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A Woodworker's View on Rot
A Woodworker's View on Rot

Tools and Wood with Bob Smalser


Brown Rot, White Rot and other Wood Pestilence





Punky that the stuff that isn't really rotted yet, but it's on it's way? Dark, even black, solid but a little soft, hard to dry out. I wonder if my floors and keel are punky at this point? How does one define it?"

Grow, mill and air dry enough lumber, plus restore several old boats and you eventually get to see it all, so I’ve saved some examples of pest damage to show you what’s what, what’s bad, what’s not, what you can do about it, and some why’s and wherefores.

I’ll start with the worst, and end with the worst.

Brown Rot

Often called “dry rot”, but there’s nothing dry about the results of this fungus attack that consumes both the cellulose that provides wood fibers their strength and the lignin that holds those fibers together. Brown Rot leaves some lignin residue that creates the distinctive cube pattern associated with this rot. Like all fungus, the spores of these species are everywhere in the environment, but they need warmth, oxygen, moisture and food to germinate and grow. Alter those conditions to inhibit it.

Temperature -  Between 75 and 90 degrees F are ideal, and below or above that range germination tapers off on a linear scale until 40 or 105 degrees where it stops entirely. Where I am, 55 degrees and lower stops significant fungus activity in wood. High temperatures kill fungus entirely, which is one reason for kilning wood to the international standard of “56/30” or 56 degrees C for 30 minutes.

Oxygen -  While oxygen is required, significant airflow deters spores from gaining a foothold, so ventilation is useful in prevention.

Moisture - Like all organisms, these need water, and 20% moisture content is their threshold. That’s another reason why construction lumber for domestic consumption is generally kilned to 19% – to prevent the stains and worse damage caused by fungus. Kilning to the 56/30 international standard for a “KD-HT” (heat treated) stamp also coincides nicely with the domestic “KD-19” (kiln dried 19%) stamp. “HT” wood here runs around 16% MC, which is adequately dry for immediate exterior or marine use without the danger of over-kilning damage.

Food - While the species that cause Brown Rot eat any cellulose, most other fungus prefer the sugars found in sapwood, where most problems begin. Saturating or coating wood with preservatives like copper and lead poison their food to prevent growth, as do the natural extractives found in the heartwood of durable species. Extractive is a 5-dollar word describing the concentration of salts, colorful resins and acids that occur as the cells of sopping-wet, weak, sugar-laden sapwood transition to the relatively-dry, tough, heartwood that provides the tree’s stem its strength.

Because it consumes both cellulose and lignin and is self-generating, Brown Rot causes severe damage quickly if left unchecked. Here you see that planing the surface didn’t remove the rot pockets, which remain discolored and spongy. If I were to plane off a little more surface, I’d arrive at wood merely discolored. This is called “incipient rot” because the fungus species causing the discoloration are still there waiting for the right conditions to grow again.  .  .

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