Appendix 2: Timber rot and moulds
Issues related to timber decay and mould identification are probably the least understood and most complex of the entire remediation process. There is relatively little information available for consultants or specialists on where to take timber framing samples, how many samples to take, the size of samples, and the ongoing need for sampling throughout the remediation repair process. This appendix expands on areas related to timber decay.
The most critical aspect of timber decay is to determine the extent of the decay. Timber that is clearly decayed is easy to identify. The difficult part is to detect decay where the timber can appear quite normal even to a specialist. It is only when a sample is examined under the microscope by a trained and experienced specialist in fungal and mould behaviour, that the type and extent of decay can be determined. It is therefore essential to work closely with an experienced laboratory specialist who can give advice based on microscopic analysis.
It is also important that samples continue to be sent for specialist analysis throughout the repair stage as more of the building is opened up.
The process of assessing decay may be seen as four phases.
1. Before inspection
2. During inspection
3. Laboratory analysis
4. Using the results
Types of rot
While identification should be made in the laboratory, the following is a general guide.
- Brown rots 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.
- White rots cause the timber to become lighter in colour and fibrous in texture without cross-checking.
- Dry rot is the common term for one brown rot Serpula lacrymans, and is relatively rare in New Zealand. However, it is also difficult to distinguish from other brown rots so field observations must be backed up with laboratory testing. The main concern with dry rot is that decay is very rapid once suitable conditions prevail - and it can move moisture considerable distances to dry wood and cause decay of timber that would have otherwise remained dry and resistant to other types of decay.
- Wet rot refers collectively to all other brown rots and white rots.
- Soft rot often shows little outward sign of decay in the affected timber (ie, classical 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 similarly).
A 2.1 Before inspection
Collect information and identify particular risks as discussed in Section 1: General information collection stage to help determine where cladding cut-outs should be made and timber or mould samples taken.
Figure 3: Common areas of weathertightness risk Section 1.1: The property, Section 1.2: Construction history
A 2.2 During inspection
The on-site inspection is part of the extensive diagnosis process described in Section 2. Before any moisture testing is undertaken, any early evidence of possible decay should be noted in order to inform the testing process (refer to 'Early indicators of decay').
Moisture readings should first be taken in the non-destructive or capacitance mode, followed by drilling holes through the cladding into the framing and taking readings in the resistance mode
Critical factors in both modes are to identify relative rather than absolute values. A useful approach is to identify a location that is known to not be affected by moisture penetration. This can be used as a control with other moisture readings compared with it.
Further information on the use of moisture meters (including limitations) is provided in Section 2.
Section 2.2.2: Step 2: Non-invasive testing
Early indicators of decay
- Occupants' knowledge of leaks, locations and length of time of moisture problems
- Occupants' comments about health problems that could be related to toxic moulds
- Visible signs of dampness, mould and decay (eg, corroding carpet fixings, swelling skirtings, cracked linings etc)
- Odours such as a distinctive mushroom smell associated with decay fungi
- Cracking in cladding and/or staining and discoloration are often reliable signs of moisture ingress and subsequent timber decay
Moisture problems can occur remotely from leak sources. It is important to allow for the possibility that evidence of moisture can show in unexpected positions and/or may be concealed.
An example of this is where (maybe due to daytime temperature fluctuations) external moisture evaporates within cavities; the vapour redistributes and then condenses as moisture elsewhere.
Taking on-site samples
The decision on how many and where samples should be taken for laboratory analysis is not straight forward.
It requires careful consideration and an understand-ing of possible moisture behaviour within building walls (refer to 'Moisture travel').
The number and location of samples taken are influenced by the following factors.
- Whether framing is known to be treated or not
- The length of time the timber has been subject to excessive moisture
- The extent of decay as assessed from on-site observations
- Information from drilling and other on-site testing (refer to 'On-site techniques')
- Whether the likely repairs are tending toward complete recladding or targeted repairs
- Whether initial assumptions on decay are confirmed by laboratory analysis
- The costs of taking extra samples at a repeat visit if initial assumptions prove incorrect
- Whether laboratory analysis is required as part of a dispute resolution process
- The consultant's or specialist's experience (refer to 'Using experience)
Useful site techniques (which should be confirmed by laboratory testing) include the following.
