Lumber Buying Guide for Deck Building

Building a deck starts at the lumber yard, and the choices you make there — on species, grade, treatment level, and size — will determine how long your deck lasts, how safe it is underfoot, and how much your total project costs. This guide walks through every lumber decision in a deck build from the ground up: pressure-treated framing requirements, joist sizing and span tables, decking board coverage calculations, the composite vs. wood debate, hardware budgeting, and what to reject at the lumber yard before it ever reaches your truck.

1. Board Feet vs. Linear Feet: Getting the Units Right

One of the most common sources of confusion when buying lumber is the difference between board feet and linear feet — two completely different units that are often used interchangeably by mistake, leading to either under-purchasing or over-purchasing materials.

A board foot is a unit of volume equal to a piece of lumber that is 1 inch thick, 12 inches wide, and 12 inches long — essentially one square foot of lumber at one inch thick. The formula is: (thickness in inches × width in inches × length in feet) ÷ 12 = board feet. For example, a 2×8×16 ft board: (2 × 8 × 16) ÷ 12 = 21.3 board feet. Hardwood lumber is traditionally sold by the board foot, which is why it's important to understand for price comparison purposes when choosing between species.

A linear foot (also called lineal foot) simply measures the length of a board regardless of its dimensions. Most dimensional softwood framing lumber (2×6, 2×8, 2×10) and decking boards (5/4×6) are sold either by the piece at a set length or by the linear foot. When suppliers list "5/4×6 decking at $2.25/lf," that's the price per foot of board length, regardless of how wide or thick it is.

Understanding both units matters for accurate price comparison: if one supplier quotes board feet and another quotes linear feet for the same species and dimension, you need to convert to compare. For framing lumber bought by the piece, simply multiply pieces × length to get total linear feet, then divide by 12 and multiply by (actual thickness × actual width) to convert to board feet if needed. In practice, for deck projects, you'll primarily be working in linear feet for both decking and framing estimates.

2. Framing Lumber: Pressure Treatment Is Non-Negotiable

Every single piece of structural lumber in an outdoor deck frame must be pressure-treated (PT). This is not optional — it is required by the International Residential Code (IRC) and by every building department in the country for good reason. Untreated lumber in contact with or near the ground, or exposed to outdoor moisture cycling, will begin to rot within 2–5 years and can cause catastrophic structural failure.

Pressure treatment works by forcing preservative chemicals deep into the wood fibers under high pressure. The two most common preservative systems in use today are ACQ (Alkaline Copper Quaternary) and CA (Copper Azole), which replaced the older CCA (Chromated Copper Arsenate) system that was phased out of residential use in 2004. The treatment level required depends on the application:

• .40 treatment (UC4A/UC4B): Required for ground contact — posts embedded in soil or concrete, any framing within 6 inches of grade. This is the higher chemical loading and provides long-term protection against rot and insects in direct soil contact.
• .25 treatment (UC3B): Above-ground applications where the wood is exposed to weather but not in contact with the ground — joists, beams, ledgers, decking boards.

Posts should be 4×4 minimum for decks under 8 feet in height with standard residential loads. Use 6×6 posts for taller decks, wider beam spans, or where local code requires it — 6×6 is actually the more common specification for most residential decks today because it provides better resistance to buckling and a more solid feel. Always use post bases with standoff design to keep the end grain off concrete, which is where moisture wicks in and rot begins.

The ledger board — the board that bolts to your house and carries the deck load — must be the same depth as your beams, and it must be lag-bolted or through-bolted to the house's rim joist at minimum every 16 inches in a staggered pattern. Nailing a ledger is not structurally acceptable and is a frequent cause of deck collapses. The ledger must also be flashed properly to prevent water from getting behind it into the house structure.

Beams are typically constructed as a 3-ply assembly of dimensional lumber — three 2×10s or three 2×12s nailed or bolted together. The size required depends on the span (distance between support posts) and the tributary load (the amount of deck area the beam carries). Always check your local code and the AWC Span Calculator for your specific situation.

