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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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