Civil Engineering Reference
In-Depth Information
Figure 9.4
Serpula lacrymans - Dry rot spores © Lutz Weidner. Reproduced under the terms of the GNU Free Documentation License
The fungus can remain active in timber with a moisture con-
tent of 20-30% and the threads enable the dry rot to spread
across inert material such as brickwork or concrete to reach
other timber locations. It is on this basis that treatment of the
rot necessitates the removal of all the timbers, plaster and fin-
ishes beyond the visible extent of the strands before treating
with fungicides. However, it is now thought that removal of
the infected timber, eradicating the source of dampness and
improving ventilation, is sufficient to prevent the fungus from
returning.
As opposed to the dry rot fungi which source moisture in
the timber to feed and grow, wet rot is typically the rotting of
the timber elements subject to long-term wetting over a period
of time with a persistently damp moisture content of 50-60%.
This is especially found where the end grain of the timber is
exposed in locations such as roof timbers, floor beams and
timber joints where the timber will stay wet due to lack of
ventilation and gradually rot away over time. The visual sign
of wet rot is a darkening (brown rot) or bleaching (white rot)
of the timber along with cuboidal surface cracking. Wet rot is
responsible for up to 90% of the wood decay in buildings and
it attacks both hard and soft woods.
punctatum
, the Powderpost Beetle
Lyctus brunneus
and the lar-
gest of all, the Death Watch Beetle
Xestobium rufovillosum
.
The Common Furniture Beetle is usually found in the sap-
wood of both hardwoods and softwoods and usually in damp
areas such as timber ground floor construction or roof voids
where the timber moisture content is above 12%. The beetles
will emerge from the timber usually between May and August
and the flight holes will measure 1-2mm in diameter.
The Powderpost Beetle attacks the sapwood of hardwoods
such as oak, chestnut, ash and elm, particularly young wood
which becomes less susceptible to attack as it ages. Wood
beyond the age of 15 years is thought to be immune from fur-
ther damage from this beetle. The beetles emerge throughout
the year with flight holes approximately 1.5 mm in diameter
creating fine bore dust.
The Death Watch Beetle is found in large-sectioned hard-
wood timbers such as oak or elm, where there is already some
form of rot present. Therefore this beetle is commonly found
in older properties with damp conditions although should the
timbers dry out then the attack will slow or even stop. Whilst
the oak and elm hardwood is considered the preferred wood of
choice by the beetle, the infestation may also attack softwood if
close by and/or if rotten. The beetle usually emerges in spring
through a 3 mm diameter hole creating gritty bore dust.
As with all these boring insects the larvae feeding within
the timber leads to the deterioration, loss of section and overall
9.5.4 Insect attack
The most common form of wood boring insects in the UK
are the Common Furniture Beetle (woodworm)
Anobium
