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This may seem obvious, but roofs and fire donít mix, and should be kept
as far away from each other as possible. The 1666 Great Fire of London,
which started in Pudding Lane and spread across London from thatched
roof to thatched roof, is testament to what can happen if the two meet.
Thankfully the introduction of the London Building Byelaws and the
Building Regulations have reduced the incidence of fires spreading
through towns and cities, and from roof to roof. But with the number of
roof fires getting lower, we are now getting complacent and leaving out
some basic fire precautions.
There are three generic fire threats to a roof:
- a fire that starts in the rooms
below the roof and spreads up into the roof
- a fire that starts in the roof and
spreads to other roofs
- a fire that starts outside the roof
and finds a way into the roof
A fire that starts in the rooms below the roof can spread up into the
roof void via holes through the plasterboard ceiling such as light
fittings, pipes and ducts and the loft hatch. Fire can also spread up
the outside of a building by melting or blowing out a window frame, and
spreading up the external wall surface, then up through the soffit
board. In the past, soffit boards were often fire resistant asbestos
cement, but since asbestos has become a controlled substance, upvc with
ventilation slots have become more popular. Any timber or plastic
material, and any form of ventilation grill or slot in the soffit, will
allow fire to spread up into the roof. It is almost impossible to allow
air into the roof without letting fire in as well.
A fire that starts in the roof (caused by faulty electrics, a careless
plumber soldering a pipe, or an owner using a candle to see into the
loft) can spread relatively quickly. The majority of underlay materials
are combustible, and many insulation materials are not fire resistant Ė
often containing fire retardant chemicals to suppress the effects of
fire (materials like glassfibre will shrivel up into a gooey mess). The
most readily available non-combustible insulation is rock wool.
Timber trussed rafters, unlike old roof timbers,
are so slender that they can burn through, rather than char on the
outside leaving a core of timber un-affected. Trussed rafter plates that
hold the timbers together can also be affected by fire, resulting in a
possible premature collapse of the roof structure. Once the timber roof
structure collapses the total weight of the roof structure and roof
covering can fall down through the ceiling and spread into the rooms
Adjacent property fire
A fire that starts in an adjacent property can spread through the
batten cavity, through a parapet wall gutter framing, or through an
eaves soffit box. Generally these areas are not adequately protected. If
the fire is above the roof and drops onto the roof, most roof coverings,
being clay, concrete or slate are incombustible and will resist the
fire. However, large gaps between the tiles or slates can allow flames
to penetrate the roof covering and set light to the battens and
underlay. Also, plastic ventilation tiles and fittings can melt leaving
an opening that will allow the fire to penetrate the roof covering.
Why do we have fewer roof fires now? The answer is that
we have better safety standards regarding the causes of fire. There are
fewer open fires to heat buildings, better working practices for
plumbing and electrics, fewer people smoking inside buildings and fewer
products containing volatile chemicals. By preventing more fires, fewer
spread into the roof. But when a fire does start, the roof is possibly
more vulnerable that ever before.
During a roof fire the fire brigade will
spray lots of water onto the affected section of roof to quench the fire
and onto the adjacent roofs to stop the spread of the fire. In many
instances this practice is very effective. However, where water is
poured onto materials that are hot, the sudden quenching of the heat can
result in micro-cracking of the material. While visually the tiles or
slates may look as if they are suitable for reuse, often they can be
badly affected by frost and disintegrate within a few years.
It is not advisable to reuse salvaged tiles from the
area affected by fire, but if the underlay is intact the temperature in
that area is unlikely to have been high enough to have affected the
tiles or slates.
What can the roofing industry do? Looking at the list of ways fire can
spread into a roof, the majority of solutions are specification related,
not roofing related.
- The roof structure itself could be
non-combustible, such as steel.
- The insulation should be
non-combustible such as rock wool.
- All apertures through the ceiling
should be sealed with non-combustible material or be manufactured
from fire and heat resistant materials.
- All soffit material should be fire
resistant, such as fibre cement or metal.
- At party walls, rock wool fire
stopping should be installed between the battens and in the soffit
box and under parapet walls where specified.
- Ventilation tiles and other
penetrations through the tiles or slates should be kept to a
- Gapping between the tiles and slates
should be kept to a minimum.
- All heat and flame producing
equipment and tools on a roof should be kept under strict control.
While we have the Building Regulation
Part B Fire, and British Standards 476 parts 3, 4, 6 and 7 that provide
some guidance and test methods, there is much that should be done to
improve our roofs from the spread of fire. Where fire is concerned,
prevention is always better a cure. A building constructed from 100%
non-combustible materials would be the ideal solution. But in the
present climate this is not economically achievable.
by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove,
Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774