How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manfuactured and modular homes
What is the average lifespan of a wood deck?
Saturday, August 4, 2018
How long does a wood deck last?
The life expectancy of wood deck boards is 10 to 20 years, with an average of 15 years. Perhaps a little longer, with limited weather exposure and maintenance coating with a weatherproofing sealer every few years. Deck boards are usually the first component to go bad and, for a while, you can replace an occasional rotted board to get more life out of your deck—as long as the joists underneath remain structurally sound. The deck structure underneath the boards will last longer than the boards, but wood rot where water gets trapped between the deck boards and supporting joist will create spots of rot that make getting a good fastener grab difficult in places.
Here’s a graph comparing wood decking to other deck/patio materials.
There are multiple factors that can affect how long you deck lasts. Here’s our list of issues to consider:
1) Location - The best location for a deck will have moderate, dappled shade for most of the day. Blazing, continuous sun deteriorates deck boards and any sealer applied to protect them. At the other extreme, continuous heavy shade keeps the surface moist for long periods and promotes wood rot.
2) Quality of lumber - Buying a better grade of lumber, with fewer knots, checks and other defects, will extend the life of a deck. The standard pressure treated southern pine lumber at the big box home improvement stores is #2 grade. But #1 grade is also available. It is stronger structurally, will have fewer knots, less twisting and cupping, and a better appearance. There is also a “Prime” or “Premium” grade available that will have an even better appearance than the standard, but the same structural rating.
3) Distance from bottom of joists to ground - A deck that is close to the ground can be visually pleasing, but it will have a short life. Moisture is constantly rising out of the soil, and a deck built just above the grade lacks adequate ventilation to disperse the moist air created. When a deck is sitting on or just above the ground, you can expect that the lifespan will be cut in half; and, in some cases, the joists will rot as fast as the deck boards above them.
The deck in the photo below is an example, built close to ground and in an advanced state of deterioration. As you look at the image, it’s easy to visualize how each joist creates a separate compartment of unventilated wetness.
4) Type of foundation - The best deck foundation is stable—with a real concrete foundation, not precast deck blocks sitting in the dirt— and has no wood in contact with the ground. But, if you do end up installing wood posts in the ground, be sure they are marked as rated for “ground contact”—which means there is more preservative in the wood—and put the uncut end into the dirt. The pressure-treatment of the wood applied at the factory does not saturate completely to the center and, when you cut off the end, the exposed core of the post will have little or no chemical treatment to resist rot.
5) Water traps - “Water trap” is a term that construction professionals use to designate any surface of a wood structure that is exposed to the weather and does not slope adequately enough to drain off any water after a rain. Examples of a water trap in a deck would a board that has cupping (concave at top surface), a loose knot, or checking cracks—all of which will allow puddling of water that accelerates wood rot in the area. Deck boards with any cupping should be installed with cupped face downward, and any boards with loose knots or checks rejected, or the bad section cut away when possible.
Also, if the top of a wood post is cut off perfectly level, standing water will soak into the end-grain of the wood and center pocket of rot will begin within a few summer seasons. Cutting the tops of posts at a slant or in a pyramid shape is better, but even then the end grain is still susceptible to rot as shown in the photo below. The best solution is installing a waterproof decorative post cap.
6) Construction Details - A ledger board, where the deck meets the house, is the most common source of failure in a poorly built deck. A good builder uses spacers to create a gap between the ledger and the wall to let rainwater flow down the wall behind the board, and avoid rot from water trapped against the wall. But a ledger should never be attached to the wall a manufactured home, since they are not designed to accept any additional loads. Decks adjoining mobile homes should be freestanding.
Also, the fasteners used to construct the deck must be rated for use with the specific material, whether it is pressure-treated or composite wood. If regular screws and nails are used, they will have a shorter life than the boards. To read about other details to make a deck stronger, safer and last longer, go to our blog post “What are the warning signs of a dangerous deck?”
7) Coating - A water-repellant wood coating with an oil or wax base keeps water from penetrating the wood surface. The fungal organisms that cause wood rot need water to survive, so a deck coating applied every couple of years keeps the wood rot at bay, at least for a while.
Why don’t wood decks last as long as other residential structures? The simple reason is that everything else has a roof over it. Deck boards are exposed directly to the sun and rain, year after year. It’s amazing that they last as long as they do when they are well constructed and maintained.
Go to our blog post What is the average lifespan of the parts of a house? for rating of other house components. To understand the basis, potential use, and limitations of lifespan ratings, see our blog post ”How accurate are the average life expectancy ratings of home components? Are they actually useful?”
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To learn more about exterior walls and structures, see these other blog posts:
NOTE: These life expectancies are based on data provided by InterNACHI, NAHB, FannieMae, and our own professional experience. Because of the numerous variables that can affect a lifespan, they should be used as rough guidelines only, and not relied upon as a warranty or guarantee of future performance.
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