What Is Dry Rot?

Dry rot, also known as brown rot (Serpula lacrymans), is a common wood-destroying fungus that grows on wood that has 28%-30% moisture content. It will not typically damage wood stored or used in normal dry conditions, as the wood in an upper level of the home will typically have a 12%-15% moisture content. Once active, dry rot can grow in wood with a moisture content as low as 20%.

Identifying Dry Rot Damage, How It's Caused, & How To Prevent It

To grow, an external source, such as a dripping pipe, leaky roof, flooding basement, or moisture passing through concrete is needed. When conditions reach a relative humidity level of 95% or higher, dry rot is able to reproduce through spores. These spores are rust-colored in appearance, and resemble a fine, brick-colored powder

Dry rot can survive in areas with lower moisture levels, going into a dormant state when the relative humidity is between 76-86%, waiting for the moisture level to rise again. Dry rot is also known to survive in areas with lower humidity, translocating moisture through "roots" through several feet of mortar, concrete, masonry, or other materials.

Wood Damage & Dry Rot

When allowed to grow on wood unchecked, dry rot can be tremendously damaging to wood cellulose. According to the US Forest Service, a 1% weight loss due to dry rot fungi can lead to a full 75% loss in the toughness of the wood.

At 2% weight loss, it loses 18% of its perpendicular-to-the-grain compressive strength, and 10% of its compressive strength parallel to the grain. All said, 20 billion board feet of wood are destroyed by dry rot, annually, in the USA alone, and has been credited with £150,000,000 in annual damages in the UK.

Dry Rot Photo Gallery

Damage from dry rot is commonly confused with damage from carpenter ants or termites, however, the differences are easy to spot.

  • Carpenter Ant Damage: Carpenter ants create smooth-sided passages or "galleries" throughout the wood.
  • Termite Damage: Termites create ragged passages, which are often much more extensive than carpenter ant galleries.
  • Dry Rot Damage: Dry rot is white and fuzzy, breaks wood into small, square chunks, and leaves behind a red spore powder.


Preventing Dry Rot

The key to preventing dry rot in wood is to keep huge levels of moisture and humidity away whenever possible. Under normal conditions, this can be as simple as running a dehumidifier in your basement, venting your attic, and sealing concrete surfaces to prevent moisture transmission. However, if your home has experienced a flood or plumbing leak, its' important to remember that dry rot has been reported to manifest itself after months or even years of dormancy.

Guidelines To Avoid Dry Rot Damage

  • Address all building defects in the home, including roof damage, overflowing gutters, blocked airbricks, leaking basement windows, leaking plumbing, vents that discharge into the crawl space or attic, etc.
  • Seal concrete surfaces behind wood studs and paneling, and treat the wood with a fungicide and/or sealant.
  • Isolate below-grade spaces by encapsulating your crawl space, closing basement windows, and avoiding any basement ventilation. Use an energy efficient dehumidifier in the basement and/or crawl space instead.
  • Take any leak in your home seriously, making sure to remove not just the affected area but also areas near the affected area. Clean off all nearby surfaces, including metal surfaces and pipes within 1.5 meters, with a wire brush, and apply a water-soluble fungicide to all materials that have not been removed.

It's always a good idea to consult a professional to evaluate the situation as well. Many professionals will provide you with a free estimate, including pricing and recommended repairs. Professionals are likely to identify existing or potential problem areas that may lead to big expenses later on.

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