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This article provides additional guidance relating to how the Functional Requirements can be satisfied in our Technical Manual.
The ‘Provision of information’ section below indicates the information we need and this is followed by guidance on how our Functional Requirements may be satisfied.
For warranty purposes we require the following:
A structural engineer shall take the thatched roof covering and the associated components, e.g. roof battens and fire resisting sarking board (required to meet the Dorset model design approach), into account as part of their structural design, and this should be provided to the warranty surveyor.
The installation and formation of the main structural roof elements should follow Section 11 of the Premier Guarantee Warranty Technical Manual.
The use of the Dorset model to comply with Building Regulations requires the rafters to be overdrawn with a micro-porous fire resistant boarding to increase fire resistance. For warranty purposes, the boarding must have appropriate third-party approval and be suitable for use in that situation.
Chimneys should conform to Building Regulations, particularly in relation to their proximity to thatch due to it combustible or conducting properties. The chimney should terminate at least 1.8m above the height of the ridge – this measurement is inclusive of any chimney pots, which should have a limited unit height of 600mm.
When following the Dorset model guidance, it is noted that the guidance recommends ceilings beneath a thatched roof need to be capable of supporting access to fight fires from inside the roof via a fire resisting hatch of 900mm x 600mm. Where this is required by Building Control, the Structural engineer must include this in the design.
Electrical fittings such recessed lighting into ceilings below thatched roofs should be avoided, with any lights within the loft space enclosed with a fire resisting bulkhead. External lighting should not be mounted under the eaves of the thatch.
Television aerial and communication cabling should not pass over or under the thatch. External aerials should be fitted to a free standing pole at least 7m from the roof.
Any plumbing within the roof space should utilise flame free jointing.
Junctions between the thatch and other building elements should be made weather tight using lead flashings. Sizing and detailing should follow recognised lead work practice.
An AVCL should be incorporated on the warm side of insulation as a means to combat vapour movement into voids and the thatch material. AVCLs must be used alongside all roof configurations and their associated ventilation measures.
Cold pitched roof ventilation
Cold pitched roofs should be served by 25mm continuous eaves ventilation strip at the eaves. As the formation of intricate detailing at the ridge in thatched roofing often results in situations where it is not practicable to incorporate ridge ventilation, it is essential that an assessment of the potential risks and the alternative means to promote air changes within the void are fully demonstrated by a designed ventilation strategy.
Insulation employed at ceiling level for an improved thermal performance must incorporate appropriate provision to allow for unobstructed air flow at the eaves e.g. eaves ventilation strips fitted under the fire resisting sarking board at the eaves location.
Warm pitched roof ventilation
Rigid insulation used at the rafter line, such as within room in a roof situations, must allow for airflow between the insulation and the underside of the fire resisting sarking board by providing a 50mm ventilated void. As above, where it is not practicable to incorporate ridge ventilation, the means to promote air changes via cross flow ventilation within this ventilated void must be fully demonstrated by a designed ventilation strategy.
Reducing the transfer of water vapour from occupied spaces
A combination of good workmanship on AVCLs, the assessment of occupancy levels, and moisture management within occupied spaces via the use of a designed ventilation system such as use of background ventilation or mechanical extraction systems, may provide options in reducing the transfer of water vapour into the roof voids.
For warranty purposes, the general Functional Requirements within the Technical Manual must still be applied to thatched roofs, along with the additional functional requirement covered within Section 11 Roofs.
At practical completion of the roof, a thorough recorded visual inspection should be carried out with representation from the general contractor and the specialist roofing contractor in attendance.
As this is highly specialised work, it would not be unreasonable for the Risk Management Surveyor to request evidence of experience of the specialist roofing contractor, which may be proved and demonstrated by membership of an appropriate trade association that sets a Code of Conduct for the service.
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Every care was taken to ensure the information in this article was correct at the time of publication (June 2022). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.