When using lumber underground, you should always apply preservative to the lumber to prevent rot and termite damage. There are several tried-and-true methods for treating lumber that have been developed in the construction building industry. These methods and can be implemented by the do-it-yourself homeowner.
Thoroughly coat the lumber with water sealer made for wood. Using a pump garden sprayer, spray one coat onto dry lumber. Allow the sealer to soak completely into the lumber grain, and allow two hours between coats. Apply the second coat of sealer to the lumber with a roller or brush. This will ensure that the sealer is applied in thick, even coats. You may apply more than two coats of sealer; however, do not apply more than will absorb into the lumber grain.
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Apply two coats of termite treatment to the lumber. These chemicals can be purchased at any hardware store. Use the pump sprayer to apply this chemical, but do not inhale the mist from this spray. Wear a mask and goggles when applying. Follow the drying time directions on the container for reapplications.
Prepare the tar you will apply to the lumber. Thin the tar with paint thinner before application. Add 2 parts tar and 1 part thinner to a mixing container, mixing with a paint mixing stick. For example, if you use 1 quart tar, then add 1/2 quart paint thinner and blend. The thinned tar will ensure thorough penetration of the wood pores.
Apply roofing tar to the lumber, but only to wood that will actually be below dirt. Using a brush, brush tar onto the lumber. One coat will suffice.
Allow the lumber treatment to dry for 24 hours after you have applied all preservatives. Add a few drops of water to the surface to test the waterproofing abilities of the chemicals. If the water beads up and stands on the surface, then your applications have worked. Otherwise, treat the lumber again with tar.