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There are three commonly used thatching materials:
- Water Reed (Also Known As Norfolk Reed)
- Long Straw
- Combed Wheat Reed (Also Known As Devon Reed)
- Others Include Flax, Heather, Broom, Sods, Marram Grass
etc - for further information please contact T.A.S.
Water Reed (Phragmites Australis) is the most durable thatching
material.
Long Straw is a winter wheat straw which has not been combed
(its name refers to a style of thatching rather than a material
achieved by any singular process although generally for
the last 90 years long straw thatching has used the processed
straw from the threshing drum). It has the shortest life
expectancy of the three commonly used materials.
Combed Wheat Reed is winter wheat straw which has had the
leaf removed and is laid in a similar way to water reed.
With modem farming methods tall strong straw has become
less readily available.
Water reed, which is the most durable thatch, can last
up to 50 years. Maintenance will include reridging every
10 to 15 years.
Combed wheat reed can have a life expectancy of 25 to 35
years. Long straw will last from 15 to 25 years. As with
water reed both these materials will require reridging at
10 to 15 year intervals.
These life expectancy figures can drop noticeably the further
west the thatched property is situated. This appears to
be due to climatic conditions. The warm, high humidity,
clean air conditions experienced in the West Country are
ideal for the microbes that begin the decomposition process.
Diagram 1: The pitch of the roof will relate directly to
the pitch of the thatch and equally the thickness of the
thatch will influence the pitch of the thatch. Thus, an
l8in coat of thatch will lie at a much slacker pitch than
a l2in coat and therefore a thicker coat will wear more
quickly. The thinner the thatch, the steeper the pitch,
however there must be adequate thickness of thatch over
the fixings, thus a 4in coat of thatch is steeper than a
l2in coat, but because the exposed stem length is longer
(and therefore wears more quickly) and because there will
be very little thatch over the fixings the thatch will not
last as long as a l2in coat. An optimum thickness for maximum
longevity would be between 9 and l5in for water reed and
9-l2in for combed wheat reed and long straw. Therefore,
the point to remember is that there are an almost infinite
number of specifications depending on pitch of roof and
length of the thatching material.
Thatching is generally recognised as a craft and as such
it is almost impossible to give a single specification for
a rethatch. However in the field of new build thatching
the majority of work is undertaken using water reed and
therefore a specification is possible (other specifications
are available through T.A.S for other styles and materials):
- The Water Reed should be laid to a thickness of approximately
12" (300mm)
- The ridge most commonly used is a block cut, patterned
and saddled type ridge
- Supply and fix, 3/4 inch (19mm), 22 gauge galvanised
wire mesh to the ridge only
- Chimney flashings are usually lead although cement flashings
are perfectly acceptable
- Finally the thatcher should be expected to leave the
site reasonably tidy
Hazel or steel runners can be applied and secured by steel
thatching nails, fixing wires, twine or spars. The method
will depend on the roof in question and the material used,
however water reed on a new roof is usually fixed with thatching
nails or stainless steel wires attached to rust proved screws.
It is advisable to set the pitch at about 50 degrees. This
is not due to the weight of the material which is in fact
not overheavy, but rather to facilitate efficient drainage.
Dormer roofs and eaves window-roofs should be at least at
a 45 degree pitch, if possible.
Thatch is inherently a good thermal insulator with 'r'
values of 11.1 and 14.3 mK/W for reed and straw respectively.
It is worth noting that generally thatch is considered a
'warm roof construction' and consequently does not require
the ventilation of a tiled roof.
The U value of a roof refers to its thermal conductivity,
i.e. how quickly heat is lost from it, and is measured in
watts per metre squared per Kelvin. The U value of 12 inches
of Water Reed on a pitch of 45 degrees is 0.35 W/sq m K.
An R value is the inverse of U. The current building regulations
state that the U value required from a new roof should be
0.25 W/sq m K. Therefore there has to be some sort of insulation
even under a thatched roof.
The calculations for U value are relatively complicated
and therefore should be left to the architects. However
it is definitely worth telling any interested parties that
Thatchbatts and Barrier Foil underneath the thatch give
a U value of 0.16.
When calculating for a roof construction a weight of 7
lbs/ft2 (34 kg/rn2) should be assumed.
