Modern UK new build housing is significantly more airtight than housing from twenty or thirty years ago. That airtightness delivers the thermal performance that Building Regulations Part L requires. It also changes the moisture behaviour of the roof void in ways that older ventilation strategies did not have to accommodate.
The physics is simple. Warm moist air produced inside the building has to leave the building somehow. In older, leakier housing, it leaves through incidental air paths through the fabric. In airtight housing, it can only leave through designed ventilation systems, and any that leaks upward into the roof void has to be able to leave the roof void through controlled ventilation before it condenses on cold surfaces.
BS 5250 sets out the ventilation strategies for pitched roofs. Cold roof, warm roof, hybrid arrangements each have specific requirements. Eaves ventilation at the low points. Ridge or high-level ventilation. Cross-ventilation through soffit vents. Vapour control layers at the ceiling to reduce moisture ingress in the first place. Each element interacts with the others, and the calculation for a specific building requires all of them to be considered together.
The most common defect in new build housing roof voids is condensation on the underside of the roof underlay, resulting from insufficient ventilation combined with vapour ingress from the living spaces below. The condensation dampens the insulation, encourages timber decay in the roof structure, and eventually produces visible staining or actual water ingress at the ceiling.
For developers and QSs, the ventilation strategy on modern housing needs to be treated as a design decision rather than a default. Standard eaves and ridge details from a decade ago may not be adequate for the current airtightness standard. Specifying a modern breathable underlay does not automatically solve the ventilation problem. The design has to be calculated for the specific building.
Globe Roofing's pre-start review on new build housing schemes includes verification of the ventilation strategy against the building's actual construction. Where the design specifies eaves ventilation to a certain area but the eaves detail as-built does not achieve that area, the discrepancy is raised. Where the ridge ventilation is specified but the underlay taping or the ridge vent detail does not deliver it, the same happens.
The installation itself matters as well as the design. Eaves ventilation is only effective if the insulation at the eaves does not block it. A common defect is insulation stuffed into the eaves right up to the underlay, blocking the ventilation path. The insulation installer intends to be thorough. The roofer signs off the eaves detail. Nobody checks the interaction. The building is compromised.
On Globe Roofing sites, the eaves detail is checked before the roof is closed. Where the insulation is blocking the vent path, the insulation is pulled back to allow airflow. The verification is documented as part of the plot sign-off.
For developers, the practical implication is that ventilation is one of the areas where the trade interfaces matter as much as the individual trades. A well-installed roof and a well-installed insulation package can still produce a badly ventilated roof void if the interface between them is not managed. Joint pre-close inspection between the roofer and the insulation installer is the mechanism that prevents this.
Condensation defects tend to emerge in the second and third winters after handover, when the building has been fully occupied and the internal moisture load is highest. By that point, the roof void has usually experienced significant condensation cycles. The visible defect that reaches the developer's warranty desk is a late-stage consequence of an early-stage design or installation failure that could have been prevented for very little cost.
Talk to Globe Roofing
To discuss roof void ventilation on your housing scheme on your scheme, contact Globe Roofing on 01223 890727 or email enquiries@theglobegroup.co.uk.
