Decking Calculator: Linear Feet & Joist Layouts
Almost always, yes. Especially if it is attached to the house or over 30 inches high. Building without a permit can force you to tear it down later
Decking Calculator: Linear Feet & Joist Layouts
A deck is the ultimate backyard upgrade, but it is also a math trap. Wood is sold by the linear foot, but your deck is measured in square feet.
If you confuse the two, you will end up with 50% of the material you actually need. Furthermore, modern Composite decking requires tighter structural spacing than traditional Pine. This engineering-grade tool by ShockTrail solves the geometry for you.
The Material Estimator
Calculate your Deck Boards (Linear Feet), Joist Count, and Fasteners instantly.
Deck Material Estimator
Boards • Joists • FastenersPART 1: The Geometry of Decking (Linear vs. Square)
The most common mistake beginners make is ordering “100 square feet of wood.” Lumber yards sell boards in Linear Feet (LF). You need to convert the area.
The Magic Multiplier: 2.2
A standard “5/4 x 6” deck board is actually 5.5 inches wide. If you include a standard 1/8″ or 1/4″ gap for drainage, the effective coverage is roughly 5.75 inches.
To cover 1 Square Foot of deck, you need approximately 2.1 to 2.2 Linear Feet of standard decking.
Formula: Square Footage × 2.2 = Linear Feet Needed (Before Waste).
The Gap Factor
Wood expands when wet. Composite expands when hot. If you install boards tight against each other without a gap, your deck will buckle and heave within 6 months.
Standard Gap: 1/8″ to 1/4″. Using a “Camo” hidden fastener tool automatically sets a 3/16″ gap.
PART 2: Real-World Case Studies
Calculating waste depends entirely on your layout pattern. Let’s analyze three common builds.
Case Study 1: The Standard Pine Deck (12×12)
The Project: A simple 12ft x 12ft deck using Pressure Treated Pine. Boards run parallel to the house. Spacing: 16″ OC.
The Math:
- Area: 144 sq ft.
- Board Conv: 144 × 2.2 = 317 Linear Feet.
- Waste (10%): Simple cuts. Total = 350 LF.
- Board Selection: Since the deck is 12ft wide, you can buy 12-foot boards and have zero butt joints. You need approx 29-30 boards.
Joists: (12ft width × 12 / 16″ spacing) + 1 starter = 10 Joists.
Case Study 2: The Composite “Picture Frame” (High End)
The Project: A 16ft x 20ft Trex Composite deck with a “Picture Frame” border (a darker board running around the perimeter). Spacing: 12″ OC (Required for composite).
The Complexity:
- Blocking: You need extra framing underneath the perimeter to support the picture frame board.
- Joist Density: Composite is more flexible than wood. It feels “bouncy” if joists are 16″ apart. You MUST use 12″ OC spacing.
The Math:
- Perimeter (Picture Frame): (16+16+20+20) = 72 Linear Feet of “Border Color” boards.
- Internal Area: Approx 300 sq ft.
- Internal Boards: 300 × 2.2 = 660 LF of “Main Color”.
- Joists: (20ft × 12 / 12″ spacing) + 1 = 21 Joists (Double the lumber of a wood deck!).
Case Study 3: The Diagonal Layout (Max Waste)
The Project: A 10ft x 10ft deck with boards running at a 45-degree angle for style.
The Math:
- Area: 100 sq ft.
- Waste Factor: Every single board must be cut at an angle on both ends. The off-cuts are triangular and often useless. You need 20% to 25% Waste.
- Calculation: 100 × 2.2 × 1.25 = 275 Linear Feet.
- Joists: Diagonal boards span a longer distance between joists. You must reduce joist spacing to 12″ OC to maintain strength.
PART 3: Material Guide (Wood vs. Composite)
The biggest decision in decking is the material. It affects the price and the framing requirements.
| Material | Cost (per sq ft) | Lifespan | Maintenance |
|---|---|---|---|
| Pressure Treated Pine | $2 – $4 | 10-15 Years | High (Stain/Seal yearly) |
| Cedar / Redwood | $5 – $9 | 15-20 Years | Medium (Oil every 2 years) |
| Composite (Trex/TimberTech) | $8 – $15+ | 25-50 Years | Zero (Wash with soap) |
| PVC (Plastic) | $12 – $18 | Lifetime | Zero (Cooler on feet) |
PART 4: Fasteners (Screws vs. Hidden Clips)
A 500 sq ft deck requires over 1,750 screws. Choosing the wrong one can destroy your aesthetic.
- Face Screwing: The old way. You see the screw heads. Cheap and strong. Use ceramic coated (green/tan) or stainless steel screws.
- Hidden Fasteners (Camo System): Screws go into the side of the board at an angle. The surface is pristine. Highly recommended for wood.
- Clip Systems: Used for grooved composite boards. A plastic or metal clip sits between the boards and screws into the joist.
PART 5: Joist Spacing Rules
The “Skeleton” of your deck dictates the board layout. Check your local building codes (IRC).
- 16″ On Center: Standard for Pressure Treated 5/4 boards running perpendicular to joists.
- 12″ On Center: Required for Composite Decking (to prevent sagging/bouncing) or any Diagonal Layout.
- Span Tables: A 2×8 joist can typically span 12 feet. A 2×10 can span 14-16 feet. Consult the AWC Prescriptive Residential Wood Deck Construction Guide (DCA6).
PART 6: Railings & Stairs
Railings are often more expensive than the deck flooring itself. Code usually requires a railing if the deck is more than 30 inches off the ground.
- Post Spacing: Usually max 6 to 8 feet apart.
- Balusters: Must be spaced so a 4-inch sphere (baby’s head) cannot pass through.
Frequently Asked Questions (FAQ)
Should I use screws or nails for decking?
Never use nails for deck boards. They will pop up over time as the wood cycles through wet and dry seasons. Always use rated Deck Screws (typically #10 gauge).
How much gap should I leave between boards?
If the wood is “Green” (wet from the store), butt them tight; they will shrink to create a gap. If the wood is dry (kiln dried) or Composite, leave a 1/8″ to 1/4″ gap for drainage and expansion.
Do I need permit for a deck?
Almost always, yes. Especially if it is attached to the house or over 30 inches high. Building without a permit can force you to tear it down later.
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