Proceedings e report - Firenze University Press

BIOLOGICAL DETERIORATION OF HISTORICAL WOODEN ROOF AND FLOOR STRUCTURES AND THEIR RENOVATION
Fig. 4. Typical floor structures: massive and hollow timber floors.
3.2. Degrading biological agents
According to [1] there are two types of biological agents degrading timber: wood destroying fungi and
insects. Fungi cause two major kinds of decay – “brown rot” and “white rot”. With brown rot only the
cellulose is extensively removed, the wood takes a brownish colour and it trends to crack across the
grain. With white rot both lignin and cellulose are removed, the wood appears whiter but doesn't
crack. Until it is severely degraded, it retains its dimensions and the deterioration is hardly visible. But
both kinds considerably lower the weight and consequently the wood strength.
The second degrading agents are insects. In this part of Europe wood is mostly attacked by powderpost
beetles. They lay the eggs in the wood pores and their larvae are feeding with cellulose, starch,
sugar, and albumin from the wood, mostly the sapwood. They make tunnels and when grown up leave
the wood packed with fine powder. The winged adults leave the wood through the surface which was
until then untouched, and we can notice their presence too late. They leave the wooden elements bored
through with substantial loss of mass and strength.
They are two major types of insects that attack the installed wood in our climatic conditions: one from
Lyctus family called “house capricorn” or “old house borer” (Hylotrupes bajulus L.), and another from
the Anobiid family (Annobium puncatum De Geer). The Lyctus larvae need for their growth higher
temperatures (28–30°C) whereas Anobiid beetle can live also on lower temperatures (22–23°C). Both
prefer high moisture (30%) but Anobiid beetle can leave also at lower moisture (15%). Climatic
conditions favoured by Lyctus beetles are exact the same as those in wet roof structures, so it is very
important to prevent leaking or condensation on the closed timber roof structures.
3.3. Roof structure - findings
A thorough inspection of practically all roof elements has been performed and a lot of damaged parts
of the structure have been found [3]. The most problematic parts of the structure were found on spots
where long term wetting was present. In these parts combined attack of fungi and wood insects
(Hylotrupes bajulus L.) caused substantial deterioration of wood, sometimes even total loss of
strength. Wetting was caused primarily by leaking at bad connections of roof planes (especially in
valleys) and the bad details at the gutters. (The same findings were reported before [2].)
In our case we found the most damaged parts in the above mentioned bottom of the gutter valleys.
These parts were practically completely destroyed by the attack of fungi causing brown rot: the wood
became brownish, cracked across the grains and totally lost its strength.
Fig. 5. Destroyed parts in the bottom of gutter valleys due to fungi attack.
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