Breathable Roofing Felt: What It Is and Why Your Warwickshire Home Needs It

The felt underlay beneath your roof tiles is invisible from the outside and rarely discussed. Most homeowners have no idea what type is under their roof, or that there are two fundamentally different categories with very different implications for how the structure manages moisture. This matters more in the Avon Valley around Stratford than in drier parts of the country.

Non-Breathable Felt: The Old Standard

Traditional BS 747 bitumen felt — used on almost every UK roof built before the early 1990s — is fully waterproof and fully impermeable to water vapour. This means warm, moist air rising from the living spaces below cannot pass through the felt. If there are no ventilation openings at the eaves and ridge to exhaust this moisture, it condenses on the cold underside of the felt. Over time this condensation accumulates in the roof void, causing batten rot, rafter degradation and mould in the loft insulation.

For non-breathable felt to work correctly, the roof needs adequate cross-ventilation — a 25mm continuous gap at the eaves across both sides and ventilated ridge tiles. Many properties built in the 1960s–80s have this ventilation partially blocked by insulation pushed too close to the eaves, or by later alterations that closed original ventilation gaps.

BS 5250 covers moisture control and the role of breathable underlays in ventilating cold roof spaces, which modern re-roofing specifications rely on to prevent condensation.

— BS 5250: Management of moisture in buildings

Breathable Felt: The Modern Specification

Vapour-permeable underlay (breathable felt) allows water vapour to pass through from the warm interior to the exterior while remaining waterproof against liquid water from above. This eliminates the need for deliberate cross-ventilation of the roof void, because moisture migrates through the felt rather than accumulating against it. Modern breathable underlays are rated by their vapour resistance (low-resistance HR or LR types) and can be used with or without the traditional eaves ventilation gap.

Why This Matters More in the Avon Valley

Stratford-upon-Avon's position in the Avon Valley creates elevated ambient humidity year-round compared to higher-ground Warwickshire towns like Harbury or Kineton. This additional moisture loading makes the ventilation performance of non-breathable felt underlays more critical — and the consequences of inadequate ventilation more severe. Roof void condensation problems are consistently more common in valley-floor properties than in equivalent properties at higher elevations.

If Your Roof Is Being Stripped, Always Upgrade

The most cost-effective time to upgrade from non-breathable to breathable felt is when the roof is being stripped for any other reason. The additional cost of breathable underlay versus like-for-like felt replacement on a standard Stratford semi is typically £150 to £300. This buys you 25 to 30 years of moisture management without deliberate ventilation management. It is one of the best-value upgrades available in any re-roof. We specify breathable underlay on every re-roof we carry out as the default.

Signs Your Existing Felt May Have Failed

• Crumbling, powdery or easily torn material visible in the loft on the underside of the roof
• Occasional damp entry during heavy driving rain that does not occur in normal rain (wind-driven water penetrating failed eaves felt)
• Condensation staining on the underside of the felt visible in the loft
• Mould growth on loft timbers or insulation not attributable to a specific leak

Failed felt is one of the three primary indicators we use at every survey to recommend re-roofing rather than repair. Once the secondary waterproofing has failed, the tile lap is the only line of defence in heavy driving rain — which is insufficient for most UK exposure conditions.