The underlay specification on new build housing is one of the roof components that gets treated as boilerplate at design stage but carries significant implications for the roof's long-term performance. Traditional sarking felt (bituminous, non-breathable) and modern breathable underlays (spun-bonded synthetics) behave very differently in service, and the specification choice interacts directly with the ventilation strategy for the roof void.
How sarking felt and breathable underlays behave differently
Traditional sarking felt is impermeable to water vapour. Any moisture that enters the roof void from below has to leave through designed ventilation openings at the eaves, ridge or soffit. The system works reliably in older housing where the fabric is naturally leaky and the roof void receives less moisture load. On modern airtight housing, the moisture load is higher and the ventilation requirements are more demanding.
Modern breathable underlays are vapour permeable. Water vapour can pass through the underlay itself and exit through natural drying paths, reducing the load on designed ventilation. This can allow simpler ventilation detailing at the eaves and ridge, and in principle allows warm roof and hybrid constructions that sarking felt would not support.
The trade-off is that not all breathable underlays perform equally. Vapour permeability, tensile strength, water resistance and long-term durability vary significantly by product. A budget breathable underlay used on a demanding site (high exposure, high internal moisture load) can fail earlier than sarking felt would have.
Matching the underlay choice to site conditions
For new build housing on regional sites, Globe Roofing's specification review considers the underlay against the specific site conditions rather than as a boilerplate choice. Exposure zone (BS 5534). Roof pitch. Ventilation strategy. Internal moisture load expected in service. Each factor influences which underlay type and which specific product is appropriate.
There is a specific consideration around fire performance worth flagging. Since Grenfell, the fire classification of roof materials on residential buildings has received more attention. Both sarking felt and breathable underlays are available with Class A, B or lower fire ratings. The specification needs to match the building's overall fire strategy, and on taller residential blocks the classification is scrutinised.
The installation sequence differs between the two underlay types. Sarking felt is nailed with defined lap widths and requires specific attention to detail at eaves, ridges and hips. Breathable underlays are typically taped at laps and require different fixing patterns. A roofer trained on one system does not automatically install the other correctly.
Specification responsibility at design and tender stage
On refurbishment and re-roofing work, the underlay decision is often more constrained. The existing rafter geometry, the existing ventilation arrangements and the existing wall abutment details limit what the new underlay can be. A wholesale switch from sarking felt to breathable underlay on a re-roof may require significant secondary work to make the change actually deliver its benefits.
For developers and QSs pricing housing schemes, the underlay specification is a small line item in the cost plan but has a large influence on long-term performance. Where the specification is left as boilerplate to the roofing contractor, quality varies. Where it is specified with a defined product and grade, quality is consistent.
The specification question is worth raising specifically at design stage rather than left for the roofing contractor to interpret. What underlay type. What product grade. What fire classification. How does it interact with the ventilation strategy. Getting the answers documented gives the developer, the QS and the roofer a common starting point.
Talk to Globe Roofing
To discuss sarking felt versus breathable underlay specification on your scheme, contact Globe Roofing on 01223 890727 or email enquiries@theglobegroup.co.uk.
