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How To Use Pressure-Treated Lumber

November 24, 2018Olen Murriel

What makes pressure-treated softwoods, such as Fir, Spruce, or Pine, different from regular lumber? As their name implies, they contain a protective chemical sealant that is forced into the core of the wood through pressure. Asa result, these woods resist insect, fungal, and water damage for as much as two decades, thus making them fit for use in exposed areas.


How Wood is Pressure-Treated

In 1940, Karl Heinrich Wolman, a wood-preservation pioneer, thought up the idea of infusing wood with a sealant. He first placed wooden boards in a large cylindrical tank from which he sucked out air, creating a vacuum. Wolman then forced an arsenic-based sealant into the tank, and therefore into the wood, under pressure. After soaking the boards, he removed the excess sealant for re-use and then either air-dried or kiln-dried them.


Drawbacks of Early Treatment Methods

This technique proved to be so effective that it not only created a wood-preservation industry but also remained unchanged for many years. Unfortunately, it also had its drawbacks. Because arsenic is toxic, the sealant also proved to be toxic, especially for woodworkers and for pets that either scratched or bit the treated wood. Asa result, the Environmental Protection Agency limited its use.


The Rise of New Sealants

In response, manufacturers changed to copper-based sealants, such as copper citrate (CC) copper azole (CA), acid copper chromate (ACC), micronized copper quaternary (MCO), and alkaline copper quaternary (AOC). Unlike arsenic, copper is not toxic.

However, it is not without its drawbacks, for it reacts with steel nails and screws, causing corrosion. To combat this effect, manufacturers have had to create special screws and nails for use with copper-treated lumber.


Buying Pressure-Treated Lumber

Now that you know the basics of pressure-treatment, the question begs: How do you buy pressure-treated lumber? After arriving at the lumberyard, the first thing that you will notice about newly treated timber is how wet it is. In fact, it will be dripping wet, and there is a reason for this.

As you already know, wood absorbs liquids like a sponge. And, when exposed to air, the wet fibers shrink and then warp more on the short axis of a board than on the long axis. Treated timber is no different, for it also warps when the sealant starts drying unevenly.

Knowing this, manufacturers get their lumber to the lumberyard as soon as possible, many times with the sealant dripping. Yet despite their best efforts, some warping still occurs.


How Warping Affects Your Buying Decision

Before buying, look closely at a pile of treated lumber. If any boards bend away from the center of the pile, especially those boards at the edges, they have begun to warp. Do not buy them. Instead, opt for those located at the center of the pile, for they will still be wet and, more importantly, straight.

However, note that straight boards may not remain straight after you install them, especially if they harbor defects. So, after settling on a board, first inspect its end grain. If the grain is arched, not straight, the board will cup in the direction of the arch. Next, look for knots and cracks, defects that also cause warping.


Installing Treated Timber

Because pressure-treated timber resists the elements, use it for any number of outdoor projects. This includes decks, deck furniture, trellises, swings, and even mailboxes. But no matter the project, follow the same installation rules.

First, try not to leave gaps between the boards, even in projects that require the gaps. Instead, butt the boards to each other. Remember that they will eventually shrink on their short axes, leaving gaps. Second, install boards with their arched grains pointing upwards. This way, the timber cups upwards, not downwards.

Otherwise, your deck, outdoor seat, or project ends up with ruts, which are not only uncomfortable to walk or to sit on but which also collect water or dirt. And to arrest the cupping, fasten the center of each board with a screw during the installation. Only make sure to drill the holes first so that you do not damage the wood.


Safety Considerations during the Installation

Although they are not toxic, copper-treated sawdust and small shavings can irritate your respiratory system. So as a precaution, work the wood outdoors, not in the garage or shed, and remember to wear a mask, especially when sawing the boards. Also, do not burn the wood in a hearth or a fire pit, causing you to breathe in the sealant. Treated lumber is strictly for outdoor woodwork. Finally, follow the standard woodworking safety guidelines.

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