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Q. Our house has patio wood deck. The deck sits on the ground and on one side the wood base is at least 3 inches below the surface of the ground. It was graded this way by the original owner to help with water run off. We are assuming it has been this wa y since the deck was built before we bought the house a few years ago. The ground in this area stays wet longer then normal due to a slight run-off problem and shade trees. The deck has green moss on it which we plan to power wash and seal. Our concern is the life expectancy of the wood below the surface. Should we be concerned?
A. Since the deck is a few years old, I'll bet that the framing and decking material used for it was pressure treated lumber. If not, it would most likely have begun to rot by now and you'd see it.
There are a few different types of preservative treatment materials available now but the most common combination used when your deck was built was Southern Yellow Pine subjected to a pressure bath of chromated copper arsenate (CCA) to a penetration leve l of four tenths of a pound of treatment material per cubic foot of wood. Recently the arsenic based preservatives have been replaced by preservatives that do not contain toxic arsenic chemicals.
They still treat Yellow Pine as the wood of choice because it is a very moisture absorptive material and accepts treatment well. You have no doubt noticed that it accepts water well and in the environment that your deck was built the wood tends to stay wet for long periods of time-- so long in fact that it’s growing algae and moss on its surface.
To check to see if your deck is in fact constructed of this type of treated material, it should exhibit the tell-tale light green hue of the treatment material somewhere and you may find a lumber grading stamp on the wood in black ink that bears the .40 CCA identification of the wood preserving trade organization.
Treated wood provides an inhospitable environment for classic wood destroying micro-organisms that consume wood and treated lumber is generally rated as being able to last 50 years in contact with the ground. Even more densely treated lumber is regularly used in marine and pile driving applications. If it’s an application where people don’t come in contact with the wood they’ll still use CCA treated wood.
If you have to replace any of the deck boards, the newer treatment materials, though not toxic, are very aggressive to metals and you have to use hot dipped galvanized or stainless steel nails or screws.
I've heard of and actually seen rotted and insect damaged treated lumber but those anomalies can generally be explained as wood being improperly treated or the treatment not being able to penetrate all the way to the center of the piece, such as thick 8 by 8 posts. But by and large, properly treated lumber will last on and in the ground.
It's a good idea to keep the surfaces from getting slippery and dangerous to walk on-- appearances aside. Some homeowners like the look of the green algae thinking it looks quaint. Wet algae is very slick and hazardous underfoot.
Unless you own power-washing equipment and are comfortable using it, I must warn that a power washer in the hands of an amateur can be disaster for any wood. If the washer stream is too strong or you hold the nozzle too close to the surface, you will damage the wood and you'll notice the surface splintering up. Power-washing is fast and fun to do but it comes at a price. And if you've done it wrong you won't be able to tell until it's dry and by then it's too late. The power washer damaged wood will look like it’s growing a fuzz of splinters that you’ll have to sand off.
There is a simple and easy hand washing technique you can employ. You can buy premixed deck wash or mix some homemade brew up yourself using one cup of bleach with a quarter cup of common laundry detergent per gallon of hot water. Use a plastic bristled scrub brush on a long handle and a garden hose to wash it off. Wear eye protection and gloves and old clothes. Be careful about getting bleach on siding, lawn furniture or plants. Wait until it's almost dry then hose off liberally.
Use only clear or slightly pigmented wood stains or preservatives and be wary of sealers that brag about being able to bead water up on the deck surface after using them. They too can be awful slippery and I have slipped and fallen on them.