Roofing Felt: One Size Does Not Fit All
Roofing felt isn't one size fits all. The correct underlay depends on the roof covering, pitch, ventilation, exposure and roof design.
Always check the roofing material manufacturer's guidance and the roofing felt manufacturer's installation instructions before starting any job.
Types of Roofing Felt and Membrane
1. Non-Breathable (Bitumen) Felt
- Traditional waterproof underlay.
- Requires roof ventilation.
- Suitable for concrete tiles, clay tiles, natural slate and approved fibre cement slates.
2. Breathable Membrane
- Allows moisture vapour to escape while remaining waterproof.
- Helps reduce condensation.
- Suitable for most modern roofs, including tiles, slate and approved fibre cement slates (often with counter battens where specified).
3. Vapour Permeable Membrane
- Controls moisture movement while remaining weatherproof.
- Used where condensation control is important.
4. HR (High Resistance) Felt
- Traditional bitumen underlay.
- Requires ventilation.
5. LR (Low Resistance) Membrane
- Modern breathable underlay.
- Often used instead of HR felt.
6. Reinforced Membranes
- Extra tear and wind resistance.
- Ideal for exposed locations.
7. Fire Resistant Membranes
- Used where fire performance is required.
- Common on timber buildings, commercial roofs and roofs close to boundaries.
8. Fire Break Membranes
- Used around party walls and compartment walls to help restrict fire spread.
- Follow Building Regulations and manufacturer's guidance.
9. Fibre Cement Slate Underlay
- Usually a breathable membrane (often counter battened where specified) or an approved traditional underlay.
- Always check the slate manufacturer's recommendations.
10. Temporary Weather Protection
- Underlay protects the roof before the covering is fitted.
- Don't leave exposed longer than the manufacturer's maximum exposure time.
11. Bat-Friendly Membranes
- Smooth membranes designed to reduce the risk of bats becoming trapped.
- Required where specified by bat surveys or ecological reports.
- Never assume a standard breathable membrane is suitable.
Trevor's Top Felt Tips
- Tape or support laps to prevent wind uplift.
- Don't overstretch breathable membranes — allow a 10–15 mm drape between rafters.
- Fit the correct overlaps and fixings.
- Protect membranes from prolonged UV exposure.
- Replace damaged or torn sections.
- Store rolls dry and out of direct sunlight.
- Follow Building Regulations.
- If the manufacturer's instructions differ from general guidance, always follow the manufacturer's specification.
Trevor's Golden Rule
The correct roofing felt depends on the roof covering, roof pitch, ventilation, exposure and roof design. Always check the roofing material manufacturer's guidance and the roofing felt manufacturer's installation instructions before installation.
Keep this information handy next time you're selecting or installing roofing felt to ensure your work stays compliant. Look out for more of Trevor's Compliance Tips coming soon!
