Building With Pressure Treated Lumber? Know The Rules

Pressure treated lumber, if installed properly, should provide a 40 year useable life. However, there are some things about pressure treated lumber that you have to know, in order to obtain that life and not have your project fall apart in a few years.

To create pressure treated lumber, raw lumber is impregnated with two waterborne compounds that are copper based chemicals. The amount of copper basically determines the usage and all pressure treated lumber, sold in the US, is supposed to have a tag stapled to the end of the board signifying its pounds of preservative per cubic foot of wood product.

Table 1 indicates how many pounds of preservative to use per cubic foot of wood based on the application of the lumber.

When pressure treated lumber is cut, you may notice that the cut ends do not have the same level of protection that the rest of the board does This is because the board does not absorb the copper throughout its entire depth. In order to avoid having the ends of cut boards absorb moisture and begin to rot, it is important to seal the ends with a wood treatment designed for pressure treated lumber.

Because the lumber is impregnated with copper, the choice of fasteners becomes very important. When two dissimilar metals come in contact with one another a galvanic reaction is created, the outcome is that the materials corrode and disintegrate. The higher the levels of copper in the lumber the faster that the corrosion will occur.

Although most people realize that they must use corrosion resistant fasteners and connectors many do not consider items such as flashings and light poles that are also in contact with the pressure treated lumber. Of course the best metals to use for fasteners, flashings and anything else that comes in contact with the deck is type 304 or 316 stainless steel or copper whenever possible. Stainless steel is expensive and copper is not conveniently available at your local lumber supply so the next best material is steel that has been hot galvanized.

Galvanized fasteners come in different grades (the thickness of the coating), better manufactures mark the box with a “G” rating. The higher the G rating, the more zinc used. You shouldn’t use less than a G-90 rating and engineers are now suggesting the use of G-185. If the box is not marked it is probably a G-60 or less and this will not provide enough zinc coating to prevent galvanization from occurring. Cadmium plated nuts, bolts and screws do not provide any protection from corrosion whatsoever.

Some manufactures have created composite coatings for use with pressure treated lumber. Read the label on the box and make sure that what you are buying is suitable for the job.

Do not use aluminum or carbon steel flashing, deck flashing must be stainless steel, galvanized or copper. Aluminum and carbon steel corrodes very quickly when in contact with copper. This also means that metals items that come in contact with the pressure treated lumber must also be protected, items such as outdoor furniture, and barbeques should have rubber or vinyl feet or wheels.

Pressure treated lumber also requires special saw blades that have been designed for wet or chemically impregnated lumber.

Update: Using the new ACQ pressure treated lumber.