Pressure Treated Wood

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April 22, 2014
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Fighting Decay in Harsh Conditions

Proper installation, coating, and maintenance are important to keep wood from rotting (see the previous post on How to Keep Wood Homes Solid). For exposed places that naturally get very wet or that touch the soil, pressure-treated wood is often used as an additional protection. It repels wood-boring insects, too.

You’ll find pressure-treated wood used most often on decks, porches, fences, and sills, particularly wall sill plates but also exterior door and window sills.

Pressure Treated Deck

Pressure-treated wood has been in use for well over half a century. Less toxic types of preservative are being used these days, but their protective value is the same as the original arsenic mixes. Even though they are less toxic, you should still wear gloves when handling pressure-treated wood. Add a dust mask and safety glasses, too, when cutting and drilling it, and always cut it outdoors. Finally, wash up well before eating.*

Pressure-Treated Wood Makes the Grade

The current formulations of preservatives have been shown to keep wood solid for two decades or longer, depending on where the wood will be located and how much preservative has been forced into it. Pressure-treated wood in contact with the ground needs the most protection, and will rot in just a few years if you use the wrong grade. If you’re planning a DIY project, make sure to tell your lumber dealer the end use, so you’ll get the right grade.

Grades of pressure-treated wood are numbered to indicate that amount of preservative per board foot. The grades start at .25 and go up to .60 (marine grade). If your wood will touch the ground or be buried, you should get the highest grade you can, up to .60 if it’s available. It will be more expensive, but you’ll save money in the long run by not having to replace it as often.

Coat Pressure-Treated Wood for More Protection

As we mentioned above, proper installation is important to keep even pressure-treated wood from rotting. In addition, it should be coated after it is installed, and the coating should be maintained just as you would the siding of your home. “Pressure-treated” does not mean “invincible!”

Rotted Pressure treated

A coating on pressure-treated wood helps keep out moisture, but it also helps seal in the preservative. This is important on wood that people will be touching, such as decks, porches, and outdoor railings.

Coating the pressure-treated wood also protects it from the sun’s ultra-violet rays, to slow down weathering. Pressure-treated lumber, like all wood, will develop checks after prolonged exposure, and renewing the sealer will help. Badly cracked boards should be replaced.

What to Use, and How

Any high-quality exterior deck paint or stain will work. Note, however, that you want a penetrating paint or stain. Non-penetrating coatings are prone to crack and flake off in the harsh conditions that call for pressure-treated wood.

If you want a natural wood look, use a clear wood sealer and preservative to retain the lumber’s rot resistance, and renew the sealer annual. Some experts recommend resealing at six months after the initial installation and coating. Additional tips:

  • Make sure the surface is completely dry before applying the paint or sealer. Sprinkle water on the wood; it it beads up, wait. If it soaks right in, it’s dry enough.
  • Some coatings can be applied to wet wood, but don’t do it unless you’re in a hurry, and make sure you use the right product!
  • Make sure to coat the ends of boards thoroughly, since end grain can be the easiest entry point for both moisture and insects.
  • When recoating, clean the surface thoroughly, and then let it dry thoroughly. Use a commercial cleaner that will kill mildew. If pressure spraying, don’t set the pressure too high, so you won’t damage the surface of the wood.
  • Use a brush, or brush out the wet coating after spraying. You’ll get a nicer finish.

* Installation Tips for DIYers

  • Pre-drill the holes for nails and screws, particularly near the ends of the boards.
  • Remember that wood exposed outside will shrink across the width quite a bit. Take that into consideration when spacing deck boards and fencing.
  • Coat the bottoms, sides, and ends of boards before you set them in place, so all surfaces will be coated.
  • For posts (fences or decks), coat the underground portion, including the end-grain, with tar.

Finally, never burn treated wood of any kind. Dispose of cut-offs or old, weathered wood with the other construction debris at your local dump or transfer station.

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