Building a Backyard Deck: Materials, Costs, and Mistakes

Let’s talk about the deck you’re about to build

I’ve framed out more decks than I can count — from little 10x10 platforms off a back door to full wraparounds with built-in benches and pergolas. And the pattern is always the same. Somebody gets excited, buys a pile of lumber on a Saturday morning, and by Sunday afternoon they’re back at the yard buying more because they guessed wrong on quantities.

A deck is one of those projects where the planning stage pays for itself three times over. Get your numbers right before you cut a single board, and you’ll avoid the two things that kill every DIY build: wasted material and wasted trips.

This guide covers the material side of the job — how much decking you actually need, how to think about lumber in board feet, and how to figure stain coverage so you’re not left with a half-finished surface when the can runs dry.

Figure out how much decking you need

Start with the obvious: the size of your deck. Length times width gives you the total square footage of the deck surface. But that raw number isn’t how much lumber you need to buy. Here’s why.

Deck boards aren’t installed edge to edge. You leave a gap between them — typically 3/16 to 1/4 inch — for drainage and seasonal expansion. That gap eats into your coverage. On top of that, you’ll have cuts at the ends of rows, boards that split when you drive the last screw near an edge, and the odd piece with a warp or twist that’s not worth fighting with.

My rule of thumb is a 10% waste factor for a simple rectangular deck and 15% if you’re doing a diagonal or herringbone pattern. Mitre cuts generate more offcuts, and angled layouts mean more of those offcuts are too short to use anywhere else.

The board dimensions matter too. A standard 5/4x6 pressure-treated deck board (which actually measures about 1 inch by 5.5 inches, because lumber sizing is its own special brand of confusion) covers a different area per linear foot than a 2x6 or a composite plank.

Use the Deck Board Material Calculator to dial in your quantities. Enter your deck dimensions and board size, and it’ll tell you how many boards to order:

Write that number down and add a couple of extras. Lumber yards will take back full-length returns, but they won’t take back the board you cut two feet off of and decided you didn’t need.

Understanding board feet — how lumber is actually priced

If you’ve only ever bought pre-cut studs and sheets of plywood, the term “board foot” might be new to you. Here’s the short version: a board foot is a unit of volume equal to a piece of wood that’s 1 inch thick, 12 inches wide, and 12 inches long. It’s 144 cubic inches of lumber.

Why does this matter for your deck? Because once you move beyond basic deck boards and start buying posts, beams, joists, and rim boards, many lumber yards — especially hardwood dealers — price stock by the board foot rather than by the piece. If you’re sourcing cedar or ipe for a premium deck surface, you’ll almost certainly be quoted a per-board-foot price.

The formula is straightforward: thickness (inches) times width (inches) times length (feet), divided by 12. But doing that calculation for every piece in your cut list gets tedious fast, especially when you’re dealing with nominal versus actual dimensions.

The Board Foot Calculator handles the arithmetic for you. Plug in your dimensions and quantity, and it gives you the total board footage:

Quick example: say you need eight 4x4 cedar posts at 8 feet each. That’s 4 x 4 x 8 / 12 = 10.67 board feet per post, or about 85 board feet total. At $8 per board foot for clear cedar, you’re looking at $680 just for posts. Numbers like that are exactly why you want to calculate before you commit.

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Don’t skip the stain — and don’t guess the coverage

Here’s the mistake I see constantly: someone builds a beautiful deck, stands back to admire it, and then “plans to stain it next weekend.” Next weekend becomes next month. Next month becomes next spring. By then the wood has greyed out, mildew has started creeping in, and the grain has raised enough that you need to sand before you can coat.

Stain or seal your deck as soon as the wood is dry enough to accept finish. For pressure-treated lumber, that’s usually four to eight weeks after installation — do a water drop test by sprinkling a few drops on the surface. If they bead up, the wood is still too wet. If they soak in within a couple of minutes, you’re good to go.

Now, how much stain do you actually need? Coverage rates vary by product and application method. A typical semi-transparent deck stain covers about 150 to 300 square feet per gallon on smooth wood, depending on how porous the surface is and whether you’re brushing, rolling, or spraying. Rough-sawn or heavily textured wood drinks up finish and will be on the lower end of that range.

Don’t forget to account for railings, stair treads, and the fascia board along the deck edge — they add more surface area than people realise. A 12x16 deck with railings on three sides can easily have 30 to 40% more stainable surface than the deck floor alone.

Use the Deck Stain Calculator to get your gallon count right:

Buy it all at once from the same batch. Stain colour can vary slightly between production runs, and if you run short and buy a second can six months later, the difference will show — especially on horizontal surfaces in direct sunlight.

Common deck-building mistakes (and how to avoid them)

After two decades in the trade, these are the errors I see most often on DIY deck jobs:

• Skipping the ledger board flashing. The ledger is the board that bolts to your house. Without proper flashing — a Z-shaped piece of metal that tucks under your siding and over the top of the ledger — water wicks into the wall sheathing and causes rot. This is the number one cause of deck failures I’ve been called to fix.
• Undersizing the joists. A 2x6 joist is fine for a short span, but once you get past about 8 feet, you need 2x8s or 2x10s depending on your joist spacing. Check your local building code or use the span tables from the AWC (American Wood Council). Don’t guess on structural members.
• Spacing footings too far apart. Posts carry the load down to your concrete footings. If the footings are too far apart, your beam sags and the deck feels bouncy. I space mine no more than 6 feet apart for a standard residential deck.
• Using the wrong fasteners. Galvanised nails and screws are the minimum for pressure-treated wood. Stainless steel is better if you’re using cedar or a hardwood, because the tannins in those species react with standard galvanising and create black stains.

The bottom line

A deck is one of the best return-on-investment projects you can do to a house. It extends your living space, looks great, and a well-built one lasts 20 to 30 years with basic maintenance. But “well-built” starts with “well-planned.” Run your numbers through the calculators above, order the right quantities, and you’ll spend your weekends building instead of running back to the lumber yard.

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