Wood and the elements do not mix. Stroll through any forest and you'll see how nature can make short work of massive fallen timber. Insects, UV rays, fungi, moisture, all conspire to turn that once-proud column of wood into a hollowed-out log, and then into wood chips.
Nice in the forest; not so nice when it's your deck, basement exterior wall, or retaining wall. One way, of course, is to build those 3 items with non-organic materials: metal, CMU blocks, and masonry retaining wall blocks, respectively.
01 of 05
02 of 05
Any Direct Contact With Earth
Earth, no matter how dry it appears, isn't dry. And even the driest earth will eventually accumulate moisture, and this moisture will be transferred to your wood.
That's why pressure treated lumber is required whenever it is embedded in, or in direct contact with, earth.
03 of 05
Basement Masonry Walls
Pressure treated wood is required whenever you attach framing lumber (as shown here) or furring strips to concrete or other exterior masonry walls below grade.
Take note that this 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.
04 of 05
Posts On Masonry
When you have a vertical wood post (or column) on masonry or concrete, and that concrete/masonry itself rests on the earth, then that post must be pressure treated.
You don't need to use pressure treated wood 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 weather-exposed locations.
As with the previous requirement, this is meaningful in basement finishing applications, but this time with regards to structural support.Continue to 5 of 5 below.
05 of 05
When discussing "earth," we are referring to the actual dirt on the ground.
This is not an exhaustive list of all instances when pressure treated wood is required. See ICC IBC (2012) 2304.11.
Building code allows for any type of preservative treated wood, not just pressure treated. It also allows for what they term "naturally durable" wood. The American Wood Council says that cedar, black locust, and redwood are a few of these types of woods.