Chapter 2

Why do timber frames fail? 4/5.

2.5 Decay - Rot - Beetle Attack .

A significant factor in timber frame failure is the decay of the stressed timbers resulting in a loss of their structural integrity. There are essentially two causes of decay: rot and beetle attack. Rot is usually confined to dry or wet rot.

Dry Rot is one of the most serious fungal 'diseases' in the built environment and occurs in timber infected by the causative organism Serpula lacrymans. This affects mostly softwoods and infestation typically occurs where wood comes into contact with stone or brickwork. Dry rot has the ability to grow through masonry. Strands can transport moisture from damp areas enabling the fungus to spread. Damage characteristics are that the infected wood is dull brown in colour (Fig 2.12) and deep cracks appear along and across the grain. The timber becomes light in weight and crumbles easily.

Rot attack in cill plate

Fig 2.12. Dry rot is visible in this sole plate in the Wheat Barn at Cressing Temple. Its cause is both rising damp and an ineffective gutter system which vented onto the ground. Remedial work included re-routing the drainage and lowering the exterior ground level.

There are many fungal species causing wet rot (Cellar Fungus, white rot, brown rot, oyster fungus, white spore or mine fungus) and remedial treatments are required for all of them. There may be signs of a darkening of the timber or a bleaching. Certain types are found only in buildings and only in specific areas. Fungi exist and grow by feeding off dead wood, causing it to break down, changing its weight and colour. The most common wet rot, the cellar fungus (Coniophora puteana), must have very damp conditions, and is most easily recognise by its dark brown threads spreading over the surface of the timber.

Both types of rot demand an unusually high water content that might be the result of, for example, a leaking rain gutter or rising damp. They prefer warm conditions but will not thrive over 25 degrees centigrade.

Beetle infestation is limited in this country to the Deathwatch (Anobiidae, especially Xestobium rufovillosum) and the wood boring beetle or wood worm (the common name for the larval stage of certain wood-boring beetles). Dead or injured trees are their natural target but they also attack structural timber and furniture. Included are the furniture beetle Anobium punctatum, which attacks older timber and the powder-post beetle genus Lyctus, which attacks newer timber.

All wood boring beetles digest cellulose and they will eat timber which has more than 10-12% moisture content. Left unchecked they will completely consume a timber until it collapses.

In the case of rot or beetle decay, ironwork will be introduced only if the infestation has been checked at an early stage and there is enough sound timber to anchor the iron to. Normally in an advanced stage of decay removal and replacement is the only remedy.

Rot and beetle infestation can be totally eradicated by drying the timbers out completely and exposing them to daylight to kill any spores.

2.6 Erosion.

Erosion of timber by constant water action or exposure to high winds normally occurs only in wind and watermills and even then is rare. Erosion by animal action (Fig. 2.13), especially rodents is very common and, historically, may have led to metal plating and strapwork to reinforce the damaged members.

Erosion  of timber by cattle

Fig. 2.13. The corner of the porch of the Wheat Barn at Cressing Temple. The timbers have been exposed to water and animal erosion. The yard contained long-horn cattle who enjoyed licking the sodden timber. This eroded the woodwork far quicker than normal.

2.7 Poor workmanship

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