Pressure-Treated Lumber Sizes in Miami: Posts, Framing and Heavy Dimensions Explained

Pressure-treated lumber sizes in Miami are easy to misread: a 3×10 is not three inches by ten inches, and a post that is fine above ground can fail in the dirt. This guide explains what common nominal sizes actually measure, which use category the job needs, how to read the end tag, and what to send us when you need a size that is not on our list.

Nominal vs Actual: What a 3×10 or 4×10 Really Measures

Lumber is named by its nominal size, not the size you measure with a tape. The Southern Forest Products Association (SFPA) standard sizing table gives the dry dressed size for each nominal thickness and width. Following that table, a nominal 2 is 1-1/2 inches thick, a nominal 3 is 2-1/2 inches, and a nominal 4 is 3-1/2 inches. Nominal widths of 4, 6, 8, 10 and 12 are 3-1/2, 5-1/2, 7-1/4, 9-1/4 and 11-1/4 inches. Boards with a nominal thickness of 1 are 3/4 inch thick.

Nominal size	Actual size, dry dressed (inches)
1×10	3/4 x 9-1/4
2×4	1-1/2 x 3-1/2
2×6	1-1/2 x 5-1/2
2×10	1-1/2 x 9-1/4
2×12	1-1/2 x 11-1/4
3×10	2-1/2 x 9-1/4
3×12	2-1/2 x 11-1/4
4×4	3-1/2 x 3-1/2
4×10	3-1/2 x 9-1/4
6×6 (timber)	5-1/2 x 5-1/2

The 6×6 follows the SFPA timbers rule, which puts pieces 5 inches and thicker at one-half inch under nominal. Figures in this table come from the SFPA table and its rules, not from International Plywood & Lumber specifications. The American Softwood Lumber Standard page says green and dry lumber are measured using separate size schedules so both approximate the same size in service, which is why the figures above are the dry dressed ones. The SFPA pressure-treated Southern Pine brochure also says most pressure-treated Southern Pine is delivered without being redried, so measure what you receive and plan around it.

Match the Size to the Job

Treated lumber is bought for a job, so start with what the piece does. These are common pairings, not engineering advice. Your designer, engineer and building official decide the size and treatment your project requires.

• Posts: 4×4 and 6×6 for fence, deck and pergola posts. Heavier timbers suit larger structures.
• Joists and beams: 2×8, 2×10 and 2×12 for deck framing, with 3x and 4x pieces where a designer calls for a heavier member.
• Rails and boards: 2×4 rails and 1x or 5/4 boards for fence and deck surfaces.
• Landscape timbers: 4×4 and 6×6 sizes for planters, walls and edging, where the piece usually touches soil.

When a search says 4×10 or 3×12, check first whether the plans really need that member. A heavier nominal size changes weight, handling and the hardware that fits it.

Above Ground vs Ground Contact

Size is half the order. The other half is the treatment. The SFPA pressure-treated Southern Pine brochure says the two most common service conditions for pressure-treated Southern Pine are Above Ground, Exposed (UC3B) and Ground Contact, General Use (UC4A), and that higher retention levels are typically required for ground contact.

The Preserved Wood use-category guide describes UC3B as exterior construction above ground, such as decking, railings, joists and beams for decks and fence pickets. It describes UC4A as ground or fresh water contact, and lists sawn fence, deck and guardrail posts, structural lumber, and joists and beams for decks and freshwater docks as typical uses. The AWPA information for homeowners states that ground contact applications require an AWPA Use Category 4A or higher.

The SFPA guide to choosing pressure-treated lumber offers a plain rule: when in doubt, choose ground contact. It names deck joists or beams within 6 inches of soil, hard-to-reach structural components and stair stringers near the ground as examples.

Thickness Matters for Ground Contact

AWPA warns that some treated lumber that is two inches or less in thickness may be treated only for above-ground uses, so it may not last long in ground contact. That is why a one-inch board is a poor choice for anything touching soil, however well it is sized. For posts and ground-contact members, confirm the treatment on the piece itself before you buy.

