How long does a wood fence last?

A wood fence lasts about as long as its posts, because posts sit in wet soil where rot starts and boards above ground can be replaced one at a time. With posts treated for ground contact, a well-kept wood fence in Virginia commonly stands for decades; with untreated pine posts, it may start leaning within a few years. Material choice sets the ceiling, and maintenance decides how close you get to it.

Expected life by wood

Virginia Cooperative Extension's table of post life gives southern pine 3 to 7 years untreated and 25 to 30 years treated.^[1] The same table, in years:

• Red cedar 15 to 20 untreated, 20 to 25 treated.^[1]
• Black locust and Osage orange 20 to 25, untreated.^[1]
• White oak 10 untreated, 15 to 20 treated.^[1]
• Yellow-poplar 3 to 7 untreated, 20 to 25 treated.^[1]
• Red oak 5 untreated, 15 treated.^[1]

As a broad range, the same publication gives treated wood posts in general an expected life of 10 to 30 years, and untreated posts 2 to 25.^[1] The publication is about livestock fences, so treat the figures as a guide to how posts in the ground age, not a warranty on a backyard fence.

Why wood fences fail

Rot is a fungus, and it needs the right conditions. The growth of decay fungi depends on mild temperatures, moisture and air,^[2] which is exactly what the top few inches of soil around a fence post supply. That is why a fence usually fails at the ground line first, then sags at the rails, while the tops of the boards still look fine.

Species alone does not settle it. Untreated sapwood of essentially all species has low resistance to decay and usually has a short service life under conditions favoring decay.^[2] Natural decay resistance is in the heartwood, and it varies even between pieces cut from the same tree.^[2] Marketable sizes of some species are mostly second growth with a high share of sapwood,^[2] and for wood under severe decay conditions, the Forest Products Laboratory generally recommends pressure-treated wood rather than resistant heartwood.^[2]

Treatment comes in grades. In the treating industry's use categories, UC3 is for exterior wood used above ground and UC4 for wood used in ground contact.^[3] Fence boards and rails can be above-ground lumber; posts should be ground-contact lumber. A post cut from the above-ground grade is likely to fail early.

Virginia's climate adds to the wear. Powhatan County's design data rates termite risk as moderate to heavy and weathering as severe,^[4] and Shenandoah County lists moderate to heavy termite damage and severe weathering.^[5] Sun and rain crack and grey the boards; termites and decay work on the posts.

How to make it last

Start with the posts. Buy ground-contact posts, set them on gravel, and finish the concrete so water runs off. QUIKRETE suggests a 6-inch gravel base under each post and a concrete top that slopes away from it^[6] as ways to protect the wood from rot. How to set a fence post in concrete covers the method.

Treat every cut. Cutting treated lumber exposes wood the treatment may not have reached. The Forest Products Laboratory's remedy is a thorough application of copper naphthenate (1% to 2% copper) to the cut surface.^[3] Put factory ends in the ground and cut ends at the top, under a cap.

Keep it dry. Keep mulch, soil and leaves off the bottom of the boards, leave a gap between the bottom of the pickets and the ground, aim sprinklers and downspouts away, and trim back vines. A fence that dries quickly after rain rots slowly.

Keep a finish on the boards. An exterior stain or water repellent slows the cracking and greying of boards and rails above ground. Recoat when water stops beading on the surface rather than on a fixed schedule; a sunny south-facing run weathers faster than a shaded one.

Inspect once a year. Push a screwdriver into each post just below grade, tighten or replace loose fasteners, and replace a split picket or rail before its neighbours work loose. A leaning post is easier to reset than a fallen section is to rebuild; see why new fence posts lean.

Old treated wood

If your fence went up years ago, it may be the older arsenic-treated kind. Prior to 2004, wood treated with chromated arsenicals was used in residential structures such as decks and playsets.^[7] When it comes down, it can usually go out with ordinary household trash, though state and local programs may have their own rules.^[7] EPA says not to burn treated wood at home, not to reuse CCA wood as mulch, and to wear goggles and a dust mask when sawing it.^[7]

Stop and call a pro if

• Many posts are soft at the ground line, or a whole run is leaning.
• You find mud tubes, hollow-sounding wood or swarming insects, which call for a licensed pest company.
• The fence is a pool barrier, or it sits on a retaining wall or slope.
• A section has fallen toward a sidewalk, driveway or neighbour's yard.

If it is time for a new fence, read what a fence costs in Virginia and whether you need a permit. Browse fence installers, all exterior answers, or exterior businesses around Blacksburg. For repairs, describe the fence problem once and local businesses send written quotes.