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Roofing Guide · Bath · March 2026
Sarking Felt and Breathable Membrane in Bath
The sarking felt — or roofing underlay — is the secondary waterproofing layer under the tiles or slates on your Bath roof. Most Bath homeowners are unaware of its condition until it fails. Understanding when and why to replace it, and what type to specify, prevents both water ingress and condensation problems that arise from incorrect specification.
Sarking felt is the membrane beneath the tiles that catches any water passing the covering and directs it to the gutter. On older Bath roofs the original bitumen felt has often perished, tearing at the eaves. A re-roof replaces it with a modern breathable membrane that manages moisture without trapping condensation.
Traditional Felt on Bath's Older Properties
The majority of Bath's Victorian and Edwardian housing stock has traditional bitumen sarking felt under the Welsh slate — typically laid during a 20th-century re-roofing that replaced the original close-boarded sarking of the Victorian period. This felt has an expected service life of 30 to 60 years and is now at various stages of its useful life depending on when the re-roofing was done. On properties re-roofed in the 1960s and 1970s, the felt is now 50 to 60 years old and may be brittle or perished in sections.
British Standard BS 5534: British Standard BS 5534 specifies underlay requirements including the support and lap needed at the eaves — the point where perished sarking felt on older Bath roofs most commonly fails.
Signs That Sarking Felt Has Failed
Failed sarking felt shows in the loft space as: large tears or holes visible from below; sections that have detached from the battens and are hanging down; areas of extreme brittleness that crumble when touched; and water staining on the felt surface without obvious active tile failure that would explain the water entry. On steep-pitched Victorian Bath roofs, inspecting the felt from the loft is straightforward and should be part of any condition survey.
The Breathable Membrane Upgrade
Why Breathability Matters in Bath
Bath's valley and canal corridor microclimates create above-average ambient humidity in roof spaces. Traditional bitumen felt, being vapour-impermeable, traps this moisture in the roof void. Combined with modern loft insulation at ceiling level — which reduces the air temperature above the insulation, increasing the dew point — this creates conditions for condensation on the felt surface. Breathable membrane allows moisture vapour to pass through to the atmosphere, substantially reducing condensation risk.
The Building Regulations Position
Current Building Regulations (Approved Document C) require low-resistance vapour-permeable underlay (breathable membrane) on new roofs and re-roofs where the underlay is fixed immediately below the tile or slate without a 25mm ventilated air space above it. For the majority of Bath re-roofing projects — close-tiled or close-slated without an air gap — breathable membrane is the regulatory requirement.
Felt Replacement During Re-Roofing
Sarking felt is replaced as an integrated part of any full re-roofing project in Bath. The tiles or slates are stripped, the felt and battens are removed, the roof structure is inspected and any timber repairs are made, new breathable membrane is installed on new treated battens, and the tiles or slates are re-fixed. The membrane is never an optional extra — it is included as standard in every Bath re-roofing specification.
Questions about sarking felt or roof underlay on your Bath property? Call 0122 528 7924. Free assessment, breathable membrane specified correctly as standard.
The simplest way to assess sarking felt condition on a Bath property is to inspect it from the loft space on a dry day. Look for: areas where the felt has torn and is hanging down between the battens; sections that are extremely brittle — they crumble when gently touched; horizontal staining on the felt surface (indicating water has been running down the outside of the felt rather than over it, suggesting a failed section above); and visible daylight through gaps in the felt that correspond to gaps between slates.
On a damp day or after recent rain, touch the felt surface in different locations. Sound felt should be slightly flexible — old bitumen felt that has aged poorly will be rigid or crumbly. Breathable membrane that is sound should be slightly pliable. In either case, visible moisture on the loft side of the felt on a day when it has not rained recently indicates condensation — which points to a ventilation issue rather than a felt failure.
Planning Felt Replacement in Bath
Sarking felt is replaced as part of a full re-roof — it cannot be replaced in isolation without stripping the tiles or slates. If your Bath survey identifies failed felt but the tiles or slates themselves are in reasonable condition, the options are: proceed with a full re-roof now (new breathable membrane, new battens, and tiles or slates re-used or replaced); defer until tiles or slates also require attention (monitor the felt and repair any active leaks as they develop); or temporary internal repairs where active ingress through failed felt is occurring. We advise honestly on which approach makes financial sense for each specific Bath property. Free assessment on 0122 528 7924.
