Felt and batten are the two roofing components developers and QSs most often treat as a commodity. They are, in fact, two of the components that most directly determine long-term roof performance and warranty exposure. Specifying them correctly at design stage is cheap. Specifying them weakly costs money across the warranty period and beyond.
The role of the underlay is more than weather protection during construction. On a modern UK roof, the underlay forms part of the controlled ventilation strategy under BS 5250. It must work with the eaves vent details, the ridge vent system and the building’s overall vapour control approach. A breathable, vapour permeable underlay is now standard on most pitched roofs because it allows interstitial moisture to leave the construction. A non-breathable underlay, used without compensating ventilation provision, traps moisture and accelerates timber decay in the roof void.
The choice of underlay also affects fixing performance. Some breathable underlays require a specific overlap depending on pitch. Some require taped joints at low pitches. Some are zone-rated for wind uplift and cannot be substituted across regions without revalidation. Where the underlay specified in the design is substituted on site for a like-for-like product, the substitution can quietly invalidate the design’s wind uplift calculation.
Batten specification is the other component that quietly carries warranty risk. UK practice requires battens to meet BS 5534, which sets criteria for grading, treatment, dimensions and moisture content. Battens that do not meet BS 5534 may pass visual inspection on the day they are fixed but will, over time, twist, split or pull through fixings. The consequences arrive at five to ten years, not at handover.
Batten fixing patterns matter as much as batten grade. The fixing schedule that defines how many nails per batten, at what spacing, and into which rafter centres, has to reflect the actual wind load calculation for the site. A schedule designed for a low exposure zone is not safe to use in a high exposure zone. Wind uplift failures rarely show up at moderate wind speeds, but they show up dramatically when the design exposure is approached, which is exactly the wrong moment for them to fail.
For developers, the practical implication is that the underlay and batten specification is not a commodity decision. It should be made against the actual exposure zone, the actual roof pitch, the actual tile or slate weight and the actual ventilation strategy of the building. Where this is not done at design stage, it is being done implicitly on site, and the assumptions are not being tested.
Globe Roofing’s pre-start process treats this as a contractual review. The underlay, battens and fixings specified by the design team are checked against the site’s actual wind exposure, the pitch of each plot type and the tile specification. Where the design is silent or unclear, this is raised before installation begins. Substitutions on site are not made without engineering check.
For QSs, the cost implication of getting this right is small. Specifying the correct underlay and batten grade at design stage typically adds little to the unit cost of the roof. The cost of getting it wrong, however, is significant. Underlay failure in the warranty period is one of the more common drivers of large-claim defects, and batten failure in service is among the more common drivers of remedial work in the post-warranty period.
For homeowners, none of this is visible at handover. The roof looks complete. The defects emerge slowly. By the time they emerge, the original developer is, in most cases, the party defending the claim, even where the original cause sits with a substituted material or a wrong fixing schedule from a decade earlier.
The practical conclusion for developers and QSs is straightforward. Felt and batten specification is a design decision worth taking seriously. The right products, properly fixed, transform the long-term performance of the roof. The wrong products, however well fixed, set a slow problem in motion that arrives many years later.
Talk to Globe Roofing To discuss felt and batten specification on your roofing scheme on your scheme, contact Globe Roofing on 01223 890727 or email enquiries@theglobegroup.co.uk.
