The George Blog – Dry Rot & Comments

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Dry Rot

I've spent much of the last two weeks replacing the floor and floor joists in our apartment, The reason: dry rot!

Actually the worst dry rot that I have ever seen. And all due to construction faults (largely mine). There was a floor in the building (which was a workshop at the time) that I had put in a couple of years previously, and it was some 110mm lower than the final floor height for the apartment. I merely laid new joists on top of the existing floor. And to cap this I insulated between the two floors with fibre glass.

Now, water leaked under the door cill into the fibre glass filled void. Add to that problem, the ventilation under the original floor was pretty poor, so the soil and timber were nicely damp, and you have perfect conditional for dry rot fungus to grow.

dry rotDry rot feeds on the cellulose in timber. And given a source of water, it can transport a water supply to otherwise dry material. So above and below the original floor we had a bell curve shaped matt of dry rot fungus about 3800mm across the base and 3300 from base to tip. It was interesting to see that the original 150 year-old timber cills that the apartment is built on were not affected at all, even though the matt of fungus was enveloping them in places. They were a superior type of timber, probably pitchpine or similar – still as hard as a rock!

The only solution is to cut back to fungus-free timber and renew everything. Unable to obtain boron salts – an excellent, reasonably non toxic dry rot killer and preventative – I sprayed the soil with bleach and then anti-freeze (yes, anti-freeze is a good dry rot killer and preventative!), and then coated all the new timber with copper naphthalate solution.

Thinking back to traditional timber boat construction, the avoidance of dry rot was always at the back of our minds, when building. That is why, for example, the beamshelf was usually kept down from the deck, so air could flow behind and over the top of the shelf. If the shelf is fitted up to the deck, as was sometimes the case, the damp still air provided a perfect place for dry rot to get a hold.

And we used timbers like Larch or Pitchpine which are resistant to dry rot, rather than Douglas Fir, which is not.

Nowadays, with wood/epoxy systems the worry of dry rot and other decay in timber boat construction is much reduced. And with modern laminated construction there are fewer places where dry rot can get a hold – the shelf for example is usually bonded directly to the hull skin, so there is no air gap between it and the hull. And epoxies do allow us to use hitherto unsuitable non-resistant timbers such as Douglas Fir.

But I have had a stern reminder that good practice and proper construction, is still of prime importance.


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