How to measure and plan
Decide the board direction first — boards usually run the long dimension or parallel to the house. Measure that as length, the other dimension as width. The calculator lays boards across the width: width in inches ÷ (board width + ¼" gap) = board count, rounded up.
Buy boards in the longest lengths your supplier stocks that minimize joints — 16-footers for a 16-foot deck mean zero butt joints. Every butt joint is a future trouble spot, so fewer, longer boards are always better. Add 10% for angled cuts, notching around posts, and the inevitable miscut.
Framing basics
- Joists: 2×8 minimum for most residential spans; 2×10 for longer spans. 16" on center is standard for 5/4 decking perpendicular to joists.
- Diagonal decking needs 12" OC joists — the angled span is effectively longer.
- Beams and posts: sized by span tables in your local code, not by rule of thumb. A 6×6 post on a proper concrete footing is the standard.
- Ledger connection is where decks fail. Through-bolts or structural screws per code — never nails alone — plus flashing to keep water out of the house.
- Footings must sit below the frost line. Sonotubes with a post base keep wood off concrete and extend post life.
Wood vs. composite
Pressure-treated pine is the budget workhorse: roughly half the material cost of composite, 10–15 year life, and it needs cleaning and staining every 2–3 years. Cedar and redwood cost more and look better but need the same maintenance.
Composite and PVC decking cost 2–3× more upfront but need only occasional cleaning and last 25–30 years. The real cost comparison includes your time: a wood deck's lifetime maintenance often exceeds the composite premium. Composite gets hotter underfoot in direct sun and requires tighter joist spacing — factor both into the plan.
What affects deck cost
DIY pressure-treated: $15–$25/sq ft in materials. Pro-built wood: $30–$60/sq ft. Composite pro-built: $60–$100+/sq ft. A 16×12 deck (192 sq ft) therefore ranges from ~$3,500 DIY to $12,000–$19,000 professionally built in composite.
Railings add $30–$60 per linear foot installed. Stairs run $500–$2,000+ depending on width and rise. Permits ($100–$500), helical piles instead of dug footings, and multi-level designs each move the number. Get quotes that itemize framing, decking, railing, and stairs separately.
Common deck mistakes to avoid
- Undersized or shallow footings. Footings must sit below your local frost line — heaved footings are the #1 cause of wobbly decks. Check your local code depth (often 36–48" in cold climates).
- Skipping ledger flashing. A ledger board bolted to the house without proper flashing rots the house rim joist — the most expensive deck failure there is. Flashing is a $30 part that prevents a $3,000 repair.
- Wrong joist spacing for composite. Most composite boards require 16" OC, and diagonal patterns need 12". Read the manufacturer's span table before you frame.
- Butt joints lined up. Stagger board end joints across joists like brickwork — aligned joints look amateur and create weak lines.
Deck FAQs
How do I calculate decking boards?
Deck width in inches ÷ (board width + ¼" gap), rounded up. A 12-ft-wide deck with 5.5" boards needs 26 boards. Add 10% for waste.
How far apart should deck joists be?
16" on center for standard 5/4 decking. Use 12" OC for diagonal patterns or per composite manufacturer specs.
Wood or composite decking?
Wood costs about half upfront but needs staining every 2–3 years and lasts 10–15 years. Composite costs 2–3× more, needs only cleaning, and lasts 25–30 years.
Do I need a permit for a deck?
Usually yes for attached decks or anything over 30" above grade. Ground-level freestanding platforms are often exempt — check local code first.
How much gap between deck boards?
¼ inch for wood (it shrinks as it dries); follow manufacturer specs for composite, typically 3/16"–¼".