13 - Building Pathology - Timber Decay

Question 1

What are the main defects typically associated with timber?

Answer 1

1. Rot (Wood-rotting fungi)
2. Insects (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)

(Check - does this change for local conditions in NZ?)

Question 3

What are the types of timber decay?

Answer 3

1. Brown rot
2. White rot
3. Dry rot
4. Wet rot
5. Soft rot

(taken from Weathertightness: Guide to Diagnosis of Leaky Homes)

Question 4

What is brown rot?

Answer 4

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

Question 5

What is white rot?

Answer 5

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.

Question 6

What is dry rot?

Answer 6

• 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:

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

Question 7

What is the life cycle of dry rot?

Answer 7

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 8

What problems are associated with dry rot?

Answer 8

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 9

How would you identify dry rot?

Answer 9

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 10

What steps would you recommend to remediate dry rot?

Answer 10

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 11

What is wet rot and how is it caused?

Answer 11

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).

Caused by a certain type of fungus present in the air latching onto the damp timber to use as a food source

Question 12

What problems are associated with wet rot?

Answer 12

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

Question 13

How would you identify wet rot?

Answer 13

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 14

What steps would you recommend to remediate wet rot?

Answer 14

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 15

What is the difference between dry and wet rot?

Answer 15

1. Location:
   - Dry rot - rare outside
   - Wet rot - stays localised to moisture source
2. Type:
   - Dry rot - a brown rot
   - Wet rot - can either be a white or brown rot
3. Hyphae:
   - Dry rot - 2-8mm in and brittle when dry
   - Wet rot - thinner and flexible when dry
4. Mycelium:
   - Dry rot - silky white cotton wool-like sheets
   - Wet rot - brown branching strands
5. 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
6. Fruiting body:
   - Dry rot - reddish brown with grey/white edges
   - Wet rot - not very common but usually greenish-brown centre with yellow margins
7. 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 16

How would you identify whether rot was wet or dry?

Answer 16

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 17

What is the difference between brown rot and white rot?

Answer 17

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 18

What is soft rot decay?

Answer 18

When conditions are particularly wet – moisture contents in excess of 60 percent– soft rot decay may occur.
Timber affected by soft rot often shows little outward sign of decay – the classic softening is absent. Sometimes the timber may
become a dirty grey to brown colour. When a sample at least the size of a matchstick is broken off, the fracture surface can sometimes look like a broken carrot (although juvenile wood without decay behaves in a similar fashion).

Question 19

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 19

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 20

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

Answer 20

No, wet rot cannot transform into dry rot when drying out; they are distinct types of wood decay caused by different fungi, and wet rot will not transition to dry rot simply because the moisture level decreases; the only way to address wet rot is to eliminate the source of moisture and treat the affected area with a fungicide.
A wet rot issue will continue to exist and progressively get worse until the source of the moisture is completely removed.

Question 21

How would you treat a rotting timber fence?

Answer 21

1. Rotten timber is highly infectious to other timber. Cut out 45cm on either side of rot and remove infected timber.
2. Thoroughly clean the affected area, and apply a wood preservative to the exposed wood
   3 .Replace the removed sections with new, pressure-treated timber; 4. Ensure proper drainage around the fence to prevent future rot by addressing any moisture issues causing the decay.

Question 22

You have found extensive evidence of dry rot in a building. How might this be dealt with to cause minimum disturbance to the building?
(Level 2 question?

Answer 22

(Check NZ terminology)

Splice repairs - rotten timber is removed and reclaimed timber sections are joined to the existing timber (often incorporating internal reinforcement rods) where required
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
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 23

What are wood-boring insects?

Answer 23

1. 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
2. Can get into buildings through open windows, doors, fresh-air vents, gaps in eaves etc.

Question 24

Why are wood-borer insects a problem? (damage)

Answer 24

Wood-borer insects cause damage to wood by tunneling through it, destroying the tissues that carry water and sap. This can weaken the wood and eventually lead to structural failure.

It is not uncommon for weather boards, floorboards, joists and other structural timbers to need replacing due to weakness caused by borer. The holes and labyrinths created by borer larvae also allow water to penetrate many timbers and increases rot by fungi.

