02 Timber Decay

Question 1

What are the main defects typically associated with timber?

Answer 1

1. Wood-rotting fungi
2. Wood-boring insects

Question 2

What is the average moisture content for internal and external timber?

Answer 2

1. Internal timber: 2-16% (depending on level of heating within building)
2. External timber: +20% (depending on weather conditions)

Question 3

What is dry rot and how is it caused?

Answer 3

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:

1. Moisture content is between 20-35%
2. Temperature is between 0-26°C
3. Space is not ventilated

Question 4

What is the Latin name for dry rot and what is its meaning?

Answer 4

Serpula Lacrymans (literal meaning: ‘creeping tears’)

Question 5

What is the life cycle of dry rot?

Answer 5

1. Spores - microscopic fungus spores omnipresent in the air land on timber surfaces
2. Germination - if the timber is damp, the spores germinate (grow)
3. 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
4. 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
5. 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

Question 6

What problems are associated with dry rot?

Answer 6

1. Timber becomes dry and crumbly
2. Reduces structural integrity of timber
3. Can spread through an entire building given the right conditions
4. ‘Softened’ timber becomes more easily attacked by wood-boring insects

Question 7

How would you identify dry rot?

Answer 7

1. Decayed wood has dull brown colour with deep cuboidal cracking along and across the grain, light in weight and can crumble between fingers
2. The rot has left no skin of sound wood
3. Hyphal strands are white/grey in colour and 2-8mm thick
4. Silk-white sheets or cotton wool-like mycelium
5. Rusty red coloured spores
6. Reddish brown fruiting body with grey/white edges, usually pancake or bracket-like in shape
7. Conditions (such as lack of ventilation, moisture content of timber etc.) should also be used in identification

Question 8

What steps would you recommend to remediate dry rot?

Answer 8

Set out in BRE 299 (Dry Rot: Recognition and Control):

1. Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
2. Locate and eliminate sources of moisture
3. Promote rapid drying of the structure through heating and ventilation
4. Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
5. Remove all rotted wood and cut away timber 300-450mm beyond last evidence of rot
6. Do not retain timber infected by dry rot without seeking expert advice
7. Strip back affected plaster and contain fungus within wall by applying surface biocides or fungicidal paints/renders
8. Apply localised superficial preservative treatment only to timbers that are likely to remain damp
9. Replace any timbers necessary only with preservative pre-treated timbers

Question 9

What is wet rot and how is it caused?

Answer 9

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

Question 10

What problems are associated with wet rot?

Answer 10

1. Reduces structural integrity of timber
2. ‘Softened’ timber becomes more easily attacked by wood-boring insects

Question 11

How would you identify wet rot?

Answer 11

1. Where species is a white rot, wood becomes lighter (as if bleached) and cracked along the grain
2. Where species is a brown rot, wood becomes darker with cuboidal cracking (but not as severe as that found in dry rot)
3. A thin veneer of sound wood remains
4. Often found where wood is repeatedly wetted (e.g. as a result of faulty plumbing or leaking gutters)
5. 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

Question 12

What steps would you recommend to remediate wet rot?

Answer 12

Set out in BRE 345 (Wet Rot: Recognition and Control):

1. Establish the size and significance of the attack, particularly if structural timbers are affected as measures may be needed to secure structural integrity
2. Locate and eliminate sources of moisture
3. Promote rapid drying of the structure through heating and ventilation
4. Introduce support measures (e.g. ventilation pathways between sound timber and wet brickwork, barriers such as DPMs or joist hangers etc.)
5. Remove all rotted wood, however there may be occasions when it can be retained (e.g. large beams or where there are conservation considerations)
6. Where retained, deeply penetrating preservative treatments should be applied to rotten areas in conjunction with other repairs (e.g. resin bonding systems)
7. Apply localised superficial preservative treatment only to timbers that are likely to remain damp
8. Replace any timbers necessary only with preservative pre-treated timbers

Question 13

What is the difference between dry and wet rot?

Answer 13

• 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

Question 14

How would you identify whether rot was wet or dry?

