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New Properties and Extentions

The Dorset Model

The following guidelines are to advice 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 on 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 Building Regulations.


Thatch Battens

1. 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).

2. 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.

3. 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.

4. The written comments of the adjoining property owner will be requested by the local authority for consideration.

5. 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.


1. The provision of a lotf hatch is recommended for fire fighting purposes. The minimum recommended size is 600mm x 900mm.

2. Advice should be sought from an approved electrical contractor regarding the most appropriate types of wiring system. Effects from rodent damage and straw debris need to be considered.

3. It is not recommended to cut in recessed lighting into the ceiling 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.

4. Spark arrestors on the flues are not recommended because they can clog and restrict the flow of flue gases.

5. 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.

6. Any metal plumbing in the roof space should use compression joints to avoid the use of blow torches.

7. In order to achieve a ‘U’ value of 0.2w/m2K for thatched roofs, the following was taken from CIBSE Guide A3:

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
Reed: 450mm
Straw: 350mm

The use of a flexible material as a fire resisting barrier is not considered acceptable for the purposes of the Dorset Model

The ‘Dorset Model’ has been jointly produced by the Local Authorities across Dorset in conjunction with Dorset Fire and Rescue Service, The Dorset Master Thatchers Association and after consultation with the National Inspection Council for Electrical Installation Contracting and the British Research Establishment Ltd. On the basis that evidence has shown that thatch can be made sacrificial in the event of fire.


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