How To Look At A House
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Does wood rot spread? Is it contagious?
Tuesday, August 14, 2018
Wood rot is definitely contagious under certain conditions. But, once you understand that wood rot is a type of fungus—along with the mechanism that the fungus uses to spread and the conditions necessary for it to infect an area of wood—then your diligent maintenance will stop rot from appearing at new areas of wood.
Here’s are our “Three Rules of Wood Rot”:
1) Existing wood rot fungus colonies are constantly sending our spores into the air. These spores are floating around pretty much everywhere in the wind. It’s not necessary to have an area of wood rot nearby for spores to land on an exposed surface of wood. There are likely already plenty of wood rot spores on the surface of any wood exposed to outdoor, unfiltered air.
2) Moisture is necessary for the spores to begin growth. It can either be in the form of extremely high humidity that raises the moisture content of the wood over time (in a poorly ventilated crawl space under an elevated wood floor, for example), or lumber that has not been sufficiently kiln-dried at the factory to reduce the moisture content to under 20%, or water soaking into the wood surface that does not naturally drain by gravity and allows water to puddle on it.
3) Warm air temperature is also necessary for wood rot to flourish, although it possible for it to grow at temperatures down to nearly freezing.
While warm temperature is a component, the main trigger for wood rot to begin is moisture in the wood. The number one type of defect we see that causes rot at exterior wood siding and trim is any surface that allows rain water to puddle on the wood without draining or to seep into any any openings between pieces of wood. This is called a “water trap” by builders, and a good craftsman works to make sure than any horizontal surfaces, such as window sills, have a slight slope to the outside so that no water puddling occurs, along with carefully caulking around windows and trim to seal out wind-blown rain out. Maintaining paint as a barrier on the surface of the wood is also important.
If you find areas of wood rot on the exterior of your home, be sure to repair the area in such a way to eliminate any future water traps. Also, rot at the bottom of plywood exterior siding is often due to splash-back of rain water off the ground from a roof overhang above. The worst location for splash-back areas of rot is where the roof drains onto a patio or driveway, and the best preventive measure is to install a gutter system over the area.
Another location where wood rot can develop is in the attic at leaking roof penetrations, such as in the photo at the top of the page of the roof sheathing below a leaky chimney flashing. Landscape sprinklers that regularly spray on the wood trim of a house can also accelerate rot, like in the photo below.
To learn about how an inspector looks for wood rot, go to our blog How does a home inspector evaluate wood rot?
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