| Roofs for New
Thatched Extensions and New-Build Thatched Buildings The
following advice is considered applicable by DFRS whether
or not the building is within 12m of its boundary and thus
falls foul of the Building Regulations which do not allow
combustible roofing materials within this distance. Fire
Resisting Barrier Tests
by the Fire Research Station (FSF) at the Building Research
Establishment (BRE) have shown that a thatched roof constructed
so that the thatch is separated from the roof void by a fire
resisting barrier is both easier to extinguish and suffers
far less damage from fire-fighting water. There is a high
probability that the roof structure will remain intact, thus
allowing subsequent re-thatching of the original rafters. The
separation can be either above (overdrawn), beneath (underdrawn),
or between the rafters, but overdrawing is easier to construct
as a fire-tight roof, particularly when a room is partly constructed
within the roof space. Underdrawing also introduces a larger
air gap between the material used for separation and the underside
thatch. This allows a bigger flue effect in a fire, possibly
leading to a fiercer blaze for Firefighters to deal with.
For these reasons, when discussing new-build construction,
overdrawing only is detailed in this note.
Impermeable Membranes
The
BRE has also investigated the effect on an impermeable membrane
in close contact with the underside of the thatch, in relation
to its moisture content, and have issued a Good Building Guide
(GBG) on the subject. A membrane is taken to mean any type
of rigid or flexible sheet between the thatch and the roof
space. An
impermeable membrane, e.g. polythene or bituminous felt is
sometimes fitted over the rafters for weather protection prior
to completion of the thatch roof. This membrane is often left
in position after thatching on the understanding that it will
reduce the risk of rain penetration. These impermeable membranes
interfere with the natural transfer of water vapour through
the thatch and their use should therefore be discouraged.
Water vapour condensing as moisture, either of this membrane,
or within the depth of the thatch, can result in acceleration
of the normal fungal decay process and lead to earlier deterioration. However,
if this membrane is also required to prevent bits and pieces
subsequently dropping into the roof space, then only a permeable
type must be used. To allow adequate ventilation of the thatch
it should be left short of the eaves and roof apex and either
be draped over the rafters, allowing a clear cavity of nominally
50mm under the thatch, or be fixed to the underside of the
roof timbers. Where
a higher risk of moisture retention in the thatch has been
identified then this cavity should be ventilated at the eaves
on both elevations by a continuous air gap equal to at least
25mm. Consideration should also be given to additional ridge
ventilation equivalent to a 5mm continuous strip. Note, however,
that with a roof no increased risk, the eaves could be sealed
to reduce the ingress of oxygen to a fire, provided that this
action is thought not to shorten the life of the thatch be
decreasing the ventilation.
Standards
Fire
resisting membranes should either have passed a 60 minutes
minimum fire penetration test, or have been subjected to the
same thatch test specification as those carried out by the
FRS. At the end of the test, where a thatched roof is allowed
to burn for 2 hours under test conditions before being extinguished,
the membrane should not have been penetrated. (At the time
of writing no manufacturer had submitted their products for
this new test). Although
the Dorset Model will allow 30 minutes fire resisting membranes
to be used, this is because the Building Regulations are primarily
concerned with the means of escape for the occupants, not
property protection. I t should also be noted that in the
FRS fire test the 30 minute generic building board used had
been penetrated by fire by the end of the test, although no
collapse of the roof structure had occurred. Bearing
in mind that the tests are carried out with no wind to increase
the severity of the fire, the length of time it takes to deal
with thatch fires and the small percentage difference in cost
to protect between 30 and 60 minute membranes, DFRS feels
that 60 minutes of fire resistance is the minimum that should
be specified. As
one of the main causes of property damage in a thatch fire
is the inevitable penetration of the property by fire fighting
water then any resisting membrane chosen should be water resistant,
unless a vapour permeable (‘breathable’) roofing
felt is also used. It would be advisable to ensure that the
membrane retained its strength and rigidity when subjected
to the application of water, so as not to sag and collapse
under its own weight when wet.
