Skip to main content

Diagnosing Wood Rot

The different types of wood rot are explained here.

Diagnosing Wood Rot

A Fine Mess

Virtually every homeowner has to deal with wood damage at some time or another. It's so common, in fact, that the materials for replacing and repairing damaged wood makes up an estimated 10 percent of all wood products annually produced in the United States.

Tool Chest

Fungi are plantlike organisms. They can't synthesize food on their own, however, so they have to absorb nutrients from other sources, such as wood. Mushrooms are a well-known fungi.

Wood rots for one simple reason: It gets wet. It can happen to wood anywhere in your house—in structural timber, door trim, eaves, exterior trim—you name it. Wood rot is also common on boats and plants and can even be found in musical instruments.

All wood has the potential for rotting, as it contains a certain amount of moisture. If moisture content is below 20 percent, rot typically isn't a concern. Anything over this provides a potential breeding ground for fungi, which is what causes wood rot.

Moisture is just one thing that fungi need to survive. They also need the following:

  • Favorable temperatures. Anything in the 40°F to 100°F range will do.

  • Oxygen.

  • A food source. The fungi that attack wood prefer carbohydrates in the form of cellulose and lignin.

Wood is made up of cellulose and lignin, and the other factors are pretty hard to control, so fungi abatement efforts generally focus on the one variable—moisture—that we can do something about.

Dry vs Wet Rot

People used to distinguish between different kinds of wood rot as being dry or wet. This was misleading, as it made it seem like dry wood could develop rot. This simply isn't possible. All rotten wood is wet wood, or has been wet at one time.

We'll repeat it one more time: Dry wood cannot decay.

Tool Chest

Brown rot causes infected wood to turn dark brown, hence the name.

Still, you'll hear people call certain types of rot "dryrot." What they're really referring to is brown rot, which is one of three types of wood rots. This rot cracks wood against the grain, causing it to split and crumble. In advanced stages of brown rot, after the rot has taken all the nutrients out of the wood, it can also become dry and powdery.

The other types of wood rot are…

  • White rots. As the name suggests, these rots cause affected wood to take on a white appearance, which can range from grayish-white to yellowish. In their advanced stages, white rots make wood look stringy and feel spongy or springy.

  • Soft rots. These are rare inside of homes, but they have been known to attack wood shingles in wet climates.

Some rots can grow inside wood for a long time and cause extensive damage before they are detected. Others develop crusts called "fruiting bodies" on the surface of the wood.

Rot vs Mold

Mold (and mildew) are also fungi that spring up around moisture. While they indicate moisture problems and can cause discoloration,they themselves don't cause wood decay. They can, however, increase wood's absorption ability, which can make it more prone to fungal growth.

Another type of fungi called "sap stain fungi" looks like surface mold. However, it, too, doesn't weaken wood structures; it only discolors them. Sap stain fungi stops growing when wood dries out. Its presence typically indicates wood that was wet atone time but no longer is.

Join the Family

Your partner in parenting from baby name inspiration to college planning.