A 4×6 is not 4 inches by 6 inches, and that surprise trips up plenty of first-time deck and pergola builders. 4×6 pressure treated lumber actually measures 3-1/2 by 5-1/2 inches once it has been dried and planed. It is sold in lengths from 8 to 20 feet, weighs roughly 5 to 8 pounds per linear foot depending on moisture, and comes in above-ground and ground-contact treatment ratings.
That combination of size and rot resistance makes 4×6 treated lumber a workhorse for short deck posts, small beams, pergola rafters, landscape borders and retaining edges. It is stiffer than a 4×4 in one direction and lighter and cheaper than a 6×6, which is exactly why it gets chosen, and also why it gets misused. Knowing which way to orient it and which rating to buy saves a lot of rework.
4×6 Pressure Treated Lumber Sizes and Weights
The 4×6 lumber actual size is the same across lengths: 3.5 by 5.5 inches. Length is where the choices come in. Weights vary because freshly treated wood is often still wet from the treating process and can be much heavier than kiln-dried-after-treatment stock. Use these figures as planning estimates for carrying and hauling.
| Board | Actual size | Approximate weight (dry to wet) | Common uses |
|---|---|---|---|
| 4x6x8 pressure treated | 3.5 x 5.5 in x 8 ft | 40 to 65 lb | Short posts, landscape edging, garden steps |
| 4x6x10 pressure treated | 3.5 x 5.5 in x 10 ft | 50 to 80 lb | Pergola posts, fence posts for heavy gates |
| 4x6x12 pressure treated | 3.5 x 5.5 in x 12 ft | 60 to 95 lb | Small deck beams, pergola beams |
| 4x6x16 pressure treated | 3.5 x 5.5 in x 16 ft | 85 to 130 lb | Pergola rafters and beams, long landscape runs |
| 4x6x20 pressure treated lumber | 3.5 x 5.5 in x 20 ft | 105 to 160 lb | Long beams, carports and shade structures (two people to carry) |
Treatment ratings
Treated wood is labeled with a use category, usually on an end tag:
- Above ground (UC3B): for beams, rafters and framing at least 6 inches above the ground that drain and dry quickly.
- Ground contact (UC4A): for posts set in soil or concrete, members close to the ground, or wood that stays damp or is hard to replace.
- Heavy-duty ground contact (UC4B): for critical structural posts, permanent foundations and harsh conditions.
When in doubt, buy ground contact. A 4×6 treated post set in the ground with an above-ground rating can rot out years early.
Posts vs Beams: How to Use a 4×6
The shape of a 4×6 makes it stiff on edge and comparatively weak on the flat. That one fact decides most of its uses.
As a beam
Set a 4×6 on edge (5-1/2 inch face vertical) for beams and rafters. In that orientation it resists bending much better than a 4×4 and is comparable to a doubled 2×6, but in one solid piece that is easy to notch for a pergola. Laid flat, it sags. Span limits depend on species, grade, load, snow and spacing, so use span tables from your local code or the lumber grading agency, or have the building department check your plan.
As a post
A 4×6 pressure treated post works for low decks, privacy screens, mailbox posts and fences with heavy gates. Many codes and builders now prefer 6×6 posts for decks, especially taller ones, because the extra section resists buckling and gives room for proper beam connections. For a deck or pergola more than a few feet tall, check what your local code requires before choosing 4×6 posts.
Landscape and other uses
Laid flat or on edge, 4×6 treated wood makes durable garden bed walls, driveway edges, step treads for landscape stairs and parking stops. For low retaining edges, stack and pin them with rebar or long spikes. Taller retaining walls need engineering and are often better built from block.
Tools and Materials
- Circular saw (7-1/4 inch blades need two passes to cut a 5-1/2 inch face) or a large miter saw
- Speed square, tape measure and carpenter’s pencil
- Drill/driver and an impact driver for structural screws
- Hot-dip galvanized or stainless steel fasteners and connectors rated for treated lumber
- Post base bracket or standoff base for posts on concrete
- End-cut preservative to brush on every fresh cut
- Gloves, safety glasses and a dust mask
How to Install a 4×6 Post Step by Step
- Call 811 before digging. Have underground utilities marked a few days ahead. It is free and required in most areas.
