One of the major reasons homeowners opt to upgrade to pressure treated wood is because it’s supposed to maintain its integrity over time (up to 40 years). That mostly includes standing up to the wear and tear of moisture, but what about changes in temperature?
Untreated wood will shrink to its smallest possible capacity in cold Winter months (contract) but also grow to its largest possible dimensions during warmer periods (expand). Does that hold true even for pressure treated wood though? Wood is wood, so of course it’s going to react to temperature changes – but here’s a more in-depth look at how the product responds:
Does Pressure Treated Wood Shrink?
Ultimately, pressure treated wood will shrink – but not as much as untreated wood. In fact if treated boards are left out in the sun for too long the boards will start to warp and twist. That is why it’s best to install the boards as soon as possible, using hot dip galvanized or stainless steel fasteners that will not corrode due to the moisture in the lumber.
Pressure treated boards will shrink about 1/4” after installation, once they are completely dried. When installing something like decking, it’s recommended to fasten the planks as tight as possible to the prior board, since a natural gap is going to occur anyways. Some homeowners are surprised when gaps appear in their once tight deck only two days later, but a small space is actually good for letting dirt and debris fall through as well as drainage.
Aside from gaps between boards, the shrinkage should be accounted for on overhangs. If you leave 1/4” overhang, the board might actually contract to flush once drying is complete. That’s not the worst thing in the world, but it could be if you’re looking for a flush edge and actually end up short.
It should also be noted that the majority of shrinkage comes in the width of the board, across the face of the grain because wood is hygroscopic (absorbs moisture from the air). There is minimal, if any, shrinkage in the length of the board. Having to cut boards to length taking into account a 5-9% shrinkage rate would be a nightmare.
Where is Pressure Treated Wood Used?
Pressure treated wood is created by removing air during the manufacturing process and replacing it with preservatives. These preservatives give the wood an extra layer of protection not only from Mother Nature, but from Mother Termite and other insects as well.
Easily the most common project where pressure treated wood is used is in building a deck. A home’s deck takes an absolute pummeling, whether it be via rain or the sun’s UV rays. In Northern climates a deck may have snow built up on it for 5-6 months straight.
Pressure treated wood is also preferred in fences/gates as well as anywhere this is contact with a ground surface such as base plates in a garage or buried fence posts.
Since the pressure treated wood will be more resistant to moisture, insects, etc. it is nearly mandatory to use on any exterior projects. The other option is to buy untreated wood, prime it, and paint it but that also requires a lot of maintenance moving forward.
Pressure Treated Wood Maintenance
Just like primed and painted untreated wood needs maintenance, so do pressure treated wood products. If you have a deck installed for instance, the recommendation is to let the wood dry (aka acclimate, settle, etc.) for anywhere from 3-6 months before any type of treatment or stain is applied.
You also don’t want to let the green treated wood dry out too much, which is why a test of sprinkling water on the surface is recommended. If the water beads up, the wood is still too wet to stain. When the water soaks in, stain can be applied. Water-based stains are more efficient than oil-based products, as they are less prone to mold growth. A clear polyurethane can also be used for a more natural look.
Even pressure treated wood used to build decks, fences, sheds, picnic tables, etc. still requires maintenance, it’s one of the reasons people are switching to the use of composite materials even though they are more expensive. You should expect to reapply stain every 1-3 years, which will also help with the wood shrinkage.
Does the Wood Really Hold Up?
There is a real noticeable difference between treated and untreated wood. Pressure treated wood has a green hue to it, which is why it’s often referred to as ‘green treated.’ Often times when you receive pressure treated wood from a lumber supplier it’s still wet, and thus much heavier. Also if you get a sliver from a pressure treated wood, it’s best to remove it as fast as possible because the chemicals will cause an infection.
These differences lend credence to the fact that pressure treated wood really is more resilient than ‘normal’ lumber. The two products have the same strength, it’s just that pressure-treated wood will resist rot better.