For a 16'6" (5.03m) span roof with a clear span a simple
form of truss is required as indicated in diagram 3. The
trusses should be set at between six to eight feet apart.
Rafters, ties, collars and diagonals should be 4" x 2" (100
x 50mm). The diagonal members should be notched so that
the purlins will be held at right angles to the rafters.
 
The size of purlin is dictated by the distance between
trusses and will vary from 5" x 2" (130 x 50mm) to 7" x
2" (180 x 50mm). Binders will vary from 5" x 2" (130 x 50mm)
to 6" x 2" (150 x 50mm). All ceiling joists should be spiked
to the binder to prevent sagging. Binders can be omitted
if there is a convenient structural wall at or near the
centre of the span. Up to an 11'O" (3.35m) span the construction
can be of a simple collar type with the collar set in the
bottom third of the roof. 4" 2" (100 x 50mm) ceiling joists
can act as collars if conveniently positioned. All rafters
must be bird-mouthed over the wall plates.
Valley
A 7" x 1 1/2" (180 x 38mm) layer board is placed over the
rafters of the main roof. The jack - rafters are attached
to this and the ridge can be set between the twin rafter
members of the truss. See diagram 5.
The truss illustrated in diagram 4 is suitable for a 24'
(7.3m) span. Ridge collars and diagonals and bottom ties
should be 4" x 2" (100 x 50mm), hangers are 3" x 1" (76
x 25mm). With a span of this size the ceiling joists require
support in three positions if the joists are to be kept
at 4" x 2" (100 x 50mm). This truss provides for three binders
which can be omitted whenever a convenient structural support
is available.
Valley
With large and equal spans, a conventional 7" x 1 1/4"
(180 x 32mm) valley rafter should be used. Jack-rafters
should then coincide as far as possible. Where rafter spacings
of the two roofs are different, a layer board should be
employed.
Ridge
Presuming 4" x 2" (100 x 50mm) rafters and a 500 pitch,
the ridge plate should be 9" x 1 1/4" (229 x 32mm) and the
upstand of the ridge should be 2" (50mm) from the ridge.
Thereafter battens should be spaced at 9" (228mm) centres.
For construction see diagrams 6, 7 & 8.
Close Boarded Raking Eave is constructed with a 15" (380mm)
fascia held by 2" x 1 1/2" (50 x 38mm) bearers and hangers.
The hangers are spiked to the rafters. See diagram 6.
Vertical Fascia & Close Boarded Soffit.
The fascia board should be I 1/4" x 12" (32 x 300mm) and
grooved to take 3/4" (19mm) tongued and grooved boarded
soffit. The soffit bearers should be 2" x 1 1/2" (50 x 38mm).
(Diagram 7) and are fixed to 2 x 11/2" (50 x 38mm) plates
which are plugged to the wall.
Open Eaves Type. See diagram 8. This roof
is finished with a 3" x 3" (76 x 76mm) tilting fillet. The
overhang is infilled with eaves boards and battens and then
rendered underneath to prevent vermin getting into the thatch.
Unlike diagrams 6 & 7 the top of the cavity is infilled
with expanded metal and mortar.
These are often necessary due to the steep pitch of the
roof and the deep overhang of the eaves. The windows can
be held in a brick spandrel above the normal wall plate.
The wall plate is continued through the spandrel to provide
support for the untrimmed rafters. These are further secured
by 3" x 1 1/2" (76 x 38mm) spacers. The top of the wall
cavity is closed with expanded metal and cement and capped
with a 3" x 2" (76 x 50mm) wall plate. The ceiling joists
above the eaves window can be extended beyond the rafters
to offer an extra securing point for the 3" x 2" (76 x 50mm)
rafters forming the roof over the window opening. These
rafters have been laid in a staggered formation to provide
a curved seating for the thatch. The eaves treatment illustrated
(diagram 9) joins onto a standard eaves type at the bottom
of the spandrel.
See diagram 10 & 11. In both cases illustrated the barge-board
should upstand the batten by 1.5" (38mm). The top of the
cavity may be closed by slates bedded in mortar with the
batten passing over this.
N.B. Built-in purlins should not project beyond the wall
into the verge. Also the last rafter is set approximately
1" from the inside edge of the brickwork.
