How to Weatherproof Treated Lumber Buried Underground

Surround the submerged wood with cement. Adding cement to the structure adds stability and seals the wood from any submerged dangers such as insects or water. This allows the exposed wood to be treated as often as necessary without concerns for the submerged aspects.

Dig the hole to the desired depth for stability. Typically, for a free-standing structure, it should be one-third the height of the structure. For structures such as a deck, patio or fence, approximately one-fourth the height should do. The width of the hole should be three times the diameter of the pole or wood beam. If the beam is 4 inches, the hole should be about 12 inches.

Pour about 2 inches of cement into the hole and lower the pole or beam into the hole. Tie ropes to it and anchor those ropes with ground stakes approximately 4 to 8 feet away from the pole. Make sure the pole is level and secured by the ropes and fill the hole with cement. Allow to dry per drying time directions on the cement bag, and remove the ropes. The submerged wood is now sealed against the elements.

Seal the wood before submerging it. Using treated lumber helps to protect from the elements but for added security, you can treat the lumber with an all-weather sealant. As the wood will be submerged, the color of the sealant is immaterial. Because you will not be able to re-apply the sealant, make the coating as thick as possible -- the sealant will wear off in time but will likely outlast the exposed elements if you seal it sufficiently.

Add rocks to the hole. The submerged wood will be resting on top of moist ground. If you are not securing the wood with cement, adding a thick layer of rock (approximately 6 to 8 inches) beneath the wood will allow moisture to drain away from the wood, further protecting it from harm.