Discovering Wood Rot in the Home Q&A


“I’m getting ready to paint my house, and I’ve been patching some of the bad spots in the wood trim around the windows and at the edges of the roof. On two sides of the house, there are pieces of wood that are starting to rot. Although the wood looked okay, when I started to scrape the loose paint off it, the wood started to crumble. What causes wood to rot, and what do you have to do to fix it?” –  M.C., Hayward


Many homeowners are familiar with wood damage caused by rot. It can be found at the base of fence posts, roof eaves and rafters, exterior doors, exterior wood trim, or decks and patio covers. Wood rot usually occurs because of excessive moisture, and is often not visible until the wood is completely damaged. To give you an idea as to how much wood is damaged by rot, the wood industry estimates that about 10% of its annual wood production will be used to repair damaged caused by wood rot alone.

Wood rot is a term used to describe the deterioration and breakdown of the cellular structure of wood that is caused by certain types of fungus. The spores of these fungi are carried through the air, and when they come into contact with moist wood, they will attach themselves to the wood, and begin to feed on its nutrients in order to grow. The fungi releases an enzyme that actually breaks down the cellular structure of the wood into a form of food it can eat. As the fungus grows, the wood decomposes and looses its strength.

One of the dangers of these fungi is that they can grow for long periods of time before producing any external evidence of their presence in the wood. Most wood decay fungi grow only on wood with high moisture content, which is normally considered 20% or above. Wood is actually a porous material that can absorb moisture if left unprotected. Although some woods such as redwood and cedar are naturally decay resistive, under the right conditions, all wood can rot.

Therefore homeowners need to check for wood rot on a periodic basis, and take preventative measures to stop it from occurring. This should be done twice a year, in the autumn and after the spring rains. Any wood member at the exterior of the house can be subject to rot, and therefore should be inspected. However, be most suspicious of wood trim, roof eaves, and any wood close or in contact with the ground, or wood that gets repeated wetting from sprinklers.

At the exterior of a building, wood rot commonly occurs at joints between wood trim and the exterior siding or windows, at the base of wood jambs for exterior doors, wood that is contact with dirt, or on the parts of wood where water can sit and not drain away. This includes most horizontal surfaces, or top edges of boards. Although wood at the exterior of the building is usually painted, moisture can get under the paint through small cracks, allowing rot to form. Because rot can form under the paint, most of the time wood rot is not visible from the surface of the wood. Therefore it is often helpful to have a small screwdriver to be able to probe for rot, and to test the firmness of the wood.

There are two main types of wood rot to look out for. In one type, the decayed area has a brownish discoloration and a crumbly appearance. As the cellular structure of the wood breaks down, it forms a cube like pattern in the wood. The other type of rot causes the wood to have a white or yellow discoloration, with the decayed wood being stringy or spongy.

Once wood rot is discovered, it must be cut out and removed from the wood. The reason for this is that if the wood is to dry, the fungus becomes dormant and stops attacking the wood. However, once the wood gets wet again, the fungus continues its destructive course. This wetting and drying cycle continues over a period of time, until the wood becomes structurally unsound.

The best way to prevent wood rot is to make sure that all exterior wood trim, siding and roof eaves are kept well painted and sealed. Preventing water from getting into joints, gaps or separations in wood, where it can be trapped and not allowed to dry out, is the secrete to preventing rot. All horizontal surfaces and edges of wood are most prone to absorbing moisture, as well as joints between wood members. All vertical and horizontal joints between the pieces of wood should be cleaned out, and filled with a good quality exterior caulking material. Loose paint should be scrapped off, and the wood primed and painted with good quality paint.

Wood rot can also occur at the interior of the building, particularly in areas exposed to water or leakage. The source of water can be a plumbing leak, such as a water line, or water splash from sinks, tubs, or showers. Homeowners should check the base of sink cabinets, or floors adjacent to clothes or dish washers, or floors in bathrooms adjacent to the tub or shower pan for signs of swelling or discoloration.

Taking the time now to inspect and locate wood rot, will allow you to make repairs before further water damage can occur. Failure to repair wood rot when it is first discovered, will allow the fungi to continue to damage the wood, requiring more extensive and costly repairs in the future.