1×6 Pressure Treated Lumber: Sizes, Lengths, Grades and Uses

The most common surprise with outdoor boards is that a “1×6” is neither 1 inch thick nor 6 inches wide. 1×6 pressure treated lumber actually measures about 3/4 inch thick by 5-1/2 inches wide once it has been dried and surfaced, and that difference changes how many boards you need for a fence, how a skirt lines up with framing, and which screws you buy. These boards are usually sold in 8, 10, 12 and 16-foot lengths, with 6-foot lengths sometimes stocked for fencing.

Beyond size, the two decisions that matter most are the treatment rating and the grade. Boards rated for above-ground use are fine for fence pickets, trim and skirting that stays off the soil. Anything buried, touching the ground or hard to replace later needs a ground-contact rating. Grade affects knots, wane and how straight the board stays as it dries.

1×6 Pressure Treated Lumber Sizes and Lengths

Here are the standard nominal and actual dimensions for the board sizes most people compare, along with common stock lengths and typical uses.

Treated lumber is frequently sold wet from the treatment process, so a fresh board can measure slightly thicker and wider than the dry size and will shrink a little as it dries. That shrinkage is why fence boards butted tight on install often show small gaps by the next season.

Choosing between 1x6x16 and shorter boards

A 1x6x16 pressure treated board is the go-to for long runs of skirting, fascia or horizontal fence boards because it spans two 8-foot fence sections or several deck posts with fewer joints. The downside is handling and straightness: 16-footers flex, can bow more, and need two people or a truck with a long bed. For vertical fence boards on a 6-foot fence, shorter lengths cut waste.

When a 1×8 makes more sense

A 1x8x16 pressure treated board covers about a third more width per board, which speeds up wide skirting or a tall raised bed. Wider boards cup more as they dry, though, so fasten them with two screws at each support and leave room for movement.

Treatment Ratings: Above Ground vs Ground Contact

Pressure-treated wood is labeled with a use category from the industry’s wood preservation standards, and the end tag on each board tells you which one you are buying.

• Above ground (UC3B): for lumber exposed to weather but kept off the soil and easy to replace. Fence boards and pickets above the ground line, deck skirting held up off the dirt, railings and trim fall here.
• Ground contact (UC4A): for wood touching soil or fresh water, embedded in concrete, or used where it is hard to replace or stays damp. Think bottom kickboards on fences, landscape edging, posts and any board within a few inches of grade where debris piles up.
• Heavy-duty ground contact (UC4B): for critical structural parts such as posts in severe exposure. Mostly relevant to 4×4 and 6×6 posts rather than 1-by boards.

When in doubt, buy ground contact. It costs a little more and removes the guesswork for boards near soil, mulch or standing water. Look for the end tag rather than relying on bin labels, which are easy to mix up.

Lumber Grades and How to Pick Good Boards

Most treated 1-by boards in the US are southern yellow pine, with other species regionally. Common grades include:

• #1 and select/appearance grades: fewer and smaller knots, straighter grain, better for visible trim and fascia.
• #2: the everyday grade. More knots and some wane (bark edge), fine for fences and skirting.
• Standard or utility: rougher boards for hidden or temporary uses.

At the store, sight down each board from the end. Skip boards with a twist, a deep crook, large loose knots near edges, or splits running from the ends. Stack boards you bring home flat with spacers (stickers) so air reaches all sides, and keep them out of direct sun until you install them to reduce warping.

Tools, Fasteners and Installation Steps

Treated lumber is chemically corrosive to plain steel, so fasteners matter. Use exterior wood screws or nails that are hot-dip galvanized or stainless steel and rated for treated lumber. Coated exterior screws labeled for treated wood also work for fences and skirting. Stainless is the safest choice near salt water.

Basic tools: a circular or miter saw, drill/driver, speed square, tape measure, level, chalk line and a scrap spacer block for consistent gaps.

1. Plan the layout. Measure the run, divide by the 5-1/2-inch board width plus any gap, and round up. Add 10 percent for culls and cuts.
2. Acclimate if wet. Very wet boards can be installed tight for fences (they will shrink), but for trim and skirting let them dry a few weeks if possible.
3. Cut and seal ends. Fresh cuts expose untreated wood. Brush on an end-cut preservative, especially on ground-contact pieces.
4. Pre-drill near ends. Drill pilot holes within 2 inches of board ends to prevent splitting.
5. Fasten at every support. Use two fasteners per board at each rail or joist, set flush, not countersunk deep.
6. Leave drainage gaps. Keep the bottom of fence boards and skirting 1 to 2 inches above soil unless they are ground-contact rated.

Safety When Cutting Treated Wood

Modern residential treated lumber uses copper-based preservatives, but the sawdust is still an irritant and the chemicals should not be inhaled or eaten. Wear a dust mask or respirator rated for fine dust, safety glasses and gloves when cutting. Cut outdoors where possible, sweep up sawdust rather than blowing it around, and wash your hands before eating. Never burn treated scraps in a fire pit, fireplace or wood stove: the smoke and ash can carry the preservative chemicals. Dispose of offcuts with regular trash per local rules. Avoid using treated wood for cutting boards or surfaces that touch food directly, and line vegetable garden beds if you want an extra barrier.

Finishing and Maintaining Treated Boards

Treated lumber resists rot and insects, but it still weathers, checks and turns gray in the sun. A water-repellent finish or an exterior stain slows that process and reduces cracking. Wait until the wood is dry enough to absorb water quickly, which can take a few weeks to a few months for wet-treated boards depending on climate. Some boards are sold kiln-dried after treatment and can be finished sooner; the end tag usually says so. Plan to clean and recoat fences and skirting every few years, and replace any board that turns soft at the bottom edge before rot spreads to the rails.

Troubleshooting Common Problems

• Boards shrank and left gaps: normal for wet-treated lumber. Plan for it by installing tight, or add a batten over the joints.
• Cupping or bowing: one face dried faster than the other. Fasten with two screws per support and avoid leaving boards in sun before install.
• Black streaks around fasteners: a reaction with the wrong fasteners. Replace with hot-dip galvanized or stainless.
• Stain or paint peeling: applied too soon. Wait until the wood is dry (water beads soak in quickly) before finishing.
• Rot at ground level: above-ground lumber used in contact with soil. Replace with ground-contact boards.

General Costs and When to Call a Pro

A 1×6 treated board is among the least expensive outdoor lumber options, with prices varying by length, grade, region and the lumber market. Longer boards cost proportionally more per foot, and ground-contact or appearance grades add a premium. For a fence or skirt, fasteners, posts and hardware can add meaningfully to the board cost.

Most skirting, planter and fence-board projects are friendly DIY work. Call a contractor when the job includes setting new fence posts on a long run, building or repairing structural deck framing, working on a slope or near property lines, or when local rules require a permit. Call 811 before digging any post holes so buried utilities are marked.