Almost all timber will deteriorate when exposed to moisture for long periods of time. Fungal decay or rot is the result. Rot in timber is most likely to be found in untreated framing in 'leaky' homes, around brick chimneys, around windows, on weatherboards on the side of the house most exposed to the weather and in framing close to the ground.
There are three broad categories of rot – brown rots, white rots and soft rots.
Brown rots eventually darken the appearance of the timber as they consume cellulose only, leaving the darker woody lignin behind. (Cellulose is a key part of plant cell walls and makes up about 40-50% of wood. Lignin binds cellulose and other substances in the cell walls.) Decayed timber may still look satisfactory but can be readily penetrated by a knife. When dried, the affected wood will darkenm and cross-grain cracks appear.
In the very early stages, brown rot fungal growth is invisible to the naked eye. Its presence can only be determined by microscopic examination by a suitably experienced mycologist (a scientist who specialises in fungi). Once progressed into early decay, it can be detected by evidence of bleaching, staining or loss of timber fibre strength. However not all staining or strength loss is due to decay.
Brown rots are considered to be more severe than other types of rots. This is because they decay timber more rapidly, and once started, they tend to operate at lower moisture levels than other rots.
One particularly malignant brown rot is Serpula lacrymans, commonly known as dry rot. It derives its name and feared reputation because of its ability to transport moisture, enabling it to attack otherwise dry timber. It can grow over masonry and cement-based materials, obtaining both moisture and nutrients from these sources. It is very destructive if established. All dry rot infected timber must be removed completely and destroyed by burning. Dry rot is less common in New Zealand than other types of rot. It grows in alkaline soils and is usually found in damp, poorly ventilated subfloors. It is uncommon in wall framing containing untreated radiata pine.
While white rots appear to bleach the timber, giving decayed timber a yellow-white fibrous appearance. The colouration is because they consume both the lignin and the cellulose. White rots commonly operate at mid to higher moisture levels. They are often found in decaying timber weatherboards and external timber joinery.
Timber infected by soft rots often shows little outward sign of decay. The only sign is that the timber may have darkened or appear to be greyish. The decay is taking place from within the cell wall and only becomes apparent when prodded with a sharp object. In advanced stages of soft rot decay, the timber can easily be carved with a sharp knife. Sometimes, the latewood bands in the timber (the growth that comes later in each season) may darken, and if a matchstick-sized splinter is snapped off, the fracture surface will look like that of a broken carrot.
Soft rots are usually found on timber in contact with the ground. They tend to be more resistant to fungicides than brown rots or white rots and need elevated moisture levels to grow. In boron-treated timber, soft rot is the common form of decay.
Rot in timber is one of the hardest issues to deal with, simply because affected timber is not always readily identifiable to the naked eye. Also, timber can appear stained and yet be sound. As a general rule, all timber exposed during remediation should be site treated to improve its future durability.
Lab analysis is the only sure way to determine the presence of rot.
Determining the extent of rot
While the NZIBS Building Surveyor’s report should provide a reasonable estimate of the extent of rotted timber, the full extent can only be determined once the cladding or internal lining has been removed. Once the building is opened up, it is recommended that timber samples be taken, under the supervision of an experienced NZIBS Building Surveyor, for testing to determine the actual extent of the timber damage or strength loss and level of timber treatment present.
Over time, designers and project managers may build up sufficient knowledge to direct the timber testing and removal regime.
Samples for laboratory testing need to be sections of timber approximately 100 mm long with the position of each sample uniquely identified and the location it was taken from recorded.
Timber that is considered, after testing, to have lost strength as a result of the rot must be cut out and replaced. Experienced NZIBS Building Surveyors advise that it is better to err on the side of caution when replacing untreated timber framing.
For a wall stud, it is likely to be easier and more cost-effective to remove and replace the affected members rather than try to cut out the rot and flitch in new framing. As a general rule of thumb, it is considered more economic to do a total replacement where more than 50-60% of the timber is affected by rot. NZS 3604 does not allow the jointing of studs, so any rot-affected stud needs to be replaced.
- remaining timber framing must be supported as required until the new framing is installed
- remaining accessible timber is treated with two coats of brush-on boric solution – including drilling and so on in accordance with the MBIE Building and Housing guide Dealing with timber in leaky buildings
- new timber is treated in accordance with NZS 3602 for its use location – a minimum treatment level of H1.2 is required where cladding is to be direct fixed
- dry storage should be provided for replacement timber
- once installed, protect from wetting.
Where wall framing damage is extensive, the repair may also require the replacement of internal linings, internal trims, insulation, wall underlays and wiring.
It may also require the demolition or removal of kitchen fittings, bathroom fittings, windows and wet area internal linings to allow existing timber to be removed and new timber installed.
Floor joists that are infected and have lost strength need to be cut back to the point where the strength loss ceases to occur. Timber that has retained its strength but has fungi present will need to be treated.
Where there are boundary joists, the visible face exposed after the cladding is removed may appear sound, but it is common for deterioration to occur between the outer and inner joist.
Depending on the design of the building, it may be possible to insert a new beam within the floor space to support the remaining length of joist and the replacement joists usually utilising joist hangers.
The amount of timber that needs to be removed may vary between units in a multi-unit development. Boundary joists that are damaged will usually need to be treated in the same way.
One area of difficulty is construction involving multiple or laminated framing members where the members in the middle are affected by rot but the two outside members are sound.
Timbers adjacent to that removed (that are accessible) should be treated with a paint-on boron-based solution, which must:
- be applied at the recommended rate to all faces
- not be used in weather-exposed situations unless coated by a well maintained paint system
- not be in ground contact
- be applied into holes drilled into built-up members.
Structural engineering advice is required where anything less than straight replacement of full members is to occur.
Updated: 27 May 2016