How to Calculate Materials for Building a Pressure-Treated Wood Deck
Building a pressure-treated wood deck is a rewarding project that requires careful planning. Before grabbing your tools, you need to accurately estimate the materials. Here’s a practical guide to help you calculate everything you’ll need, with simple examples adaptable to any dimensions.
1. Define the Deck Area
Start by measuring the length and width of the deck to get the total square footage.
Formula:
Area = Length × Width
Example: A deck measuring 12 feet by 16 feet gives a total area of 192 square feet.
2. Calculate Decking Boards
The boards used measure 1-1/4" × 6", but the actual usable width is about 5-1/2 inches, or 0.458 feet.
Formula:
Linear feet required = Total area ÷ 0.458
Example:
192 ft² ÷ 0.458 = approx. 419 linear feet of boards.
Then, choose your board length (8', 10', 12', 14', or 16') to convert linear feet into board count.
Example:
419 ÷ 12 = approx. 35 boards of 12 feet.
Always plan for 10% extra for cuts and unexpected needs.
3. Calculate Joists
Joists are typically spaced 16 inches apart (1.33 feet). You’ll need one joist at each interval, plus one at each end.
Formula:
Number of joists = Deck width ÷ 1.33 + 1
Example:
For a 16-foot-wide deck:
16 ÷ 1.33 + 1 = approx. 13 joists.
Each joist should span the depth of the deck (e.g., 12 feet for a 12-foot-deep deck).
4. Calculate Beams and Posts
Beams:
The number depends on the span between supports (usually 6 to 8 feet). For a standard deck, install 2 to 3 beams parallel to the house.
Posts or Concrete Blocks:
Each beam needs a support every 6 to 8 feet.
Example:
For a 16-foot-wide deck, plan for about 6 to 9 posts depending on the structure.
5. Hardware and Accessories
Don’t forget:
- About 30 to 40 wood screws per board.
- 1 joist hanger per joist.
- Beam anchors and other fasteners depending on your method.
- Extra boards for finishing edges or skirting.
6. Quick Calculation Summary
- Area = Length × Width
- Linear feet of boards = Area ÷ 0.458
- Number of boards = Linear feet ÷ chosen board length
- Joists = Width ÷ 1.33 + 1
- Beams = Based on span (usually 2 to 3)
- Posts = 1 every 6 to 8 feet under each beam
- Recommended surplus = 10% extra for each main material
Conclusion
By following these simple and flexible steps, you can adjust the calculations to any deck size. Always check your local building codes before starting construction.
