Thatch Building Regulations

Fire Retardants for Thatch
With an increase in new build properties and extensions to existing thatch properties and the changes in building regulations there is a great deal of confusion relating to the fire retardants / barriers that can be used on thatched properties, the information below has been put together to help builders, architects, designers, building control and property owners understand the current regulations.

Current Building Regulations
Thatch should be regarded as having an AD/BD/CD designation under current building regulation B4 Part 2 in table 1 if performance under BS476 Part 3 cannot be achieved; consideration can be given to thatched roofs being closer to the boundary than shown in table 1 if, for example the following precautions are incorporated into the design.

a. The rafters are overdrawn with construction having not less than 30 minutes fire resistance.
b. The guidance given in Approved Document J combustion appliances and fuel storage is followed; and
c. Smoke alarm installation is included in the roof space.

Table 1 Limitation on roof coverings

Designation of covering
Minimum distance from any point on relevant boundary
of roof or part of a roof
Less than 6m
At least 6m
At least 12m
At least 20m
AA, AB or AC
BA, BB or BC
CA, CB or CC
o (1)(2)
o (1)
AD, BD or CD (1)
o (2)
DA, DB, DC or DD (1)
o (2)

o acceptable x not acceptable

(1) Not acceptable on any of the following buildings:

a. Houses in terraces of 3 or more houses
b.Industrial, storage or any other non-residential purpose group buildings of any size
c. Any other buildings with a cubic capacity of more than 1500m2

(2) Acceptable on buildings not listed in note 1, if part of the roof is no more than 3m2 in area and is at least 1500m from any similar part, with the roof between parts covered with a material of limited combustibility

Each category designation consists of two letters e.g. AA, AC, BB these being determined as follows.

First Letter
A Those specimens, which have not been penetrated within 1 hour.
B Those specimens, which are penetrated in not less than half hour.
C Those specimens, which are penetrated in less than half hour.
D Those specimens which are penetrated in preliminary flame ignition test

Second Letter
A Those specimens on which there is no spread of flame.
B Those specimens on which there is not more than 21 inches (533mm) spread of flame
C Those specimens on which there is more than 21 inches (533mm) spread of flame
D Those specimens which continue to burn for 5 minutes after the withdrawal of the test flame or spread more than 15 inches (381mm) across the region of burning in the preliminary test.

The ‘Dorset Model’
“The Dorset Model” was a formulated design guide put together in 1995 within Dorset to try and meet the aspirations of planning departments and designers. The model was drawn from a variety of different disciplines, including, Thatcher’s Building Control, Designers, Dorset Fire and Rescue, Insurance Companies, Scientists and Thatch Property Owners.

“The Dorset Model” is cited in the Approved Document Part B Appendix G sub Para B4 as a way of meeting Building Regulations B4(2) “TDM” has recommendations to overdraw the thatch with a minimum 30 minute fire barrier (integrity and insulation) this barrier should also be water resisting,
50 x 25mm counter battens are recommended on a micro-porous boarding which will allow the thatch
to Breath.

It is important to note that The Dorset Model is only one way of achieving compliance with building Regulations B4(2) External Fire Spread (roofs) and any product would be suitable providing it meets the requirement of regulation B4(2).

U Values
A U-Value figure is determined by indicating the rate at which heat energy passes through certain wall, roof or floor structures, the figure is derived from the number of watts of heat energy which will pass through 1sq m of material for every 1 degree C temperature difference between one side and the other.

Current Building Regulations require a U Value of 0.18 watts/sqm/degreeC for new build and extensions, a water reed thatched roof, 300mm (12in) thick at a pitch of 45 degrees has a
U-Value of 0.35.

Thatch Protect
Fire Retardents
Fire Resistant
U Value
8mm Thatch
Barrier Board
60 minutes
1220 x 2440 x
35 boards =
100 sqM
Thatch Flexi
60 minutes
1.3m x 50m
61 sqM
Thatch Insulation
Batts 50mm
30 minutes
1200 x 400 x
6 piece =
2.88 sqM