Answer 14

1. Location - if outside, likely to be wet rot
2. Conditions - if space is unventilated, could be dry rot
3. Appearance - deep cuboidal cracking, no skin of sound wood, rusty red spores, fruiting body and mycelium would indicate dry rot
4. Smell - mushroom smell would indicate dry rot
5. Moisture content - dry rot would be between 20-35%, wet rot would be 45-60%

Question 15

What is the difference between brown rot and white rot?

Answer 15

1. White rots - cause wood to become lighter in colour and fibrous in texture, without cross-cracking
2. Brown rots - cause wood to become darker in colour and crack along and across the grain, forming cubes in the decaying process. When dry, very decayed wood will crumble to dust

Question 16

Timber decay is often found to suspended timber floors in traditionally constructed buildings. What does the occupier commonly do to the property to lead to this problem?

Answer 16

1. Often block the air bricks
2. Inadequate ventilation will allow the RH of the air below the floor to rise (particularly if the ground is wet), as the air steadily absorbs water
3. At the same time, the moisture content of the floor structure will slowly settle into equilibrium with the damp air, causing its moisture content to rise, thus putting it at risk of decay

Question 17

Is there a potential for wet rot turning to dry rot when drying out?

Answer 17

???

Question 18

How would you treat a rotting timber fence?

Answer 18

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Question 19

Your client owns a Grade I listed building and has found evidence of extensive dry rot. How might this be dealt with to cause minimum disturbance to the building?

Answer 19

1. Splice repairs - rotten timber is removed and reclaimed timber sections are joined to the existing timber (often incorporating internal reinforcement rods) where required
2. Resin bonding systems - used for localised repairs (e.g. window frames) where the rotten timber is removed and a 2-part epoxy resin is mixed and applied to the timber, effectively ‘filling in’ the voids left by the rotten timber, hardening and sanded to a smooth finish
3. Introducing supplementary structures - allows existing timbers to be left in place whilst new supplementary supports perform its function (e.g. doubling up timber, fixing discrete metal plates etc.)

Question 20

What are wood-boring insects and why are they a problem?

Answer 20

• Wood-boring insects are insects that use wood as a food source, as well as a habitat, and can cause damage to timbers within buildings by eating away at it
• Can get into buildings through open windows, doors, fresh-air vents, gaps in eaves etc.

Question 21

What is the general lifecycle of a wood-boring insect?

Answer 21

1. Larva (worm): 1-5 years
2. Pupa (larva to adult): 6 weeks
3. Adult (beetle): 2-3 weeks

Question 22

Name some of the common wood-boring insects.

Answer 22

1. Common furniture beetle (aka ‘woodworm’)
2. Deathwatch beetle
3. House longhorn beetle
4. Lyctus powderpost beetle
5. Ptilinus beetle

Question 23

What are the two most common wood-boring insects?

Answer 23

1. Common furniture beetle (aka ‘woodworm’)
2. Deathwatch beetle

Question 24

What are the key indicators to identify different wood-boring insects?

Answer 24

BRE Digest 307 (Identifying Damage by Wood-Boring Insects) contains extensive guidance, including:

1. Size of flight holes
2. Bore dust (frass) colour
3. Geographical location
4. Type of wood attacked

Question 25

Where is the Longhorn beetle most known to attack?

Answer 25

Only common in the south-east, particularly Surrey

Question 26

Name some harmless insects to timber.

Answer 26

1. Woodlice
2. Silver fish
3. Ants
4. Earwigs
5. Millipedes

Question 27

What works should be undertaken to eliminate and deal with the effects of wood-boring insects?

Answer 27

1. Depends on the type of insect and the extent of the attack
2. Often down to a specialist company
3. Usually involves treatment with an organic solvent, emulsion or paste (or sometimes smoke for the deathwatch beetle)
4. Sometimes when wood is so wet and rotten, preservative treatment is pointless and timbers require replacing
5. New timbers may need to be placed alongside affected timbers, or completely replaced depending on the extent of the damage

Question 28

What changed within the building regulations to deal with a certain wood-boring insect?

Answer 28

House longhorn beetle - Approved Document A prescribes geographical areas where softwood timber for roof construction must be treated against infestation

Question 29

When must the BRE be notified of attacks from wood-boring insects?

Answer 29

BRE must be notified of every house longhorn beetle attack