Reading the End Tag

Do not assume. Read the tag. The SFPA pressure-treated Southern Pine brochure says to confirm each piece was treated for the application by reading its plastic end tag or ink stamp, and to look for the end-use wording, such as Above Ground, Ground Contact, UC3B or UC4A. It calls the most common misapplication lumber treated for above-ground use only that ends up where the service condition simulates ground or fresh water contact. The SFPA specification guidelines say each piece should carry a quality mark or end tag showing:

• The inspection agency
• The end use application
• The use exposure
• The preservative used
• The retention level
• The treating company and year of treatment

If the tag does not match what your plans call for, raise it before the piece is installed. The AWPA U1 standard excerpt lists sawn fence posts, sawn deck posts, sawn guardrail posts, and structural lumber, joists and beams for decks among UC4A examples, and says deck joists and beams need UC4A treatment when they are difficult to maintain, repair or replace and critical to the structure. Use it as a cross-check when you read your own specification.

Treated Lumber Moves

The SFPA pressure-treated Southern Pine brochure says most pressure-treated Southern Pine is delivered without being redried, and that drying after treatment, either kiln dried after treatment (KDAT) or air dried after treatment (ADAT), can be specified to return the wood to its dry condition. If your work is sensitive to shrinkage, such as tight deck board spacing or finished millwork, say so when you order and confirm what you will receive. We do not make drying promises on any size, so ask when you call.

Cutting and Drilling on Site

Every cut exposes untreated wood. The SFPA specification guidelines say jobsite cuts and borings should be field-treated with copper naphthenate, with a minimum 2 percent metallic solution, in line with AWPA Standard M4. Treat the ends of posts and any drilled holes before you set them, and follow the instructions on the product you use.

Which Sizes We List and How to Order Others

International Plywood & Lumber is a plywood and lumber wholesaler serving trade customers in South Florida. Our pressure-treated lumber page lists these sizes at our Doral warehouse: 4×4 in 8, 10 and 12 foot lengths; 6×6 in 8, 10 and 16 foot lengths; 2×4 in 8 and 16 foot; 2×6 in 8, 12 and 16 foot; 2x8x12; 2x10x12; 2x12x16; 5/4×6 in 12 and 16 foot; 1x6x16; and pressure-treated plywood 4×8. Call for current stock on any of them.

Sizes outside that list, including 3×10, 3×12, 4×10 and 1×10, and other lengths, are not on the page. Call (305) 884-0860 and ask. When you send a request, include:

1. Size and length, written the way your plans write them
2. The use category the job needs, above ground or ground contact
3. Quantity
4. Whether you need as-treated lumber or want to specify drying after treatment
5. Your needed-by date
6. Delivery ZIP code

Serving Miami-Dade, Broward and Palm Beach

We are serving Miami-Dade, Broward, and Palm Beach counties, with delivery available. Contractors near the county line can see our Fort Lauderdale plywood and lumber page, our Broward County delivery page and our pages for Pompano Beach and Homestead. Request a trade quote on our contact page or call (305) 884-0860. Hablamos Español.

Frequently Asked Questions

What is the actual size of a pressure-treated 4×10 or 3×12?

Following the SFPA dry dressed table, a 4×10 is 3-1/2 by 9-1/4 inches and a 3×12 is 2-1/2 by 11-1/4 inches. The SFPA brochure also says most pressure-treated Southern Pine is delivered without being redried, so measure what you receive.

Is one-inch pressure-treated lumber OK for ground contact?

Usually not. AWPA says some treated lumber that is two inches or less in thickness may be treated only for above-ground uses, so it may not last long in ground contact. Check the end tag and confirm the treatment before you buy.

What does UC4A mean and when do I need it?

UC4A is the AWPA ground contact, general use category. AWPA says ground contact applications require UC4A or higher. Preserved Wood lists sawn fence, deck and guardrail posts, and structural joists and beams for decks and freshwater docks among its typical uses.

Can I use above-ground lumber for posts?

No, not if the post touches the ground. The SFPA brochure calls lumber treated for above-ground use only, but used where conditions simulate ground contact, the most common misapplication. Read the end tag on every piece.

Do you carry 3×10, 3×12 and 4×10 pressure-treated lumber?

Those sizes are not on our pressure-treated lumber page. Call (305) 884-0860 for current stock and tell us the size, length, quantity and needed-by date.

How do I treat cut ends?

The SFPA specification guidelines say jobsite cuts and borings should be field-treated with copper naphthenate at a minimum 2 percent metallic solution, in line with AWPA Standard M4. Follow the instructions on the product you use.