What is Dry Rot? (fungus disease)

Dryrot and Decay

Fungus diseases in the wood are also known as dry rot or wet rot.  This is caused by the growth of fungus in the wood. Wood-destroying fungi require a food source, oxygen, and favorable temperature in order to thrive. The cycle of promoting more dry rot is fungi increase wood permeability, this permeability allows more moisture to penetrate the wood and leads to more decay fungi, and the cycle continues.

There are 3 types of wood fungi : mold, sap stain and wood-decay fungi.

Wood Fungi

Brown rot, white rot and soft rot are three classes of fungi that are listed according to the type of decay they cause.

Brown Rot

Wood decayed by brown rot fungi looks like dry leather and breaks easily into small cubical pieces. The strength of the wood decreases as the growth spreads. Most of the damage to structures is caused by brown rot. Soft rot occurs in situations where wood is wet over a long period of time, such as an earth-to-wood contact and look similar to Brown Rot.

White Rot

Wood decayed by white rot often assumes a bleached appearance, frequently has black lines through it and feels spongy. The strength of wood attacked by white rot decreases gradually with little loss in strength during early stages of decay. If caught soon enough, White rot may be treated by spray of a bleach and scraped away. It is important to correct the moisture problem that caused the wet wood in the first place.

Soft Rot

Soft rot fungi looks like brown rot but the affected wood softens gradually from the surface inward developing cavities (invisible to the naked eye) within the wood cell walls.

Four principles of decay prevention

1. Always build with wood that is properly seasoned.
2. Keep wood dry and allow for ventilation where necessary.
3. Break contact of untreated wood and soil.
4. Where soil must contact wood, use properly pressure treated lumber. Concrete counts as soil.
5. Don't paint mildew resistant paint on window sills or places that kids could ingest the paint chips.

Condensation & Moisture Build Up

One of the most frequent assaults on my sensitive nose and allergies is the odor of mold and mildew. Frequently, when called by a client to estimate recommended repairs by a termite company, the most important tool turns out to be my nose. The odor of mold inside a home is a sure sign that a moisture problem exists.

We have encased ourselves against the elements in structures that attempt to create a comfort envelope against the elements. Condensation on the skins of this envelope is caused by temperature differentials and can lead to dry rot in flooring to paint discoloration from mold growth.

Prevention of Condensation

One way to eliminate condensation is to increase the thickness of dead air space between heated/cooled spaces and the varying temperature outdoors. Many styles of insulation come with a built in vapor barrier that helps keep humidity (the ability of air to hold water) down indoors. As humidity decreases (dry air) we feel more comfortable at lower temperatures.

Cold weather condensation can be prevented by placing moisture barriers on the warm side (inside) of insulated walls and ceilings. Warm weather condensation can be treated by increasing the air flow by use of fans and by decreasing the humidity of crawl spaces, by use of adequate ventilation, soil drainage, soil covers and mechanical dehumidification. In theory warm weather condensation should also be treated by placing a vapor barrier on the outside (warm) of insulated walls. However, research has shown that an impermeable moisture barrier installed over studs will cause problems with sidings in areas of very hot moist conditions. Walls wrapped with permeable membrane (i.e.; Tyvek house wrap) out performed less permeable materials such as 30# felt, because they allow the moisture laden air to escape.

When mold and decay occur over broad areas not associated with a specific source of water (like a plumbing leak), the problem is either water conducting decay fungi or conventional decay fungi growing on wood that has been wetted by condensation. In the latter case the fungal growth will feel powdery or stringy. Water conducting fungi feels leathery and can often be peeled off in sheets. Unfortunately the only way to get rid of molds and decay fungi embedded in wood is to remove the piece. Frequently extracting a rotted structural piece can be an expensive proposition. Prevention is far cheaper.