- Check permits and HOA rules. Decks, pergolas attached to a house and tall fences usually need a permit.
- Choose your footing. For decks and pergolas, a concrete footing below the frost line with a post base bracket keeps the end grain out of the soil and lasts longer than burying the post. For fences, many builders set ground-contact posts directly in concrete or gravel.
- Cut and seal. Cut to length, then brush end-cut preservative on the cut end, especially any end that will sit near the ground.
- Set and plumb. Fasten the post to the bracket with the specified nails or screws, then check plumb on two adjacent faces and brace it.
- Connect the beam. Use rated post-to-beam connectors or through-bolts with washers. Avoid toe-nailing structural connections.
- Let it dry before finishing. Wet treated wood needs several weeks to several months before stain will absorb; sprinkle water on it, and if it beads, wait longer.
Anchoring is the safety-critical part. A pergola or shade structure catches wind like a sail, and snow adds load to beams. Anchor every post into a proper footing, connect beams to posts with rated hardware, and size members for the wind and snow loads in your area.
Troubleshooting 4×6 Treated Lumber
- Twisting and checking: wet treated lumber shrinks as it dries and can twist or crack. Pick straight boards with tight grain, fasten them soon after purchase, and store them flat and stickered if they wait.
- Corroded fasteners: modern copper-based treatments corrode plain steel and some coated fasteners. Use hot-dip galvanized (G185) or stainless.
- Rot at the base: usually an above-ground rated post set in the soil, or end grain sitting in water. Replace it with ground-contact stock on a standoff base.
- Sagging beam: the 4×6 may be laid flat, overspanned or overloaded. Add a mid-span post or upsize the beam.
- Stain won’t take: the wood is still wet. Wait and retest.
4×6 Pressure Treated Lumber Prices
Prices swing with region, species, treatment rating and the lumber market. As a general range, a 4x6x8 pressure treated board often costs somewhere around $15 to $30, a 4x6x12 roughly $25 to $50, and a 4x6x16 or 4x6x20 can run from about $40 to $100 or more. Ground-contact and kiln-dried-after-treatment stock costs more than standard above-ground boards. Lumberyards often stock longer lengths and better grades than general retail shelves.
Southern yellow pine is the most common species for treated 4×6 lumber in much of the US. In the West, treated Douglas fir and hem-fir are common and may be incised (small slits along the surface) to help the preservative penetrate.
Buying Tips for 4×6 Treated Wood
Pick through the stack when you can. Sight down each board for crown, bow and twist, and skip pieces with large loose knots near the middle of a span, wane (bark or missing wood) along an edge, or deep splits at the ends. For beams, place any slight crown facing up so the load straightens it over time.
Read the end tag on every piece of 4×6 pressure treated lumber you buy. It shows the use category, the preservative type and whether the wood was kiln dried after treatment. Kiln-dried stock costs more but is lighter, moves less and accepts stain sooner. If you are building a pergola or exposed beam that people will see up close, that extra cost often pays off in straighter, cleaner-looking members. For buried posts, focus on the ground-contact rating first and appearance second.
When to Call a Pro
Hire a licensed contractor or have a structural engineer review the design for elevated decks, decks attached to the house, pergolas carrying heavy loads, carports, or anything more than about one story off the ground. The building department can tell you which projects need stamped drawings. A pro is also worth it when the soil is soft, frost lines are deep, or the structure sits in a high-wind or heavy-snow area.
Frequently Asked Questions
What is the actual size of a 4×6 pressure treated board?
A 4×6 measures 3-1/2 by 5-1/2 inches. The nominal size refers to the rough-sawn dimensions before drying and planing.
How much does a 4x6x8 pressure treated board weigh?
Roughly 40 to 65 pounds, depending on species and how wet the wood is from treatment.
Can I use a 4×6 as a deck post?
For low decks it can work, but many codes and builders prefer 6×6 posts. Check your local code for the height and load of your deck.
Should a 4×6 beam be installed on edge or flat?
On edge. With the 5-1/2 inch face vertical, a 4×6 is far stiffer than when laid flat.
Do I need ground-contact rated 4×6 lumber?
Use ground-contact rated lumber for any post in soil or concrete, for wood within 6 inches of the ground, and anywhere it stays damp or is hard to replace.