3. Joist Sizing and Span Tables

Joists are the horizontal framing members that run between the ledger and the beam (or between two beams), and they directly support the decking boards. Getting joist sizing right is critical — undersized joists will feel springy underfoot and may fail to meet code; oversized joists are wasteful. The maximum allowable span for each joist size depends on spacing and lumber species/grade.

The following spans are for Southern Yellow Pine #2 (the most common PT framing species) at 40 psf live load, the standard residential deck design load, from the IRC span tables:

Tightening the spacing from 16" OC to 12" OC buys you roughly 2 additional feet of span at each joist size — useful if you're trying to avoid adding a mid-span beam. However, closer spacing also means more joists and more cost, so there's a trade-off to evaluate against the cost of an additional beam and post.

Cantilever limits: If your deck design calls for a cantilever (the joist extends past the last beam), IRC limits the cantilever to a maximum of one-quarter of the allowable joist span. A 2×10 that spans 15 ft between supports can cantilever a maximum of 3 ft 9 in beyond the beam. Exceeding this limit can cause the deck to bounce or fail under load.

4. Decking Board Coverage Calculations

Calculating how many decking boards you need is straightforward once you understand the actual net coverage width of each board — which is different from the nominal width printed on the tag at the lumber yard.

5/4×6 decking (the most common choice): The nominal width is 6 inches, but the actual milled face width is approximately 5.25 inches. With the standard 1/8-inch spacing gap between boards (required for drainage and wood movement), each board covers 5.375 inches of deck width. Converting: 5.375 inches ÷ 12 = 0.448 ft per board.

Therefore: Lineal feet of 5/4×6 needed = deck square footage ÷ 0.448

Worked example — 16×20 ft deck (320 sq ft):

• Base lineal feet: 320 ÷ 0.448 = 714 lineal feet
• Add 15% waste for end cuts, bad boards, and trim: 714 × 1.15 = 821 lineal feet
• Number of 16-ft boards needed: 821 ÷ 16 = 51.3 → order 52 boards

For 2×6 decking: Actual face width is approximately 5.5 inches. With 1/8-inch gap = 5.625 inches = 0.469 ft per board. Formula: sq ft ÷ 0.469. The same 320 sq ft deck would need 320 ÷ 0.469 = 682 lf base + 15% = 784 lf (49 boards at 16 ft). The 2×6 slightly reduces your board count because each board covers slightly more width.

If you're running boards diagonally (a popular design choice), increase your waste factor to 20% — diagonal cuts generate more end waste. If you're doing picture-frame borders, calculate that lumber separately as a perimeter run.

5. Composite vs. Pressure-Treated Wood: True Cost Comparison

The composite vs. natural wood debate comes down to upfront cost versus total lifecycle cost — and the math is more nuanced than it first appears. Here's a complete comparison to help you make the right decision for your project and budget.

For our 16×20 deck example requiring ~821 lineal feet of decking:

• PT pine at $1.50–$3.50/lf: $1,232–$2,874 material cost for decking boards only
• Composite at $3.50–$8.00/lf: $2,874–$6,568 material cost for decking boards only

That $1,600–$3,700 upfront premium for composite sounds significant — but consider the maintenance cost of PT pine: cleaning and resealing every 2–3 years runs $1–$3 per square foot in materials and labor. For a 320 sq ft deck, that's $320–$960 every 2–3 years. Over 20 years, you'll spend $3,200–$9,600 in maintenance on PT pine alone. Composite has near-zero maintenance cost over the same period — just an occasional rinse with a garden hose or mild soap.

Additionally, PT pine will often need a full replacement after 15–20 years (total deck tear-off, disposal, and new decking installation labor), while composite carries manufacturer warranties of 25–30 years with no expected replacement in that window. When you account for these factors, composite frequently wins on total lifecycle cost for decks intended to be permanent features of a home.

The verdict: choose PT pine for budget builds, temporary structures, rental properties, or projects where you plan to sell the home within 5 years. Choose composite for permanent primary residences, pools and high-moisture areas, or any application where maintenance-free performance is worth the premium.