- Observing timber hardness when drilled and the nature of the drillings and comparing this with a control point in known sound timber.
- Probing timber with a sharp tool such as a chisel. If the timber breaks off into short splinters ('brashness test') when levered by the probe it is usually an indication of decay and loss of strength. Softness of the timber is also a useful indicator of decay (although juvenile heartwood may be soft irrespective of the presence of decay).
- Striking the timber with a hammer or something similar. A hollow sound from a larger timber member, or a change in note along a length of timber might indicate decay.
It is important to confirm (or otherwise) the results of on-site testing with laboratory analyses of representative samples.
Samples of untreated timber
The use of untreated kiln-dried timber for external wall framing was very common from 1996 to 2004.
If on-site testing indicates that timber is untreated and decay is widespread, then only a few samples may be required because the need for recladding and major timber replacement will be obvious.
If in doubt - check
The golden rule is, if in doubt - check, either by taking another sample or seeking another opinion from another remediation authority. Remember that remediation is a team effort, with the underlying feature being the provision of adequate information to all those involved at every step, so that risk analysis is applied effectively.
Lack of information can quickly become a major and perhaps insurmountable problem. The experience from overseas on these issues has taught us that failure to conduct adequate investigation greatly compounds problems.
In such cases, the main reasons for sampling will be to ensure the owner has sufficient evidence that a reclad is required. Reasonable evidence of untreated timber includes observing markings on the timber and/or spot tests. Laboratory analysis can confirm that the timber is untreated and also the extent of decay.
A very experienced remediation specialist:
- will be confident as to what is sound timber and what is decayed from observations and testing
- may take 2-3 samples per elevation for analysis at the fringes of decay
- will, if initial assumptions are not confirmed, take further samples for analysis.
A less experienced consultant may take more samples because of uncertainty about the extent of decay. However, consultants should remember that some decay is impossible to detect without the backup of laboratory analysis.
Samples of treated timber
Because timber can look sound but be decayed, it may
be more cost-effective to take further samples initially (to be sure of the extent of decay) than to place reliance on experience that may prove to be wrong (and therefore lead to additional site visits and sample collecting having to be undertaken).
If the timber is treated, more samples may need to be taken than for untreated timber. Untreated timber exposed to excessive moisture is likely to have severe decay after 3 to 12 months, but treated timber may be satisfactory for 2 to 5 years.
If treated timber has widespread decay because of leaks over a long time, recladding and major timber replacement will be necessary (as discussed above, relatively few samples will be required). However, if the leaking is relatively isolated (with limited decay), further samples may be required in order to reliably establish the extent of decay. Further samples will be required when targeted repairs are an option, because the aim is to determine the limits of the decay (taking into account the rule-of-thumb that all timber within 1 metre of the outer limit of the decay must be removed).
Size and nature of samples
Practices vary regarding the size and nature of timber samples that are taken.
Although laboratory analysis of very small samples is possible, it is best to submit as large a sample as possible in order to maximise the potential forensic information (which includes measuring the degree of decay, how long it has been occurring for and the extent of leaching of preservative across the section of timber and so on).
The largest framing sample would normally be about a 100 mm in length. Samples should be included which are taken from timber that is considered to be at the least decayed end of the spectrum, to set benchmarks.
The nature and size of samples also depends on the forensic information required. If the aim is to determine the length of time that the building has been leaking at a particular location, it may be important to send a sample from a clearly wet area and one from a reliably dry area. The length of time that the building has been leaking can then be estimated by the degree of leaching of the preservative compared with the piece of timber that has not been subject to moisture. The type, location and extent of decay will also enable the duration of the leak to be estimated.
Sellotape sampling for moulds
The simplest way to take a sample is to use a piece of sticky tape, which is pressed down on the mould or fungi, transferred to a grease-proof paper envelope - and then placed in a sealed plastic bag for sending to the laboratory.
While 'Sellotape' samples are satisfactory, more useful forensic information can be obtained by sending a sample of the actual material (eg, building paper or plaster board etc) with the mould attached to the laboratory for analysis.
