In 2010 revisions to Part L were introduced. Specifically, roofers are now further challenged when a house is to be re-roofed or at least 50 percent of the roof area is to be repaired and a requirement is triggered in Part L1B to upgrade the thermal performance.
It may however not be possible to deliver a thermal performance upgrade to Building Regulation standards in all instances – perhaps the roofline cannot be raised since the house is either semi-detached or terraced, or a historic building whose appearance cannot be altered – and there may be a need for the roof insulation to remain breathable.
Relevant excerpts from Part L1B
L1B – Conservation of Fuel and Power
Approved Document L1B of 1st October 2010 defines re-roofing as “renovation of a thermal element” – stripping down the roof to expose the rafters and then rebuilding. The performance of the roof should be improved to achieve a U value of 0.18 W/m2.K.
When renovating elements of traditional construction particular considerations apply – the aim should be to improve energy efficiency as far as is reasonably practicable without prejudicing the character of the building or increasing the risk of long-term deterioration of the building fabric. The risks of interstitial condensation must be considered.
Buildings which are listed, in a conservation area or monuments are exempt from energy requirements, to the extent where compliance would unacceptably alter the character or appearance of the building.
If achievement of the 0.18 U value is not technically or functionally feasible, or would not achieve a simple payback of 15 years or less, the element should be upgraded to the best standard that is feasible, and which can be achieved within a simple payback of no greater than 15 years. Part L1B Appendix A: Practical considerations with respect to an increase in structural thickness (particularly in terraced dwellings) may necessitate a lower performance target.
If it is not technically feasible to upgrade the roof to U = 0.18 W/m2.K, but it is necessary to improve the roof as much as possible without introducing a risk of condensation.
Renovation of a thermal element is distinct from a retained thermal element, where an existing element becomes part of the thermal envelope. In the case of a roof that is a retained thermal element, it is only necessary to upgrade the thermal performance if it does not meet a threshold value of 0.35 W/m2.K. If it is not feasible to upgrade to the required standard, a lesser performance should not be worse than 0.7 W/m2.K.
C2 – Resistance to Moisture
Approved Document C2 Resistance to Moisture (6.10) states that a roof will resist damage from interstitial condensation if it is constructed in accordance with BS5250. A condensation risk analysis should be carried out on any proposed renovated roof structure. In addition, excessive moisture transport into the roof through moist air movement must be avoided by sealing junctions, gaps and service penetrations.
Regulation 7 Materials and Workmanship states that building work should be carried out with adequate and proper materials. To ensure a material is suitable, it should comply with British or other national standards, be covered by a certificate issued by an ETA issuing body (such as BBA or DIBt), have a CE mark, or be certified by a UKAS accredited body.