Felt Replacement on Bath Victorian Properties — The Conservation Consideration
When a Victorian Welsh slate roof is re-felted during a full re-roof in Bath, the specification of the replacement underlay has planning implications on conservation area and listed properties. Traditional bitumen felt is vapour-impermeable and has been the standard underlay on Victorian roofs for over a century. Modern breathable felt is technically superior for moisture management but its installation constitutes a material change that, on a listed building, may technically require listed building consent.
In practice, Bath & North East Somerset Council generally accepts breathable felt as a like-for-like improvement in the context of a full re-roofing project on listed properties, provided the external roof covering remains unchanged. The key is that the change is internal and not visible from the public realm. We advise on the consent position for any material changes during the survey stage.
Modern Sarking Felt — What the Products Mean
The sarking felt market now contains products ranging from basic polypropylene breathable membranes to high-performance vapour-permeable underlays with significant differences in performance. For Bath period property re-roofing, we specify a mid-range breathable felt with a vapour resistance appropriate to the specific roof construction — the correct specification depends on whether the property has loft insulation at rafter level or at ceiling level, and whether the roof void is intended to be warm or cold. We explain the felt specification in writing in every re-roofing quote. Call 0122 528 7924 to arrange a survey.
Free Re-Roofing Survey in Bath — How to Book
We cover Bath and all surrounding areas for re-roofing survey work across the full BA1, BA2, BA3, BS31, BA11, BA14, BA15, SN12, SN13 and SN14 postcode areas. Free assessments within 2 to 5 working days. Emergency attendance within 60 to 90 minutes for Bath city addresses. All work quoted in writing before starting. 10-year workmanship guarantee. No call-out fee on any visit.
Every Bath re-roofing project includes a sarking felt specification matched to the specific roof construction and ventilation regime. The felt specification is explained in the written quote before any work starts.
Call 0122 528 7924 to arrange a free assessment. Our team will confirm availability immediately and give you an honest assessment of what is needed — no upselling, no pressure, no obligation to proceed. If repair is the right answer we will say so. If replacement is the right answer we will explain why.
We cover the full Bath area and surrounding Somerset and Wiltshire for all the topics covered in this guide. Free assessment visits available within 2 to 5 working days. Every assessment includes a written condition report and a fixed-price quote before any work starts. NFRC registered. Fully insured. 10-year workmanship guarantee. No call-out fee. Call 0122 528 7924 to arrange. We answer 24 hours a day for emergencies and during working hours for assessments and general enquiries. Our team will give you a straight answer on whether repair, maintenance, or replacement is the right approach for your specific property — no upselling, no scare tactics, no inflated quotes. Bath's housing stock is what we know, and giving honest advice on it is what builds the long-term customer relationships that sustain our business.
FAQ
Frequently Asked Questions
Sarking felt (or roofing underlay) is the secondary waterproofing layer installed between the battens and the roof structure. It catches any water that penetrates the primary tile or slate layer and channels it to the eaves. Building Regulations require it on new roofs and re-roofs in the UK.
Sarking felt is replaced as part of a full re-roof. If the felt has failed (torn, perished, or completely brittle) without the roof being re-tiled, it may be possible to replace sections through the loft space — though this is not always achievable on traditional close-boarded roofs. The correct time to replace felt is at the next full re-roofing opportunity.
Traditional bitumen felt is vapour-impermeable — it stops moisture from passing in either direction. Breathable membrane (vapour-permeable underlay) allows moisture vapour to pass from the roof void to the atmosphere above, reducing condensation risk in cold roof constructions. Breathable membrane is the current Building Regulations specification for new installations.
Yes. Breathable membrane reduces condensation risk in cold roof spaces — a significant issue in Bath's humid valley locations. When re-roofing a Bath Victorian terrace with added loft insulation, breathable membrane is the correct specification.
Yes. Breathable membrane is not designed to be permanently submerged. A blocked valley or outlet that causes standing water on the membrane will eventually cause it to fail through prolonged saturation. Keeping outlets clear protects both the membrane and the tile/slate covering above.
Breathable membrane costs approximately 15 to 25 percent more than standard underlay. For a typical 3-bed Bath semi, the additional cost is £150 to £300. The condensation risk reduction benefit makes it the correct specification for Bath re-roofing projects.