Photograph all decay samples, both close up and from further back to show relative locations of samples on the building, as this helps with interpreting test results.
When sending samples to the laboratory, it is helpful to provide photographs showing where each sample has come from. Photographs should also be provided which give an overall perspective of the type of building under investigation (showing cladding etc), in order to assist with the forensic analysis of the samples.
Health and safety issues
When extracting samples, it is important that precautions be taken to guard against any potential hazards to those involved with the remediation work, including occupants of the building.
Diagnosis of the potential for adverse health effects from mould and other micro-organisms (eg, actinomycetes and bacteria) and their by-products is often not straightforward.
The amount of affected material and its location, and the type of micro-organisms can all have an effect. Experience is important, and advice from a specialist in fungus and mould should be sought to keep people informed of potential risks.
When taking samples:
- be careful to use suitable protective equipment including appropriate breathing masks and gloves
- remove cladding from the outside of the building rather than from the inside wherever possible (to allow any potentially dangerous fungi such as stachybotrys to be released into the atmosphere rather than into a living space inside the house)
- disturb the mould and fungi as little as possible (for instance, stachybotrys atra is far more dangerous when it has dried out and the spores readily become airborne. When wet, the spores tend to stick together and are less likely to become airborne and breathed in by the building user)
- carefully seal off any voids that may have been opened up as part of the investigation process.
A 2.3 Laboratory analysis
Identifying timber treatment
Laboratory testing is important to determine clearly the type and level of timber treatment, because doing this on site can be difficult.
Site testing for boron or copper-based preservatives can be successful, but accepted guidelines must be followed and the limitations and reliability of such tests need to be appreciated.
Moulds and fungi
The analysis of moulds and fungi found on site can only be undertaken by experienced specialists. The results of laboratory analysis can be used to provide information on issues such as the type of mould, its toxicity, how long it has been in place, and forensic details of the type of moisture elevation scenario.
Some moulds (such as stachybotrys atra and Chaetomium globossum) also cause decay in particular situations, and specialist knowledge is necessary to establish their significance in any given scenario.
Decay fungi can remain dormant in dry timber for many months and up to several years in some situations. This should be no surprise considering how long bakers yeast (also a fungus) can remain dormant in its dry form.
Laboratory testing can determine if decay was recently active or not, or if it is still viable in the case of very old infected or decayed wood.
Moulds and fungi can grow on any surface. While many do not pose health risks, stachybotrys atra and some other types of mould are toxigenic and have been implicated in building sickness syndrome. Stachybotrys atra is most commonly found on gypsum paper board, fibre-cement board, building paper and other cellulose containing materials.
Specialist laboratory advice
Advice can be available on the following.
- Type and extent of decay
- Presence of wood preservative
- Retention levels and type of preservative
- How long the decay has been present
- How long the leak/fault has been present
- How quickly decay will continue to develop
- If no decay, future risks of wood failure
- How much framing needs to be replaced
- Type of replacement framing to be used
- Speed of drying measures needed
- Appropriateness of insitu preservative application
- and the type recommended
A 2.4 Using analysis results
The results of the laboratory tests (combined with the systematic off-site and on-site investigations) can provide valuable information (refer to 'Specialist laboratory advice').
The information will allow conclusions on the:
- extent of timber damage and therefore extent of replacement timber to be estimated
- time restrictions to be established for remediation measures to be put in place
- extent and type of treatment required for replacement timber to be decided.
In this way, on-site information and laboratory testing allow an outline scope of work to be established, together with rough estimates of likely costs.
Further sampling and analysis will be required through the remedial construction stage to confirm the integrity of timber to be retained.
Some concentrated preservatives (applied by brush or airless spray) are commonly used in New Zealand. Boron-based in-situ preservative requires pre-dampening of the timber.
LOSP (Light Organic Solvent Preservative)-type preservatives can be suitable in some situations, provided the wood is essentially dry.
In some instances, it may be appropriate to under-take targeted repairs, that is, leaving the timber in-situ and applying suitable preservatives.
In such cases, it is important to follow the advice of a laboratory specialist who is experienced in the use and effectiveness of in-situ timber treatment.
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