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What is Wood Decay/Rot and What can it do?



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By : Martin Henderson    99 or more times read
Wood decay/rot can destroy your home. Discover what decay/rot is, and what you can do to minimize moisture related damage.


What is wood decay/rot?

Wood decay/rot is simply the deterioration of wood due to the enzymatic activities of microorganisms. For all practical purposes, fungi are the only agents of wood decay.

Four conditions are necessary for the development of wood decay producing fungi:

  1. An adequate supply of oxygen

  2. A favorable temperature (generally above freezing; if it's comfortable for you, it's conducive for fungi growth)

  3. Moisture in excess of the fiber saturation point (greater than 25%)

  4. A suitable food source (wood).


Eliminate any one of these and decay fungi cannot survive.

For wood located in the exterior environment, the amount of oxygen surrounding the wood, atmospheric temperature, and exposure to water is virtually impossible to control.

In the interior environment, the amount of oxygen is still difficult to regulate. However, temperature and moisture can be regulated. Research indicates that if wood is maintained at less than 20% Moisture Content, it will not rot. Therefore, in most cases, because of the "conditioned" interior environment, wood used indoors need only be properly dried, before installation, to provide protection from rot.

Building crawl-spaces and attics are not normally "conditioned", and, therefore, can experience great variations in temperature and humidity. These varying conditions can affect the moisture content of the wood structural members installed there. Providing controlled temperature and humidity in these spaces is possible, but can prove expensive. Ventilating these spaces is normally much less costly. Ventilation only slightly affects temperature, but aids tremendously with evaporation.


What can wood decay/rot do?

A building owner, a non-profit organization, telephoned after only three years of occupancy, saying "The floor is caving in!". Visiting the project, it was discovered that, in fact, the floor was caving in, and in several places!

Much like a typical wood framed house, the floor construction consisted of masonry piers supporting wooden beams which supported wooden floor joists. A plywood floor deck with quarry tile installed directly over the plywood was installed on the floor joists.

Opening the crawl-space access door revealed an absolutely amazing condition. It was beautiful! There were fungi, some as long as 24", of all colors and descriptions, hanging from the floor structure and floor decking. These fungi were rapidly consuming the wooden structure, and causing massive deterioration.

What is "massive deterioration"? Entire floor joists were completely consumed, and were no longer present. Entering the crawl-space revealed that no vapor retarder had been installed. (Proper installation of a vapor retarder can reduce the required ventilation by as much as 90%.) Further exploration revealed that although not present during the visit, water in great quantities, had been present in the crawl-space at some time in the recent past. The telltale sign of the tremendous amount of water was a dark ring approximately eight to ten inches up on the concrete masonry foundation wall, which corresponds with the sill height of the crawl-space access door opening.

Since this project was a camp’s dining hall, with only a drinking fountain in the main dining area, as a possible source of the water, there was no logical explanation for the apparent tremendous amount of water that had been in the crawl-space, especially since there was no active leak at the water fountain. Additionally, there was no indication that there had been a leak below the adjacent kitchen floor slab, since there were no water stains along the common wall between the kitchen and the dining area, nor any signs of erosion to the earth along that wall.

Except for the dark ring previously mentioned, nothing indicated a high moisture condition. Further, the water supply to the building had been turned off during the cold weather months, minimizing the likelihood that an undetected leak or ruptured water pipe had caused the flooding. Confusing the matter further, the building sat atop a hill, so the surrounding grade generally sloped away from the building in all directions, so storm water was not a likely suspect.

The building was not equipped with gutters and down-spouts, and this could have contributed to water being immediately adjacent the building's concrete block foundation walls. Concrete blocks are porous, and will absorb water, which might explain a certain amount of water in the crawl-space, but not likely the quantities required to cause the dark ring observed. Had gutters and down-spouts with splash-blocks been installed, the storm water would have been directed away from the building rather than allowing the water to fall against the exterior foundation wall.

To remove the crawl-space humidity, a large exhaust fan was installed at the crawl-space access door, drawing air in through the foundation vents and out the access door, and was operated for approximately a week. Visiting the project a week later, it was discovered that the fungi were almost completely eliminated. Most were gone completely. Even the largest ones, which had been 24” long, or longer, were now less than 10” long, and they were dry, indicating that they were no longer active. This air circulation was adequate to dry the crawl-space sufficiently to destroy the fungi growing there, but was too late to save the floor system. Replacement of the entire floor system was necessary, at the cost of thousands of dollars.

The source of the excessive moisture remains a mystery, but no deterioration of the structure has been reported in the past 15 years.

This example should serve to illustrate how important adequate ventilation in the crawl-space is.

What can you do to protect your home? Start with VENTILATE, VENTILATE, VENTILATE!
Your questions, comments, or suggestions for future topics are welcomed. Visit http://ngahomeinspector.webs.com, and enter your comments, questions, or suggestions on our “Contact Us” page. Helping you protect your most valuable investment, by providing helpful home preservation information, is our goal.

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