Properties and Extentions
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.
proposal will be considered on its merits, therefore early
consultation with a member of the Building Control staff is
approach to thatched buildings is now being advocated across
Dorset where compensatory requirements are considered acceptable
to achieve compliance with Building Regulations.
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
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.
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
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
use of a flexible material as a fire resisting barrier is
not considered acceptable for the purposes of the Dorset Model
‘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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