03 Timber Decay Flashcards
What are the main defects typically associated with timber?
- Rot (Wood-rotting fungi)
- Insects (Wood-boring insects)
What is the average moisture content for internal and external timber?
What are the types of timber decay?
- Brown rot
- White rot
- Dry rot
- Wet rot
- Soft rot
(taken from Weathertightness: Guide to Diagnosis of Leaky Homes)
What is brown rot?
Brown rots (at advanced stages) usually cause wood to lighten in colour prior to becoming dark brown, and to crack along and across the grain (although only
once dry). When dry, very decayed timber will crumble to dust
What is white rot?
White rots at well-advanced stages cause the timber to become lighter in colour and fibrous in texture without ‘cross checking’ along and across the grain
What is dry rot?
- Common term for a brown rot, serpula lacrymans (creeping tears). - Relatively rare in New Zealand but it is a serious problem when found.
- Difficult to distinguish from other brown rots, so field observations must be backed up with laboratory testing.
Serpula lacrymans does not attack dry wood. It cannot decay wood at moisture content values below 18 percent. It can, however, move
moisture over considerable distances from wet areas to dry areas via thick visible mycelial cords and can also spread across wide fronts on initially dry wood if very high atmospheric humidity prevails (above 85 percent and optimally close to 100 percent) and alkaline conditions are present (such as in fibre-cement base materials). If the air is moving and relative humidity values are no more than 75 percent, this is usually sufficient to retard dry rot growth across dry
wood.
The main concern with dry rot is that decay is very rapid once suitable conditions prevail.
- Type of brown rot that thrives and spreads rapidly in damp buildings and caused by a certain type of fungus present in the air latching onto damp timber to use as a food source under specific conditions, namely:
- Moisture content is between 20-35%
- Temperature is between 0-26°C
- Space is not ventilated
What is the life cycle of dry rot?
- Spores - microscopic fungus spores omnipresent in the air land on timber surfaces
- Germination - if the timber is damp, the spores germinate (grow)
- Hyphae - upon germination, the spores begin to grow fine white strands (not unlike cobwebs) known as hyphae. Hyphae reach out in search of moisture and it is through this process that it feeds on the timber and causes decay
- Mycelium - a mass of hyphae forms, known as mycelium, which continues to feed on the organic matter and is capable of spreading over large distances, including through masonry and plaster
- Fruiting body - a sporophore within the mycelium develops which thrives on the moisture being brought back to it from the hyphae. The fruiting body releases further spores that travel on air currents to other susceptible areas for the lifecycle to start over
What problems are associated with dry rot?
- Timber becomes dry and crumbly
- Reduces structural integrity of timber
- Can spread through an entire building given the right conditions
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify dry rot?
- Decayed wood has dull brown colour with deep cuboidal cracking along and across the grain, light in weight and can crumble between fingers
- The rot has left no skin of sound wood
- Hyphal strands are white/grey in colour and 2-8mm thick
- Silk-white sheets or cotton wool-like mycelium
- Rusty red coloured spores
- Reddish brown fruiting body with grey/white edges, usually pancake or bracket-like in shape
- Conditions (such as lack of ventilation, moisture content of timber etc.) should also be used in identification
What steps would you recommend to remediate dry rot?
Set out in BRE 299 (Dry Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood and cut away timber 300-450mm beyond last evidence of rot
- Do not retain timber infected by dry rot without seeking expert advice
- Strip back affected plaster and contain fungus within wall by applying surface biocides or fungicidal paints/renders
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is wet rot?
Wet rot refers collectively to all other brown and white rots.
Type of white or brown rot that thrives in wetter conditions (mainly external joinery) and is caused by a certain type of fungus present in the air latching onto the damp timber to use as a food source
What problems are associated with wet rot?
- Reduces structural integrity of timber
- ‘Softened’ timber becomes more easily attacked by wood-boring insects
How would you identify wet rot?
- Where species is a white rot, wood becomes lighter (as if bleached) and cracked along the grain
- Where species is a brown rot, wood becomes darker with cuboidal cracking (but not as severe as that found in dry rot)
- A thin veneer of sound wood remains
- Often found where wood is repeatedly wetted (e.g. as a result of faulty plumbing or leaking gutters)
- Hyphae, mycelium and fruiting bodies differ between species, however typically:
- Hyphae (not always present) is thinner than dry rot and flexible when dry, usually creamy-white in colour
- Mycelium (usually not present in daylight areas) is generally creamy-brown in colour
- Fruiting body is not very common but where present is usually flat and plate-like with a greenish-brown centre and yellow margins
What steps would you recommend to remediate wet rot?
Set out in BRE 345 (Wet Rot: Recognition and Control):
- Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
- Locate and eliminate sources of moisture
- Promote rapid drying of the structure through heating and ventilation
- Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
- Remove all rotted wood, however there may be occasions when it can be retained (e.g. large beams or where there are conservation considerations)
- Where retained, deeply penetrating preservative treatments should be applied to rotten areas in conjunction with other repairs (e.g. resin bonding systems)
- Apply localised superficial preservative treatment only to timbers that are likely to remain damp
- Replace any timbers necessary only with preservative pre-treated timbers
What is the difference between dry and wet rot?
- Location:
- Dry rot - rare outside
- Wet rot - stays localised to moisture source - Type:
- Dry rot - a brown rot
- Wet rot - can either be a white or brown rot - Hyphae:
- Dry rot - 2-8mm in and brittle when dry
- Wet rot - thinner and flexible when dry - Mycelium:
- Dry rot - silky white cotton wool-like sheets
- Wet rot - brown branching strands - Decaying wood:
- Dry rot - deep cuboidal cracking with no skin of sound wood
- Wet rot - smaller cuboidal cracking and a thin veneer of sound wood remains - Fruiting body:
- Dry rot - reddish brown with grey/white edges
- Wet rot - not very common but usually greenish-brown centre with yellow margins - Conditions for growth:
- Dry rot - 20-35% timber moisture content, 0-26°C
- Wet rot - 45-60% timber moisture content, -30 to +40°C
