Rot in Timber: Causes, Testing, and Treatment

Rot in Timber: Causes, Testing, and Treatment

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Published by TOP4 Team

Ever noticed small growths or stains on your timber a day or so after rain? These are signs that timber rot may have started, which if left untreated, could cause some serious damage to your home.

Rot is a fungal attack and is nature’s way of breaking down dead wood for decomposition. Sadly, fungus doesn’t recognise that your house isn’t meant to be recycled.

If you see fan-like or lace-like strands and fibres or brackets on the surface of timber, chances are it’s a fungal attack. Broadly speaking, there are three types of rot:

Brown rot – breakdown – in the form of cubic shapes – and darkening of the timber. This commonly occurs in softwoods.
White or soft rot – pale discolouration and softening (with spongy appearance) of the timber. It mostly occurs in hardwoods.
Dry rot – this cool-temperature rot needs moisture, and results in a powdery disintegration of the timber.

Where does it occur?

Timber that has more than 20 per cent moisture is a good candidate for rot. Few timbers are naturally immune to an attack. Even preservative-treated timbers can be affected if not given the necessary treatment.

The area under a house is a common place for fungal attack, particularly in older homes that have inadequate ventilation.
Fascias and bargeboards often rot at the corners or where there are joints that harbour moisture in the end grain.
Fencing is often attacked near the ground where air and moisture are prevalent.
Windowsills and frames exposed to elements without a good coat of paint can also rot as the timber grain opens.
Wall frames behind leaky showers are at high risk of rot and structural damage.
Exposed balconies that have rotten joists can be dangerous, especially if they are cantilevered and do not have posts at the outer edge.

The deep joists are often made of non-durable Oregon pine, the top surface of which can’t be maintained as it’s covered by decking. When the joists rot, the balcony can be severely weakened.

How to test for rot

To test for rot, take a splinter of wood from the surface of the timber with a penknife. If it goes into soft or brittle timber, of the splinter is brittle and breaks off easily, the rotting process has already taken place. If there are fibres clinging to the timber, the wood is sound.

How to treat rot in timber

The good news is timber that is slightly affected by rot can still be treated. If the timber is slightly affected by rot, scrape it off, improve ventilation, control the moisture, and coat with a timber preservative such as Blue 7, or a copper naphthenate-based preservative oil.

Timber that has been greatly weakened by rot should be replaced. Rot in windows or door frames can often be treated or partly replaced. Cut tack 600mm beyond where you see the rot occurring.

Professional help

If you feel that the timber replacement project would require the help of a professional tradie, make sure to hire the right one. Always check for licensure and ask for referrals to ensure reliable, high quality service.

For more home improvement tips, check out our Trades and Services page.


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