Dorset Model

Technical guide to planning requirements for thatched properties.

Thatch is enjoying a resurgence. Architects, builders and owners increasingly acknowledge the favourable advantage of thatch insulation and aesthetic qualities. Recognising this increasing popularity the 8 building control authorities throughout the county of Dorset reviewed current building controls for a thatched roof.

The Dorset Technical Committee set up undertook extensive research with the Fire authorities and all the Thatching services. The Dorset Technical Committee produced a Design Guide for thatchers and architects known as the Dorset Model. The Department of the Environment are reviewing it. Architects, builders and owners now have a definitive guide to explain the controls.

The result of these controls will

• Either protect to the main structure of the building if a fire occurs.
• Or provide better protection for the roof members if a fire occurs.
• Require an acceptance to treat the thatch as sacrificial in a fire.

Companies specialising in thatch roof insurance welcome the “Dorset Model” and recognise that it will reduce the premiums paid.

The Dorset model details

FIRE RETARDANTS FOR THATCH

Thatch as a modern-day roofing material is becoming increasingly popular throughout the UK, because of its favourable insulation properties and aesthetic qualities. With an increase in new build properties and extensions to existing thatch properties, plus the changes in building controls, there is much confusion about the fire retardants / barriers suitable for thatched properties. The information below will help builders, architects, designers, building control and property owners understand the current rules.

Current Building Regulations

Regard Thatch as having an AD/BD/CD designation under current building regulation B4 Part 2 in table 1. If performance under BS476 Part 3 is not possible, consider thatched roofs being closer to the boundary than shown in table 1. However the following precautions need incorporating 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 complied with
c. Smoke alarms installed in the roof space.

Table 1 Limitations on roof coverings

Designation of covering of roof minimum distance from any point on relevant boundary
Or part of a roof less than 6m At least 6 m At least 12m At least 20m

AA, AB or AC . . . .

BA, BB or BC x . . .

CA, CB or CC x . (1)(2) . (1) .

AD, BD or CD (1) x . (2) . .

DA, DB, DC or DD (1) x x x . (2) ___________________________________________________________________________ . . Acceptable 1. Not acceptable on any of the following buildings:

X Not acceptable a. Houses in terraces of 3 or more houses:
    b. industrial, storage or 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 1500mm from any similar part, with the roof between parts covered with a material of limited combustibility.

Designation

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 ½ hour.
C. Those specimens which are penetrated in less than ½ 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” is a design guide produced in 1995 within Dorset to try to meet the ideals of planning departments and designers. The model has drawn from various disciplines, including, Thatchers, Building Control, Designers, Dorset Fire and Rescue, Insurance Companies, Scientists and Thatch Property Owners.

“The Dorset Model” referred to 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, and use recommended 50 x 25mm counter battens on a micro-porous boarding to allow the thatch to Breathe.

Please note 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 regulation B4(2).

U Value

A U-Value figure is determined by showing the rate at which heat energy passes through certain wall, roof or floor structures. Calculate the figure 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 (2006) need a U Value of 0.20 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.

TAS Fire Retardants Water resistance	Fire Resistance	Dimensions
Thatch Firewall LFB60 yes	60 minutes	1.27m x 50m 61sqM
Thatch Fireboard yes	90 minutes	1220 x 2440 x 8mm 35 boards= 100sqM
Thatchbatts and Firewall 50mm U-Value 0.16 yes 100mm U-Value 0.14 yes	30 minutes 60 minutes	1200 x 400 x 50mm  6 piece = 2.88sqM 1200 x 400 x 50mm 12 piece = 2.88sqM
Thatchsayf Spray no	30 minutes	N/A ½ litre per sqM

For further information on fire retardants available including PDF downloads of fire tests please see the Products section and click onto the required information.