6. Hardware Requirements and Budget

Hardware is the most underestimated cost category in deck budgets. First-time builders routinely under-budget for connectors, fasteners, and structural hardware — and then face sticker shock at checkout. Plan for hardware to add $1.50–$3.00 per square foot of deck area to your total material cost.

Joist hangers: You need one joist hanger per joist end, at both the ledger and the beam. For a 16×20 deck with 2×10 joists at 16" OC, that's approximately 13 joists × 2 ends = 26 joist hangers. The Simpson Strong-Tie LUS series is the standard in the industry — LUS26 for 2×6, LUS28 for 2×8, LUS210 for 2×10. Engineered load ratings mean you can document your structural connections for permit inspection.

Post bases: One post base per post. Use a standoff post base (Simpson ABA or ABU series) that holds the post end above the concrete pier or footing surface. This keeps end grain — the most vulnerable part of any piece of wood — from sitting in water and rotting. Even pressure-treated wood will eventually fail if end grain sits in standing water.

Ledger bolts: Use ½-inch diameter lag screws or through-bolts (structural hex bolts with washers) at maximum 16" OC, staggered in two rows to avoid splitting the ledger. Follow IRC Table R507.9.1.3(1) for the required bolt pattern based on ledger depth and joist span — the table accounts for both shear and withdrawal loads on the fasteners.

Decking fasteners: For PT wood decking, use stainless steel or hot-dipped galvanized structural screws, 3-inch length. Standard stainless deck screws (Cortex, Camo, or Fasten Master brands) resist the corrosion caused by the copper preservative in PT wood — standard zinc-coated screws will corrode within 2–5 years. For composite decking, most manufacturers require their proprietary hidden fastener system (Trex Hideaway, TimberTech SecureStart, etc.) to maintain warranty. Budget approximately 1 fastener clip per 32 square inches of deck surface for hidden fasteners.

Summary hardware budget estimate for a 320 sq ft deck:

• Joist hangers and post bases: $80–$150
• Structural screws (ledger bolts, beam bolts): $60–$100
• Decking fasteners (3,000–4,000 screws or hidden clips): $120–$350
• Post caps, misc connectors: $50–$100
• Total hardware estimate: $310–$700 (roughly $1.00–$2.20/sq ft)

7. Lumber Defects to Reject at the Yard

One of the most valuable skills a deck builder can develop is the ability to reject bad lumber at the yard — because once it's delivered to your job site, any defects become your problem to work around. Most lumber yards allow customers to hand-select boards, and you should always exercise this right for structural members in particular.

Here's what to reject:

• Bow (end-to-end curve in the flat plane): Hold the board flat and sight down the length from one end. Reject any board bowed more than ¼ inch over 8 feet. Bow in decking boards can be forced flat during installation, but bow in joists or beams compromises structural performance.
• Crook (side-to-side warp along the edge): Stand the board on edge and sight down it. Reject any crook over ⅜ inch per 8 feet on structural members. A crooked joist will cause high spots in your decking surface.
• Excessive knots on structural members: Small, tight knots are acceptable, but avoid any knot that exceeds one-third of the board's width on joists or beams. Large knots represent a discontinuity in the wood fibers that dramatically reduces bending strength — exactly the property you need in a joist.
• Wane (missing wood or a rounded, bark-side edge): Wane on the edges of a structural board reduces its effective cross-sectional area and its rated load capacity. Reject any structural board with wane covering more than ¼ of any face.
• Twist: Set the board on a flat surface and check if all four corners touch. Any board that rocks on a flat surface has twist — reject it for framing use. Twisted joists create an unlevel deck surface that cannot be corrected during installation without shimming every course.
• Checks (longitudinal surface cracks from drying): These are normal in PT lumber and are not a structural concern. Checks are surface-level cracks that form as the wood dries after treatment. They are expected and acceptable — they will not affect the strength of the member and typically close up as the wood stabilizes in place.

For decking boards, some bow and minor crook is acceptable since you can force them flat and straight against the joists during installation with clamps or a pry bar. But severely cupped or twisted decking boards will create a wavy, unprofessional-looking surface — invest the extra time at the yard to select the straightest, most uniform boards available.