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New Properties and Extensions
The following guidelines are to advise you of certain requirements if you are considering extending or constructing a thatched roof building less than 12m from your boundary.
Each proposal will be considered upon its merits, therefore early consultation with a member of the Building Control staff is recommended.
A uniform approach to thatched buildings is now being advocated across Dorset where compensatory requirements are considered acceptable to achieve compliance with the Building Regulations.
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Requirements
- Rafters are to be overdrawn with a minimum 30 minute fire barrier (integrity and insulation) and this barrier should also be water resisting. 50 x 25mm counter battens are recommended on a micro-porous boarding to allow the thatch to breathe. (Dorset Fire and Rescue Service recommend a 60 minute barrier under the thatch for property protection).
- The chimney, including the pot, should terminate at least 1.8m above the height of the ridge. Due to the risk of condensation forming as hot gases cool, the chimney pots should be limited to a maximum height of 600mm.
- A domestic mains and battery powered, interlinked smoke alarm system will be required with one smoke alarm fitted in the roof void. The system should generally be in accordance with that specified in Approved Document B to B.S. 5839 Pt. 6.
- The written comments of the adjoining property owner will be requested by the Local Authority for consideration.
- A terrace may not consist of more than three thatched dwellings together.
In addition to the measures that may be required by Building Control there are a number of recommendations that you should carefully consider at an early stage.
Recommendations
- The provision of a loft hatch is recommended for fire fighting purposes. The minimum recommended size is 600mm x 900mm.
- Advice should be sought from an approved electrical contractor regarding the most appropriate type of wiring system. Effects from rodent damage and straw debris need to be considered and the National Inspection Council for Electrical Installation Contracting have issued guidance to their members.
- It is NOT recommended to cut in recessed lighting into the ceilings below the thatch. Light fittings within the roof space to be in a bulk head fitting. External floodlights should not be located just under thatch.
- Spark arrestors on the flues are NOT recommended because they can clog and restrict the flow of flue gases.
- It is recommended that an external water tap supplied from the rising main, is fitted with a hose capable of reaching all parts of the roof.
- Any metal plumbing in roof space should use compression joints to avoid the use of blow torches.
- In order to acheive a 'U' Value of 0.2w/m2K for thatched roofs, the following was taken from CIBSE Guide A3:
Thatch:
Reed = thermal conductivity 0.09, and 'r' 11.1
Straw = thermal conductivity 0.07, and 'r' 14.3
This gives a 'U' Value of 0.2w/m2K for the following thickness's (thatch alone) Reed 450mm and Straw 350mm.
The use of a flexible material as a fire resisting barrier is not considered acceptable for the purposes of the Dorset Model.
Further advice may be sought from the Dorset Master Thatchers Association on how these matters may best be incorporated into their traditional craft. Additional fire safety advice is available free of charge from Dorset Fire and Rescue Service.
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