Roofing Felt

Choosing the Right Roofing Felt & Roof Underlay

At County Building Supplies, we supply roofing felt and breathable roof underlay for professional roofers, builders, contractors and DIY customers. Whether you're installing a pitched roof, repairing a shed roof or replacing an existing roof covering, choosing the correct material helps protect the structure from moisture and weather exposure.

Our range includes Cromar Vent 3 Classic breathable roofing underlay and IKO mineral roofing felt for suitable shed and roofing applications.

Breathable Roofing Felt vs Traditional Mineral Felt

Different roofing felt products are designed for different applications:

• Breathable roofing underlay: Installed beneath suitable roof tiles or slates as part of a pitched roof system. Vapour-permeable membranes help manage moisture while providing a secondary barrier against water penetration.
• Mineral roofing felt: Bitumen-based felt with a protective mineral finish, commonly used as part of specified shed and other roofing systems.

These products are not directly interchangeable. Always choose roofing felt suitable for the roof construction, pitch and manufacturer's installation requirements.

Cromar Vent 3 Classic Breathable Roof Underlay

County stocks Cromar Vent 3 Classic 115gsm breathable roofing underlay in 1m x 50m and 1.5m x 50m rolls.

This type of membrane is designed for suitable pitched roof applications beneath tiles or slates. The correct installation depends on the roof construction, including the required overlaps, fixing method and ventilation arrangements.

Breathable underlay does not automatically eliminate the need for roof ventilation. Always follow the roof design and manufacturer's technical guidance.

IKO Shed Felt & Green Mineral Roofing Felt

For suitable sheds, garden buildings and other roofing applications, County stocks IKO Shed Felt Green Slate and IKO Trade Felt Green Mineral Top Sheet.

These products provide a mineral-finished roofing surface, but their suitability depends on the intended roof system, substrate and roof pitch.

Check whether the product is designed for the proposed application and whether additional layers, adhesives or fixings are required.

How Much Roofing Felt Do You Need?

Start by measuring the roof area in square metres, taking account of the actual roof slope rather than just the building's floor area.

County's listed roll sizes include:

• 1m x 50m: 50m² nominal roll area.
• 1.5m x 50m: 75m² nominal roll area.
• 1m x 10m: 10m² nominal roll area.

These figures represent the total material area before allowing for overlaps, edges, cutting waste and installation details. The effective coverage will be lower.

Always use the manufacturer's recommended overlap dimensions when calculating the final quantity.

Installing Roofing Felt Correctly

Correct installation is essential for reliable weather protection. Ensure the roof structure and substrate are suitable, and follow the manufacturer's guidance on overlaps, fixing methods and weather conditions.

For pitched roofs, the underlay forms part of a complete system with the roof covering, battens and any required ventilation. For mineral felt roofing, the number of layers and application method depend on the specified system.

Roofing work at height should be carried out using appropriate access equipment and safety precautions.

Need Advice?

Not sure which roofing felt or underlay is suitable for your project? Visit your local County Building Supplies branch or contact our team for product information, quantities and availability.