Got Rot?

Actually, I prefer the phrase “wood deterioration,” as it seems more genteel.  Unfortunately, no matter what name you use for it, wood rot, wood deterioration, dry rot, wet rot, brown rot, or even white rot is not good news.  In most cases, when the wood moisture exceeds about 30 percent, fungi begin growing – feeding off of and consuming the wood itself1.  Wood rot is a first cousin to that other four-letter fungus known as mold.

Given that most structures are made of wood, the missing ingredient for deterioration is moisture.  Contrary to the name “dry rot,” wood cannot be both “dry” and “rotting” at the same time.  Moisture or water itself can be introduced from a variety of sources.  The most common source of moisture is an improperly vented crawlspace.  Most building codes require crawlspace venting on a 1:150 basis or up to 1:1500 basis if a vapor barrier is present2.  However, what the code assumes is that no water is draining into the crawlspace, there are no leaking pipes or clothes dryer exhaust vents routed to the crawlspace, the vents are actually venting, and/or that the vapor barrier is actually acting as a barrier and not as a sieve..  The reality is that some crawlspaces technically meet the code, but that are still “wet.”  Don’t assume that because something meets the applicable building code that there aren’t any problems with humidity.

Under-vented attics are a second common area of high humidity.  Like crawlspaces, most codes require venting on a 1:150 basis, which can be increased to 1:300 if certain criteria are met.  However, the importance of insulation cannot be overlooked.  It is sometimes assumed that adding more insulation is the correct thing to do.  Improper installation of insulation over eave vents can do more harm than good.  The secret of properly venting an attic is getting air to move through the space.  Simply having a ridge vent or any number of roof vents will generally not provide proper airflow.  In most cases, they will only act to alleviate contraction and expansion of the attic atmosphere.  The lack of ventilation will not only increase the temperature of the attic, but also impede accumulated humidity from escaping.  Any vapor that is not directly ducted to the outside, such as that from cooking and bathing (are the bathroom vents actually routed outside or just inside the attic?), becomes trapped inside the attic.

The third source of water is leaks.  These can include improper weatherproofing – a condition that starts on day one.  It can also originate from storm damage or ongoing construction, such as the removal of gutters.  Water leaking onto or into a building is especially detrimental.  As opposed to vapor, liquid water is concentrated, flows by gravity, and can soak deeply and quickly into wooden members.

In most cases, the application of water or vapor is periodic.  A leak through the siding will only occur when it’s raining.  Between rains, the area may dry completely. As noted above, when the wood moisture decreases below about 30 percent, the fungi growth ceases.  As a result, it is difficult to accurately determine how long the deterioration action has been going on.  One factor to consider, especially in crawlspaces, is the presence of heavily damaged members that have been “sistered” or supported by more competent members.  This is a clue that the problem has been ongoing for an extended amount of time.

Unless the deterioration is constant, the damage action ceases between wettings.  Once fungi have invaded a wood product, when the wood dries, they form spores.  This puts the fungus in a passive state and ready for the next period of high moisture. 

1. This is not 30% atmospheric humidity, but wood moisture.
2. The CABO was consulted as a code “standard” for this article.  This refers to one square foot of ventilation for each 150 square feet of crawl space.

As a result, wood which is periodically wet and deteriorating can rapidly re-activate the spores contained within and deteriorate faster than wood which becomes wet initially.

Once started, wood deterioration is difficult to stop.  In the case of crawlspaces, the construction techniques which originally caused the conditions are difficult and costly to remedy.  In most cases, the building owner is simply unaware of the condition until failure has occurred.  When failure, or partial failure, occurs, replacement is generally the only option.  One reason for this is because in crawlspaces or attics, the bottoms of structural members are attacked first and more heavily.  The fibers in the bottoms of these members are in tension.  As a result, when this part of a structural member is damaged, its capacity to support a load is greatly diminished.  When the bottom inch of a 2-inch by 8-inch floor joist is comprised, it is no longer a 2 by 8, but a 2 by 6 – and can become a 2 by 4 as time goes on.  In some crawl spaces you may see a number of “supplemental” supports placed there by previous owners in order to shore up the floor.

The key to identifying the source of damage is identifying the source and avenues of water entry.  Things to consider include:

Figure 1

In crawlspaces:

• Is water draining in from the outside?
• Is the earthen floor eroded from water?
• Are there “supplemental” supports?
• Are the vents open and operating?
• Is the plumbing leaking?
• Is the clothes dryer emptying into the crawlspace?
• Does it feel damp?
• Is the vapor barrier intact?
• Are there areas where the crawlspace floor is exposed?

Figure 2

In attics:

• Where are the vents? At the eaves? Gables? Roof?
• Are the eave vents free of insulation and operating?
• Are powered vents such as in the kitchen and bathroom emptying into the attic?
• Are there any leaks (especially around the chimney)?

When investigating a structure for the cause of wood deterioration, all sources of moisture must be considered.  In areas where the wood is wetted periodically then dries, damage can proceed incrementally and take years to manifest itself.  In other areas, such as crawl spaces, which can be habitually wet, the action may be continual. As a result estimating the time required for the damage to occur is difficult.  Also be aware of lifestyle changes such as different tenants, number of people living in the building and new equipment that can have an effect on this deterioration process.