Question 25

Name some of the common wood-boring insects (to NZ)

Answer 25

1. Common House Borer (Anobium punctatum) - aka ‘woodworm’ or ‘furniture beetle’)
2. NZ Native House Borer (Leanobium flavomaculatum)
3. Two-toothed Longhorn Borer (Ambeodontus tristis)
   There are upto 7 species of borer in NZ.

Question 26

What distinguishes the Common House Borer and NZ Native House Borer from others?

Answer 26

Both prefer to eat softer sapwood timber and don’t attack harder heartwood timbers. It is common to see many borer holes in a part of a weatherboard and not other parts; the attacked part will be the softer sapwood.

The flight holes are approximately 2 mm in diameter for Common Borer and 3-4 mm for Native Borer.

Question 27

What is the Long-Toothed Longhorn Borer?

Answer 27

1. It is a much larger beetle and its flight holes are larger, up to 7mm, and more oval in shape.
2. Less numerous than the common and native borer.
3. It can be a real concern in house timbers as its larvae will attack heart wood structural timbers and the larger holes they create can do more damage.
4. This type of borer is less likely to be found in buildings.

Question 28

What is the life cycle of the common borer?

Answer 28

1. Adult females lay up to 100 eggs on bare timber or in old flight holes.
2. The eggs hatch after 4-5 weeks and the larvae bore through the wood, eating it and using yeasts in their stomachs to help break down cellulose in the wood. 3. After 3-4 years the larvae will pupate in a chamber near the surface, then 4-8 weeks later the adult beetle exits the wood by eating its way to the surface creating a ‘flight hole’.
3. The adults mate and begin the life cycle over again.

Question 29

What is the life cycle of the Two-Toothed Longhorn Borer?

Answer 29

1. Can remain in the wood for up to 11 years before exiting in autumn.
2. Damage is usually severe from two tooth borer and timber will often have to be replaced. Because two tooth borer will also attack living trees, they can be more common in bush clad areas.

Question 30

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

Answer 30

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

Question 31

Name some harmless insects to timber.

Answer 31

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

Question 32

Where can wood-borer be found?

Answer 32

1. They like seasoned or moist untreated timber.
2. They are often found on the south side of buildings - prone to damp.
3. Found in floor timbers - prone to damp.
4. Fond of soft (sapwood) or untreated wood
5. Common in untreated native timbers in older homes.

Question 33

How can you prevent wood-borers?

Answer 33

1. Macrocarpa wood is naturally resistant to insect attack.
2. Provided it remains dry, radiata pine is suitable for internal framing.
3. Heart-quality macrocarpa and eucalypt species are suitabel for weather-exposed timbers.
4. For piles, the Building Code Acceptabel Solution requires timber treated to at least H5.
5. Using untreated wood would require an Alternative Solution under the Building Code.
6. Sawn timber is also more prone to borer attack than a smooth surface.

Question 34

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

Answer 34

1. Depends on the type of insect and the extent of the attack
2. Often down to a specialist company
3. Treat the wood (any time) - - Usually involves treatment with an organic solvent, emulsion or paste. This treatment will penetrate deep into the timber and will kill borer larvae as they eat the wood and prevent adults from laying eggs on the surface
4. Inject flight holes with specialist aerosol fluid. The aerosol forces insecticide into the labyrinth created by the borer larva killing any larva in the labyrinth or nearby. It also prevents adult beetles from laying eggs in the flight hole.
5. Bomb the Borer (October-March) - During each flight season, set off a fumigator ‘borer bombs’ in living spaces, roof voids and sub-floor areas. These knockdown adult beetles that would lay eggs on the exposed timbers and give protection to the surface of the timbers.
6. Dry the wood (all the time) - Borer prefer timber that has some moisture in it. Ensure dry and well-ventilated. Check for leaks.
7. Sometimes when wood is so wet and rotten, preservative treatment is pointless and timbers require replacing
8. New timbers may need to be placed alongside affected timbers, or completely replaced depending on the extent of the damage

Question 35

What are the concerns over timber treatment?

Answer 35

1. The toxic effect on human beings
2. The effective dose which will destroy the infestation
3. The risk to the building if the treatment is not successful.

Question 36

What damage can wood-borer insects do to a building?

Answer 36

Can significantly damage a building by creating tunnels and galleries within the wood structure, weakening the timber and potentially leading to structural instability.