Even though wood is a product of nature, structural wood and the outdoor elements do not mix well. Stroll through any forest and it is evident how nature can make short work of massive fallen timber. Insects, UV rays, fungi, and moisture all conspire to turn that once-proud column of wood first into a hollowed-out log, and then into wood chips.
What is expected in the forest is not beneficial when it comes to your deck, basement exterior wall, or retaining wall. One easy way to avoid rot is to avoid wood altogether: build with non-organic materials such as metal, CMU blocks, and masonry retaining wall blocks. But if you want wood, then choose pressure-treated wood, preservative-treated wood, or naturally durable wood.
At the home center, pressure-treated wood can usually be identified by its numerous incision marks. It is easy to imagine that the copper azole, type C, preservative has been injected into the wood at those points. Actually, the incisions help open the wood and allow the preservative to be forced into the wood cells under high pressure in massive metal tubes nearly half the length of a football field. While pressure-treated wood is twice or even three times more expensive than conventional kiln-dried lumber, your gain is the peace of mind in knowing that your project will not be affected by moisture or pests such as termites or carpenter ants.
Building code requires pressure-treated wood in numerous applications. Following are selected instances where you might be required by code to use pressure-treated wood for your home remodeling projects.
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Basement Masonry Walls
Pressure-treated wood is required whenever you attach framing lumber or furring strips directly to concrete or other exterior masonry walls below grade.
Note that this requirement is only for exterior walls, as these may wick moisture onto the lumber. Interior walls are within a climate-controlled environment and are presumed to be free of moisture. This requirement is especially relevant to basement finishing.
Bypassing this code requirement can lead to the basement wall members soaking up moisture. Because basement walls are hidden behind drywall or other wall covering materials, the wall framing members are not exposed to light and air. In a closed environment of this type, wood rot and devastating mold can quickly set in.
02 of 07
Wood on Concrete or Other Masonry
Concrete and another masonry, unless treated or otherwise coated, is highly porous. Water can pass through porous materials vertically via capillary action.
When you have a vertical wood post or column resting on concrete or another masonry, and that concrete/masonry itself rests directly on the earth, then the post/column must be pressure-treated.
Pressure-treated wood is not required if an impervious moisture barrier and a 1-inch metal or masonry pedestal separate the post from the earth by a total of 6 inches in basements or in weather-exposed locations.
As with the requirement about basement masonry walls, this is meaningful in basement finishing applications, but this time with regards to structural support.
Sleepers and sills of any nature contacting concrete or masonry must be pressure- or preservative-treated.
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Direct Contact With Earth
Earth, no matter how dry it may appear to be, is rarely 100-percent dry. Even the driest earth will eventually accumulate moisture, and this moisture will be transferred to your wood. This is why pressure-treated lumber is required whenever the lumber is embedded in, or in direct contact with, earth. Fence and decking posts are the most common types of wood that come into direct contact with the earth in residential properties.
04 of 07
If you build an exterior retaining wall from wood, that wood must be pressure-treated. By their very nature, retaining walls are constantly in contact with earth. Not only that, water tends to collect behind retaining walls, increasing the chance of wood rot.Continue to 5 of 7 below.
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06 of 07
Subflooring and Joists
Wood flooring, including subflooring, and joists within 18 inches of exposed ground (as with crawlspaces) must be treated against decay or constructed of naturally decay-resistant wood. Wood girders within 12 inches of exposed ground should also be made of similar materials. This applies only to wood within the perimeter of the building.
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Legalities and Permitting
Consult your local permitting agency for code requirements. Codes are particular to each municipality and it is not possible to generalize for all areas. Model codes are usually adapted by each community for its own needs.
This is not an exhaustive list of all instances when pressure-treated wood is required. See ICC IBC (2012) 2304.11 for more applications of pressure-treated wood with respect to home renovation and building.
Building code in your area may allow for any type of preservative treated wood, not just pressure treated. It also allows for what is termed "naturally durable" wood. The American Wood Council says that cedar, black locust, and redwood are a few of these types of woods that are naturally durable.
All references to "earth" mean the actual dirt, not ground-level or grade.