 
See diagrams 12, 13 & 14. A Velux window is installed on
top of the thatch by constructing a box, the height of which
above the rafters is determined by the thickness of thatch
(a new single thickness of combed wheat reed or water reed
is usually 12" (300mm).
At the rear of the box, sloping lay boards overlain with
lead can be placed to facilitate the run-off of water onto
the side and front flashings which are on the surface of
the thatch.
 
See diagram 15. The construction of the dormer roof is
similar to that of the eaves roof except in our examples
3" x 2" (76 x 50mm) ceiling joists have been incor-porated
to make a flat ceiling above the window aperture. At the
foot of the dormer window the rafters are trimmed with an
8" x 2" (203 x 50mm) upstand. The bottom of the sill should
be a minimum of 1 '6" (457mm) from the structural roof level.
As with verge details a fascia is required with an upstand
of 1 1/2" (38mm).
It is important that the pitch should be at least 45 degrees
and on no account less than 40.
See diagram 16. The tilting fillet should be 2'6" (192mm)
from the chimney (as shown) and a minimum of 12" (300mm)
above the bed of the gutter. This fillet should be 2" x
3" (50 x 76mm) and the gutter bed and upstand should be
1" tongued and grooved boarding. The substructure consists
of 3" x 2" (76 x 50mm) bearers, struts and plates. The rafters
below and above the chimney should be trimmed into either
4" x 2" (100 x 50mm) or 4" x 3" (100 x 76mm) trimmers depen-dant
upon load exerted on these trimmers.
N.B. Care should be taken that the gutter discharges above
the level of the thatch and that the pargetting to the lower
portion of the chimney is not lower than the thatch.
 
For some years now various Building Control divisions throughout
the county of Dorset have grappled with the local Planning
demands for new thatch properties within districts and the
inappropriate distance from the boundary requirements of
Regulation B4 and Table 17.
Generally speaking the authorities, including my own West
Dorset, have varied the provisions considerably relaxing
distance requirements and replacing them with conditions
which give better protection for the occupancies.
The approaches were therefore, bound to be different from
district to district and this was obviously causing confusion
to designers and specifiers. With this problem in mind a
technical committee was set up involving representation
from all of the eight Dorset Building Control authorities.
Meetings and research were extensively carried out in conjunction
with the Dorset Fire & Rescue Service, the Dorset Master
Thatchers' Association, the Building Research Establishment
and the National Inspection Council for Electrical Installation
Contracting. Other points of references included various
insurance companies specialising in thatch, Rank Hovis MacDougal
scientists and the Thatch Advisory Service. As a result
a design guide suitable for all the authorities within Dorset
was drawn up and for the lack of a better name, was called
The Dorset Model which satisfactorily met the criteria of
all the aforementioned specialist advisors. This guide is
at present being formally drawn up and will be submitted
to the Department of Environment Building Regulation Division
for their attention. The result of these requirements will
give protection to the main structure of the building including
the roof members should there be a thatch fire - the thatch
being treated as sacrificial.
Companies specialising in thatch roof insurance welcome
"the model" and recognise that this approach will reduce
premiums paid by their customers. Although obviously subject
to future 'tweaking' and monitoring, the basis of the model
is set out below.
Head of Building Control
West Dorset District Council
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 to within 12m of 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.
Requirements
- Rafters are to be overdrawn with a minimum 30 minute
fire barrier 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 also 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 a 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 floodlight
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 glues.
- 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.
Further advice may be sought from the Dorset Master Thatchers
Association on how these matters may best be incorporated
into their traditional craft.
This leaflet has been jointly produced by the Local Authorities
across Dorset in conjunction with the Building Research
Establishment Ltd, Dorset Fire & Rescue Service, Dorset
Master Thatchers Association and the National Inspection
Council for Electrical Installation Contracting.
Application
Thatchbatts are fitted between the rafters, supported from
galvanised steel 'top hat' brackets as illustrated. The
fixing sequence is as follows:
- Secure the 'top hat' brackets to the rafters with galvanised
nails at 300mm cen tres, started 150mm from the eaves.
Ensure 150mm dimension is also main tamed at the ridge,
if exceeded fit an additional bracket.
