Is Pressure-Treated Wood Water Resistant?

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So many of us use decks to extend living space to the outdoors, with a slider allowing easy movement from kitchen to them.  Patio furniture, and maybe a pergola, finish off that outdoor sitting space.

A concrete pour, perhaps with different colored pieces of slate, or maybe patio blocks on a level sand foundation, are common.  For woodworkers, though, the choice is usually wood, with planks laid in various patterns and angles to create visual interest.  A chop saw, a power drill, some deck screws, a tape measure, and you’re pretty much ready.

For our deck, we chose wood.  Not just any wood, though, we chose pressure-treated wood.  A few 2 x 6 planks for the frame, 4 x 4 pieces for legs to keep level, and 1 x 6 x 16 planks for the surface, and our materials were assembled.  After a time, a deck stain for color and protection, and the project we complete.

But, what is pressure treated wood?  And why would that be the right choice for an outside deck project? Is it water-proof?  Or water-resistant?  Can it touch the ground? 

What is Pressure-Treated Lumber?

Pine is the most commonly pressure-treated lumber.  It’s an environmentally friendly choice of wood because it is fast-growing.  Cedar is another wood frequently pressure-treated.

After being broken down into lumber of various sizes and shapes, the wood is processed under high pressure to force a solution of water and chemicals deep into wood fibers.  This process and the chemicals commonly used are intended to extend the lifespan of the lumber.

The chemicals used in the high-pressure process include:

  • Alkaline copper quaternary (ACQ);
  • Copper Azole (CA); and/or
  • Micronized copper azole (MCA)

These chemical preservatives react with the wood fibers to slow down decay and frustrate termites, fungus growth, moisture, and other elements that promote rot or lead to structural degradation in the wood.  

Wood absorbs a great deal of liquid during the process, and will often arrive at lumber yards still wet.  It takes up to a month for the lumber to dry.  The helpful chemicals remain in the wood after the moisture evaporates.

In the case of our deck, the lumber included those dimensional pieces we mentioned above.  The pressure-treated lumber was still wet when we built the deck, so we allowed a couple of weeks for it to completely dry before we applied a coat of red deck stain to it.

Types of Pressure-Treated Lumber

Selection of Wood in Lumberyard

There are two basic types of pressure-treated lumber: above-ground, and ground contact.  Each piece of pressure-treated wood will have a tag stapled to its end identifying its type.  They are exactly what their names imply.

  • Above-Ground PT wood should be used only when it can be more than 6” above the ground, and when there will be good air circulation and drainage.
  • Ground Contact PT lumber can be used in either ground-contact or above-ground applications.  It has twice the chemical concentration after evaporation than above-ground PT wood.  It should be used when there is less than 6” of ground clearance or there is poor circulation.  

Within these types, there are also grades, just as there are in other lumbers.  The highest grade, #1, is free from defects like knots and presents a clear surface appearance.  The lower grade, #2, will have some scars, and the occasional knot.  The use to which you’ll put the wood in your project will determine which you will choose, and there is a price difference between the grades, just as there is with other lumbers.  

Uses of Pressure-treated Lumber

Obviously, decks are common PT wood projects, and so are fences and pergolas.  But it doesn’t stop there.  Wooden walkways, ramps for accessibility, fresh-water docks, and other outdoor projects that will be exposed to the elements are common.  

Fence posts that will sit in the ground for a long time are prime candidates for ground-contact wood, and if the fence itself will be 6” or closer to the ground, you’ll want ground-contact for that purpose, too.  Caps will be well above the ground, and so you have the choice between the two types.  

A pergola or other suspended architectural feature of your outdoor project can be built with above-ground PT lumber. 

Ground contact PT lumber is often used for fence posts, as well as raised garden beds.  In the case of our deck, and because of the topography of the land on which it was to be built (it’s a free-standing deck in the midst of a meditation garden surrounding it), one end of the 16’ span was simply laid on the ground, and then gradually along the span was supported by 4 x 4 PT in ever-longer pieces to some concrete sauna tubes to maintain level for its full length.  The entire deck was built with ground contact PT lumber, even though the 4 x 4 supported end rose to 14” above the ground.

Pressure-treated lumber should never be used for any indoor projects.  It’s a strictly outdoor application product where rot and decay from water, insects, and fungus are a concern.  Its chemical treatments shouldn’t be a part of any indoor work.

Pressure-treated lumber can be painted or stained, but it’s important that the wood be allowed to dry sufficiently so it can accept a coating of either.  In our case, we allowed a couple of weeks following construction of the deck, and fortunately, there was no rain during that time.  The fall breezes helped in the drying of the deck too.  

To determine if your PT lumber is sufficiently dry, you can sprinkle water onto the wood:  if the droplets are absorbed, the wood is ready for paint or stain.  If the water beads on the surface, it’s not quite ready and you will need to be patient for a few more days for applying your choice of finish.

One note worth making is that PT lumber will split and warp a little during the curing and drying process.  It has been completely wetted during the treatment, and the drying can warp it.  We’ve noticed that warping in some 4 x 4 spans we used building benches around our free-standing deck.  It isn’t a major thing but should be expected. 

Waterproof, Water-Resistant, and PT Lumber

Pressure-treated lumber is not waterproof.  The high-pressure process and the chemicals used are for protecting the wood fibers from common causes of structural degradation – termites, fungus growth, and for slowing down inevitable rot.  The intention of pressure treatment is to prolong the useful lifespan of the wood.

That being said, there are some pressure-treated lumber products that have water-repellent chemicals applied during the treatment process in addition to the usually used chemicals.  These water repellents will enhance the water-resistant qualities of the wood, but still, leave it short of being water-proof.  The tag on PT lumber will identify water repellents in the treatment just as it will identify above-ground and ground contact type.

The pressure treatment also does not prevent weathering.  However, the treatment does extend the lifespan of the wood, and with PT lumber, you are looking at a 20-year grade lifespan for your deck, fence, etc.  

You can choose to seal your PT lumber as a means of making it more water-resistant, or even waterproof.  You have three choices available to you:

Solid stains.  The color of your choice, solid stains will add that measure of water-resistance any PT lumber without repellents in the chemical treatment can use.  A solid stain will fade from a lot of foot traffic and will need follow-up coats each year to maintain water resistance quality.

Semi-transparent stains.  Not the color pop of a solid stain, but suitable for a deck’s surface or other high-foot traffic surface that won’t suffer from fade.

Clear.  With no pigment, there is no color fading.  But, while they do offer protection from the elements, they do not prevent the wood color from facing.

For a higher measure of protection from moisture, there are also wood sealants, common on docks, for instance.  Marine sealers will provide that waterproof protection, products like Thompson’s Waterseal or Seal-Once Marine Sealer.  

We found a very interesting and informative video for you to watch about the process of pressure treating wood.  You will learn things you didn’t know about the process, and it’s worth a watch.

A coat of stain or paint, or a marine sealant, once per year, to extend the lifespan of a product that has already been life-extended is a pretty easy price to pay for a deck or fence that will last you 20 years.  Its bigger picture meaning is that less wood during that time will be used, reduced deforestation is the result, and you’re using a wood that is environmentally friendly because of its rapid growth.

Concrete or patio block areas are nice, yes.  But, for us at Obsessed Woodworking, we prefer wood for our deck, with a yearly coat of stain easily applied.

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