How To #82

Pressure Treated Lumber


Pressure treated lumber has unique advantages over non-treated lumber. It can be used in contact with concrete, the ground, or even water without the normal fear of rot and decay. The chemicals are simply retained inside the wood fibres for life.


The use of the lumber depends upon its rating, and how it was treated. Every pressure treatment is rated in terms of "RETENTION", measured in "POUNDS/CUBIC FOOT". The higher the number, the greater the treatment against decay. The wood is stamped with the correct usage information.

A typical above ground application of wood for example, has a .25 lbs/Cu. Ft. retention rating, or higher, established by the industry, association, and federal standards. This type of treated wood is ideal for deck surfaces and frames; where it does not have direct contact with the ground.

Wood that is used on and partially in the ground, such as for landscaping, posts or other projects exposed to water need a .40 retention or greater. Wood foundations, utility poles and other wood that is buried into the ground requires and even higher retention rating of .60 or better. A fresh water piling needs a 1.00 rating and a salt water piling a 2.50 rating, as other examples.


Pressure treated lumber provides many times the life of untreated and hand treated lumber. Some estimates place the average life of pressure treated lumber at over 50 years. Some manufacturers guarantee the life for 30 years.

Actual life depends on the kind of wood and how it is used. Structural framing, for example, requires stronger wood than a Cedar or Redwood. Cedar and Redwood are naturally resistant to decay, but weak compared to Fir or Hemlock. Some Fir is twice as strong as Cedar.

Hem/Fir is a mixture of Hemlock and Fir lumber on slings of dimensional lumber such as 2x4's. It is naturally strong but also prone to decay. When pressure treated, it will match or outlast Cedar in life and out perform Cedar in strength. A pressure treated wood deck built from Hem/Fir may use less wood than a comparable Cedar deck.


The cost for treated lumber is about the same as the cost for Cedar/Redwood. The treated lumber for outdoor applications is available pre-stained, saving you even more time and expense.


There are two basic formulas used to pressure treat lumber, marketed under a variety of trade names. One is called the OXIDE formula, the other the SALT formula.

The SALT formula was introduced first, using some chemicals that react with each other, without treating the wood. This can cause a greenish-white powder to form on the surface when it dries.

The OXIDE formula was introduced later and eliminated these deposits. OXIDE treated wood is slightly cleaner to handle and more natural in appearance than SALT treated wood.

While both treatments may cause a greenish tint to the wood, both are efficient wood treatments.

Fence posts that carry electrical current may be more conductive with SALT wood than OXIDE wood. This means that lightly galvanized metal may corrode a bit faster than on Oxide wood, but hot-dipped galvanized or zinc metals should work well.


Both types of treatment are safe and NON-LEACHABLE. This means that no chemicals can leave the wood and enter the ground or your body. Also, treated wood is fully paintable and may be stained. It has the same paintable properties as untreated wood.

The reason they are not leachable is the way in which the chemicals are injected into the wood. The lumber is submitted to pressures of many atmospheres under which the chemicals are injected into the wood. The longer the wood is exposed to this treatment, the better the retention, and permanence of the chemicals.

Some types of wood are notched by machines to increase the saturation rate of the chemicals into the wood. These incisions are usually found on "ground contact" lumber, with a .40 rating or better.

This pressure saturation does not affect the wood's ability to perform normally. The structural integrity of the wood, and its ability to swell or shrink with moisture is the same as for untreated wood.


Handling, cutting and fastening is the same for untreated lumber. Gloves are recommended to prevent cuts, splinters and reaction to the chemicals.

Keep treated wood away from public drinking water as well as the drinking water of livestock or domestic animals. Prevent prolonged bare skin contact and situations where it may cme into contact with human or animal food sources. Never burn pressure treated wood: the chemicals can cause toxic fumes!


Though treatment begins with kiln dried lumber, the wood absorbs moisture at the lumber yards. It can check, crack, or change shape, the same as untreated wood that is exposed to the weather.

When installed and exposed to the weather, add a clear sealer that retards fast drying that can cause the wood to crack. Spray, brush or roll the sealer on all exposed wood as soon as it is installed. This also beautifies the natural look of prestained-treated wood. Read the sealer's directions on how long to wait before applying stain/paints on freshly sealed and/or treated wood.

Oil based stains/paints may be used immediately on treated lumber. They will also retard the drying and subsequent checking of the wood. Follow all directions for surface preparation first.

Use caution when staining/painting over previously sealed pressure treated wood. The sealers may repel the stain/paint you want on the wood. Be sure to read the directions on every can of sealer, stain and paint.

When end are cut, soak or brush on additional preservative. Any cuts or drilled holes should be treated to insure the life of the wood. Some brands of treated wood require cut-ends to be treated to validate the manufacturer's warranty; others recommend it.

You cannot see the pressure treatment on end-cuts and many people choose to hand-treat the cut ends anyway: it cannot hurt the treated wood.



To install a pressure treated fence post into the ground, first have a ground contact rated post, a .40 rating or better as determined by your local codes. Next, remember the 1/3 rule: that is 1/3 of the post length should be buried. A 6' post for example requires 2' of post below the ground.

STEP 1 Dig the hole with a post hole digger, and then widen it out at the bottom, with a small shovel. A 10" flare at the bottom is considered ideal.

NOTE: A post larger than a 4x4 should have a proportionally larger hole.

STEP 2 Dig the hole about 6" deeper than needed for the post. Set rubble, a rock or gravel in the bottom of the hole first and then set the post on top.

NOTE: This raises the post off of the bottom of the hole; preventing water saturation of the fence post base.

STEP 3 Align posts to guide string and fill the hole half-way with the dirt removed. Pack the dirt well around the post until it can support the post. Keep adding/packing dirt until the dirt is about 2/3 up the post or 8" below ground level.

STEP 4 Brace posts in 2 directions with fence rails; usually 2 - 2x4's. Plumb posts, keeping them aligned to the guide string. Stake the base of the braces to prevent any shifting or movements.

STEP 5 Pour concrete. This should rise 2" above ground level. Taper the concrete collar so that when it dries, rain water will run down the concrete collar and away from the post hole.


Pressure treated sill plates are used to fasten untreated wood to concrete and or the ground. For plates that are hidden from view use a #3 grade of treated lumber or as accepted by local codes. This wood is unappealing but less expensive than the prestained treated lumber and adequate.

Exposed plates, such as a base for a wood deck, need the incised and prestained treated lumber. This wood should have a retention rating of 0.40 or better, as established by your local codes.

Interior plates, such as for a basement partition, should also use the prestained #2 wood. It is straighter and drier. Which will help your job.

Installing plates to concrete is done either before or after the concrete has dried. A "J" bolt is set in wet concrete, leaving the threaded end above the concrete exposed. Drill holes through the plate to accept the "J" bolt end. A wider hole drilled half-way on the top of the plate will allow the nut to sit flush with the top of the plate.

You can shoot steel pins though plates and into a slab with a gun powder charge; available for rental.

On cured concrete use construction adhesive, concrete nails/pins, or expansion bolts. Bolts require holes drilled into the concrete, while nails/pins are hammered/shot through the plate.

Construction adhesive may be used alone or with fasteners. Try using dry plates on freshly cleaned concrete to maximize the adhesive's bond.

Wilway Lumber Sales Inc.