, intact felt is the exception, not the rule.
Yes — Part L and Part C of the building regulations require new roofs to meet current specification standards, which include breathable underlay. All re-roofs we carry out in Winchester use BS 5534-compliant breathable membrane as standard. This is not a premium option — it is the baseline.
Winchester re-roof with correct modern specification
Breathable underlay, stainless fixings, flexible mortar ridge bedding as standard on every re-roof. Free survey and fixed quote.
📞 01962 809171Victorian and Edwardian Winchester properties were built without underlay at all — the tiles were laid direct to battens. Underlay was added retrospectively when roofs were re-roofed in the 20th century, typically with bitumen felt. When these properties are re-roofed today, breathable underlay is the correct modern specification.
There is one nuance for Winchester conservation area and listed buildings: breathable underlay is installed on the cold side of the roof (between tiles and rafters). In whole-house retrofit insulation projects where insulation is installed between the rafters, the underlay position changes. We advise on the correct specification for each Winchester property at survey stage.
On most Winchester post-war properties we re-roof, the original bitumen felt has decomposed entirely. What we find is a dark powdery residue on the battens rather than an intact membrane. On properties from the 1960s-70s, the felt has typically been gone for 10-20 years — the tiles alone have been keeping the weather out. This is both more common and less immediately catastrophic than many homeowners expect — a good clay tile roof provides significant weather protection even without felt.
The practical implication: do not panic if a survey tells you your felt has failed. It is a reason to plan a re-roof, not an emergency. Call 01962 809171 to discuss the right timing for your Winchester property.
Whatever roofing work your Winchester property needs, these are the mistakes we see most frequently from homeowners who have been through a bad experience:
Accepting a verbal quote. Any job over £300 should have a written, itemised quote. Verbal quotes are unenforceable and tend to grow significantly by the time the invoice arrives.
Paying the full amount upfront. A deposit of 10–25% is standard. Full payment before work is complete removes all your leverage if the work is unsatisfactory.
Not checking NFRC registration. A roofer claiming NFRC membership who is not listed at nfrc.co.uk is not registered. Takes 30 seconds to check.
Choosing on price alone. Three quotes where one is 35% below the others is not a bargain — it is a signal that scaffolding, waste disposal or material quality is being cut from the scope.
Not asking about mortar specification. For Winchester period properties, the wrong mortar causes brickwork damage. Always ask: lime mortar or modified cement?
The next step after reading any roofing guide is a free professional survey. We cover all of Winchester and the surrounding Hampshire area — Eastleigh, Romsey, Chandler's Ford, Bishop's Waltham, New Alresford, Kings Worthy, Twyford, Colden Common, Otterbourne and Compton.
Call 01962 809171 or email info@winchesterroofers.co.uk. Free survey. Written condition report. Same-week appointments. No call-out fee. No obligation.
Can I see if my Winchester property has breathable felt by looking in the loft? Yes — breathable polypropylene underlay is typically white or light grey and translucent. Bitumen felt is black and opaque. If you can see through the underlay to the batten shadows when you hold a torch to it from below, it is breathable. If it is uniformly opaque and black, it is bitumen felt.
Does breathable felt require any special installation for Winchester lofts? No special requirements beyond correct lapping and correct batten gauge. The breathable function works in any correctly installed roof without requiring airspace above or below the membrane.
Will my insurer care what type of underlay is on my Winchester property? Some insurers ask about roof condition and materials on high-value properties. A roof with breathable underlay installed in the last 20 years is in better condition than one with 50-year-old bitumen felt and will typically be assessed more favourably. A re-roof with breathable underlay can improve your EPC rating and reduce insurance risk profile simultaneously.
What happens to breathable felt in Winchester's climate? Modern polypropylene breathable underlay is UV-stabilised and designed for 40–60+ year service life. Winchester's climate — temperate, moderate humidity, low UV extremes — is well within the design parameters. There is no special Winchester-specific consideration for breathable underlay performance.
Breathable roofing underlay (also called breathable membrane or vapour-permeable underlay) is a modern polypropylene underlay that allows water vapour to pass through from inside to outside while remaining waterproof against liquid water. It manages roof space condensation without separate ventilation requirements.
Yes for modern installations. Breathable underlay manages condensation without ventilation gaps, carries a longer design life (40–60+ years versus 30–40 for bitumen), is lighter and easier to install, and is the current BS 5534 standard. Bitumen felt is no longer the correct specification for new installations.
Any full re-roof we carry out in Winchester uses breathable underlay as standard. It is the current BS 5534 specification and is required under building regulations for new roofs.
Yes — that is the key advantage over bitumen felt. Breathable underlay allows vapour movement through the membrane itself, so the additional tile ventilators or ridge ventilation required with impermeable bitumen felt are not needed.
40–60+ years for modern polypropylene breathable underlay under normal conditions. This significantly exceeds the lifespan of bitumen-based felt and is designed to outlast the tile battens and often the tiles themselves.
No call-out fees. No obligation. An honest quote from a local roofer.
01962 809171