Roof Felt Underlay Showing Signs of Failure: Diagnosis Guide
The felt (underlay) under your roof tiles is the second line of defence, when wind-driven rain or snow gets past the tiles, it should run down the felt to the gutter. When that felt has perished, every minor tile leak becomes a ceiling stain. This guide covers how to diagnose felt failure and what your options are.
Signs of Felt Failure
Old bitumen felt sags as it ages. Mild sag is normal; pronounced drooping with visible "swags" between rafters means the felt is structurally tired. Felt that snaps or crumbles when touched is past its life, healthy felt should flex slightly when gently tested at the eaves.
Look at the felt where it crosses over rafters, at chimney junctions, around vent pipes, and at ridge level. Splits at these stress points are often the first failures. Dark patches on the felt underside indicate water has been getting through. Any hole big enough to see daylight through is letting weather through, and multiple holes mean the felt is end-of-life.
What Causes Felt Failure
- Age, bitumen felts typically last 30-50 years
- UV degradation through poor original tile coverage
- Mechanical damage from foot traffic, aerial fitting, or chimney work
- Animal damage (rodents, squirrels, even nesting birds)
- Spray foam, bonded foam destroys felt during application or removal
- Chemical attack from wood preservatives or roof cleaning sprays
Your Options
Targeted patch repair suits localised damage (£150-£400). Felt section replacement is for larger failed areas of 2-5 sq m (£600-£1,500). Full strip and re-felt is essentially a re-roof when the felt is failing across the whole roof, costing £4,500-£12,000+ depending on size.
The economic tipping point: if more than about 25% of the felt has failed, full re-felting is usually better value than chasing patches.
Modern Breathable Membrane vs Old Bitumen Felt
If you are re-felting, the modern choice is breathable membrane (Tyvek, Klober, or similar). It allows moisture vapour out while keeping water in, which dramatically reduces loft condensation. Old bitumen felt does not breathe, which is why so many older Kent properties have condensation problems alongside failed felt. Breathable membrane is now the standard specification.
Get a Felt Inspection in Kent
We do paid felt and underlay inspections across Canterbury, Margate, Faversham, and Folkestone. Inspections include loft examination, photographs, and a written report with options. Call 07562690714 or request an inspection.
Frequently Asked Questions
How long does roof felt underlay last?
Original bitumen felt typically lasts 30-50 years before failing. Modern breathable membranes are rated for 50+ years and often last the life of the new roof covering above them.
Can roof felt be repaired in patches?
Yes, small areas of damage can be patched by lifting the local tiles, inserting fresh felt, and re-laying. Patch repairs cost £150-£400 and are appropriate when the surrounding felt is still in good condition.
How do I know if my roof felt is failing?
Look in the loft for visible sagging between rafters, brittle felt that snaps when gently flexed, dark staining or holes, splits at junctions, and any daylight visible through the felt. Multiple signs together usually indicate end-of-life.
Should I replace bitumen felt with breathable membrane?
Yes, modern breathable membrane is the standard specification for any re-felting work. It prevents loft condensation while still keeping water out, and lasts longer than traditional bitumen.
How much does it cost to re-felt a whole roof in Kent?
Full re-felt is essentially a re-roof job. Costs typically range from £4,500 for a small bungalow to £12,000+ for a large detached property, including stripping, new felt and battens, and re-tiling.
Will my home insurance cover roof felt failure?
No, felt failure is wear-and-tear and not covered by standard home insurance policies. Storm damage that tears felt may be claimable if the underlying felt was in good condition before the storm.
Need Professional Roofing Services?
StoneRidge Roofing provides expert roofing services across Canterbury and Kent