Choosing a fence material for Durham's clay and termites

Durham's soil and climate are hard on fence posts. Here is what the soil scientists, the building code and the wood treatment standards say, and how the materials compare on price.

Durham sits on clay

• Piedmont soils form from weathered bedrock, and the layer below the topsoil usually holds more clay (NC State Extension).
• Cecil, the soil NC State uses as its example of a typical Piedmont soil, has a firm, sticky red clay layer starting about 8 inches down (NC State Extension, USDA soil survey).
• Clay is sticky when wet, hard when dry, and can shrink and swell (NC State Extension).
• Parts of Durham County also have Triassic basin soils. One of them, White Store, is rated "very high" for shrink and swell (USDA soil survey, White Store).

For a fence, that means post holes that can hold water and soil that moves with the seasons. Ask each contractor how they set posts and how they let water drain from the bottom of the hole.

How deep the posts go

North Carolina's building code sets a 12-inch frost line for building footings (NC Residential Code). There is no code rule for the depth of ordinary fence posts. A common rule of thumb from Quikrete is a hole one third of the post's length deep, plus 6 inches for a gravel base (Quikrete). For an 8-foot post, that is about 38 inches. Ask every contractor to write the post depth into the quote.

Termites

North Carolina's building code rates the state moderate to heavy for termite damage and requires termite protection everywhere in the state (NC Residential Code). NC State Extension says nearly every property owner in North Carolina will meet the eastern subterranean termite at some point, and that wood touching soil gives termites an easy way in (NC State Extension, termites).

Treated wood: the rating to ask for

• Wood posts set in the ground: Use Category 4A or higher (AWPA).
• Fence pickets, which stay above ground: Use Category 3B or higher (AWPA).
• Use Category 4B, the heavy duty rating, is meant for high decay risk or constant wetting (Southern Pine guide), worth asking about for posts in wet, low spots.
• The rating is printed on the end tag or the treatment stamp of each board (AWPA), so you can check it on delivery day.

How the materials compare on price

A few Durham rules decide the choice before price does:

• In a local historic district, the guidelines say chain link, wire and vinyl fencing are generally not appropriate (Durham historic review criteria).
• A chain link pool barrier needs mesh no larger than 2-1/4 inches (NC Fire Marshal).
• In residential areas, barbed and razor wire are only allowed in the Rural Tier (Durham UDO 9.9).

See the cost guide for gates, removal and the other add-ons, and the rules page for heights.