- Starting at the eaves, make a mitre cut in the first
batt for wall plate/soffit and install the first layer
of Thatchbatts, ensuring a good fit between the rafters.
A mitre cut should also be made in the batt at the ridge.
Part batts, eg 300-600mm length, should be supported on
at least two brackets.
- install the second layer of Thatchbatts. Horizontal
joints must be staggered with the first layer.
- Secure the layer of Foil 341 over the Thatchbatts with
a minimum 50mm lap on the horizontal joints.
- Secure battens over rafters for thatch as standard instructions.
- Thatch Foil 341 should only be used in conjunction with
Thatchbatts and does not act as a stand alone fire barrier.
Packaging
1200 x 400 x 50mm (400 crs rafters)
14 pieces per pack (6.72sq m at 100mm thickness)
Handling and Storage
Thatchbatts are very light and easy to cut any shape with
a sharp knife. They are compression wrapped in polythene
for short term protection. For long term pro-tection they
should be stored indoors or under waterproof covering and
away from rodents.
Thatch firewall developed jointly between Rentokil and
TAS Ltd was manufactured to the "Dorset Model" specifications
as recommended by Building Control.
This product provides a one hour fire barrier (B.S. 476:
Part 3 1958) Classification EXT. S.AC. Fixed between the
thatch and roof construction. In the event of a thatch fire,
the thatch is sacrificial and untreated.
Application
Thatch Firewall is laid horizontally across the rafters,
similar to roofing felt with a 50mm/75mm overlap. Thatch
Firewall can be either stapled or nailed to rafters. Finally
fix the first batten 5" from the arris rail followed by
12 inch centres thereafter.
Benefits
- 1 hour fire barrier (B.S. 476. Part 3 2004)
- Easy application (Quick to apply, saving on labour costs)
- Waterproof
- Strong and Durable
Standards and Approvals
- Thatch Firewall, flexible partition which forms the
fire barrier, provides effective compartmentation against
the spread of fire.
- The Firewall (LFB6O) is a lightweight glass fibre fabric
coated both sides with a polyurethane polymer containing
micronised aluminium. It's silver-grey in colour and it
weighs 29kg.
- An official report on the test has been prepared by
the Loss Prevention Council, ref TE201673, a copy is available
on request.
Dimensions
Supplied in 50m X 1.27m Rolls, which covers approximately
61 sq. metres (660 sq.ft) including overlap (50mm).
Supply
Thatch Firewall is available exclusively through the Thatching
Advisory Services Ltd, Andover who are part of the NFU Mutual
or through Master Thatchers (its network of franchise thatchers
distributed throughout the UK). A list of franchisees is
available on request.
CURRENT BUILDING REGULATIONS /
U-VALUES
FIRE RETARDANTS FOR THATCH
Thatch as a modern day roofing material is becoming increasingly
popular throughout the UK due to its favourable insulative
properties and aesthetic qualities, 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 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” 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 Value
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 (2006) require 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 Fire Resistance Dimensions Coverage
water resistant
Thatch Firewall LFB60 60 minutes 1.27m x 50m 61sqM yes
Thatch Fireboard up to 3 hours 1220 x 2440 x 8mm 35 boards=
100sqM yes
Thatchbatts and Foil
50mm U-Value 0.16 30 minutes 1200 x 400 x 50mm 10 piece
= 4.8sqM yes
10mm U-Value 0.14 60 minutes 1200 x 400 x 50mm 20 piece
= 4.8sqM yes
Thatch Foil 1m x 50m 42sqM yes
Thatchatts and Thatch Foil are installed as a system; Thatchfoil
should not be used as a fire barrier independently.
Thatchsays Spray 30 minutes N/A ½ litre per sqM
no
For further information on fire retardants available including
PDF downloads of fire tests please go to www.thatchingadvisoryservices.co.uk
or call 01264 773820 for a free information pack.
Further information, help and advice on these products
and their application, is available from:
Thatching Advisory Services Ltd
The Old Stables
Redenham Park Farm
Redenham
Andover
Hampshire
SP11 9AQ
Tel: +44 (0)1264 773820 Fax: +44 (0)1264 773836
Click Here To E-mail Us
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