The Method
New Builds
The
diagram opposite shows the usual method of roof construction
when using solid fire resisting boards. The counter-battening,
using the same battens as the rest of the roof, provides the
necessary air gap ventilation. Careful back-filling of the
thatch will ensure that the gap is uninterrupted. To
help thatchers avoid penetration of the fire resisting board
when fixing through to the hidden rafters below, the counter-batten
should be fixed down the length of every rafter by aligning
one edge of the counter-batten with the centre line of the
rafter below. The same edge, left or right, should of course
be used throughout. Thatchers using metal fixings/spikes/crooks
to fix the base coat, rather than tying it to the rafters,
can then align them down the side of the counter-batten. Once
the roof is counter-batten cross-battens are then fixed overall
at normal spacing. The
boards can be butt jointed throughout, but vertical joints
must coincide with a rafter. With some flexible and/or thinner
boards, horizontal joints can be overlapped for extra protection. If
a fire resisting flexible curtain type membrane is used instead
of boards the inherent drape of the material might provide
the necessary ventilation gap without counter-battens. The
material should be waterproof and, if fitted in close contact
with the underside of the thatch without counter battening,
‘breathable’. It is important that manufacturer’s
joining and fixing instructions are followed. The
use of a 50º roof pitch will encourage the use of single
layer thatch, or base coat and single layer, and avoid the
necessity of thatchers applying multi-layers to correct a
low pitch. Lead flashings are more efficient than mortar in
the prevention of rain water access to the flue/thatch interface
of brickwork joints and are ideal when used with single layer,
or base coat and single layer.
Where New Thatch Meets Old
The
design of the roof construction for a new extension should
be such that a single layer of thatch, or base coat and single
layer, will give sufficient depth to meet the existing roof
line, even where this is a deep multi-layer roof. The common
method of constructing the extension’s timbers to meet
the original roof timbers should be avoided, unless the existing
is already an adequately pitched single layer roof. Where
an extension has new ‘protected’ thatch butting-up
to existing ‘unprotected’ thatch some thought
needs to be given as to how to prevent fire spread from one
to the other. A fire in the extension’s thatch could
easily enter the roof void by travelling horizontally and
ignite the old, unprotected roof, and vice versa, completely
negating the fitting of the fire resisting membrane. As
overdrawing would only be possible on existing roofs if it
was necessary to strip the thatch back to the timbers, underdrawing
of the existing thatch with a fire resisting barrier within
the roof space is one possible solution. The fixing of rigid
fire resisting board to the underside of pole rafters is not
easy to achieve, although by no means impossible, by the utilisation
of packing strips. However, these are various flexible fire
resisting membranes which would be easier to attach. If one
of these is used it should be extended where the new thatch
meets the old, so as to pass over the new membrane fixed to
the top of the new rafters by at least 300mm. It should, however,
be noted that no tests have yet been carried out on this method.
Unless this underdrawn membrane is waterproof it is advisable
to supplement it by one of the ‘breathable’ roofing
‘felts’.
Netting
If
galvanised wire netting is used to protect the thatch against
bird and vermin attack then it should be laid vertically from
ridge to eaves with selvedges flush, and not overlapping,
before the edges are joined by being twisted together with
a metal hook. This facilitates easier removal in case of fire. Although
plastic netting is available, as it is fitted as a single
roof covering it cannot be removed in the same manner, but
has to be cut free. It
should be noted that the fast removal of thatch is one of
the main techniques that the Fire Service use to prevent fire
spread. Anything that slows down this removal is obviously
not going to be of assistance in this. Deep thatch, netting
by a non-standard method, or the use of steel sways, instead
of the traditional hazel, (the horizontal rods under the top
layer used to fix the thatch to the rafters), all means that
the thatch will take longer to strip off.
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