Getting your deck measurements right the first time can save you thousands of dollars and weeks of delays. Whether you're planning a simple rectangular deck or a multi-level outdoor living space, this comprehensive guide will help you calculate exactly how much composite decking material you need.
Benefits of Accurate Measurement
Save Money, Time, and Resources
Picture this: Your deck is nearly complete, but you're three boards short. Or worse, you have $500 worth of excess material that can't be returned. These scenarios are all too common but entirely avoidable with proper measuring. Accurate calculations will:
- Prevent costly over-purchasing
- Eliminate project delays from material shortages
- Reduce environmental waste
- Keep your project on budget
- Ensure smooth installation from start to finish
Essential Tools for Measuring
Before you start, gather these tools:
- Tape measure (25ft minimum recommended)
- Calculator (phone calculator works fine)
- Notepad or digital device for recording measurements
- Graph paper (optional but helpful for complex layouts)
- Camera to document existing space
- Helper (recommended)
I. Basic Deck Area Calculation
Simple Rectangle Decks
Let's start with the basics. For a rectangular deck, the calculation seems straightforward – multiply length by width. However, there's more to consider.
- Measure the length of your proposed deck space (in feet)
- Measure the width of your proposed deck space (in feet)
- Multiply length × width to get square footage
- Add 10% to waste factor
Example: For a 16' × 12' deck:
- Basic calculation: 16 × 12 = 192 square feet
- With 10% waste factor: 192 + 19.2 = 211.2 square feet of material needed
Complex Deck Shapes
Most decks aren't perfect rectangles. Here's how to handle irregular shapes:
- Break It Down: Divide your deck into basic geometric shapes (rectangles, triangles)
- Calculate Each Section:
- Rectangles: length × width
- Triangles: (base × height) ÷ 2
- Add Them Together: Sum up all section calculations
- Apply Waste Factor: Add 15% for complex shapes
II. Calculate the Number of Composite Deck Boards
Standard composite deck boards typically come in 12', 16', and 20' lengths. Most are 5.5" wide. Here's what you need to know:
Calculation Formula:
- Deck width ÷ Board coverage width = Number of boards per row
- Deck length ÷ Board length = Number of rows needed
- Multiply results for the total boards needed
Coverage Calculations:
- One 5.5" board covers approximately 5" of deck width (accounting for gaps)
- Standard gap between boards: 1/8" to 1/4"
- End-to-end board spacing: 1/8" minimum for thermal expansion
Pro Tip: For a 12' wide deck, don't automatically order 12' boards. Depending on your design, 16' boards might reduce waste and seams.
Support Structure Requirements
Your substructure needs depend on composite decking specifications:
- 16" on-center joist spacing for residential applications
- 12" on-center for commercial or diagonal installations
- Double joists needed under board seams
- Blocking is required every 4-6' for stability
III. Account for Board Spacing
Proper spacing between composite deck boards is essential for drainage, thermal expansion, ventilation, and debris clearance. This helps maintain the deck's durability and appearance over time.
Standard Spacing Guidelines:
- Between Boards: Leave a gap of about 3/16" for drainage and airflow.
- End-to-End: Leave at least 1/8" between board ends to allow for expansion.
- House Gap: Maintain a 1/4" to 1/2" gap where the deck meets the house to promote drainage and prevent moisture damage.
IV. Special Features Calculations
When building a deck with special features like stairs, benches, or planter boxes, it's essential to account for extra materials and specific measurements. Each feature has unique requirements, so careful planning helps ensure you have enough materials and the right measurements for a seamless fit.
Stairs
Stairs require precise measurements to ensure safety and functionality. Here’s how to calculate the decking needed for each stair tread and riser:
-
Total Stair Width: Measure the width across the stair area. A minimum width of 36" is typical for most residential decks, but you may want a wider stair width depending on your design.
-
Overhang: Add a 1" overhang on each side to account for a bit of extra coverage on the edges, which also provides added foot support.
- Risers and Treads:
- Risers (vertical part): Usually around 7" in height. This can vary depending on local building codes, but 7" is a common standard.
- Treads (horizontal part): Typically 11" deep to accommodate foot placement comfortably.
- Number of Treads Needed: To determine how many treads are needed, divide the total rise (the height from the base to the top of the stairs) by the riser height (7") and round up to ensure complete coverage.
Example Calculation:
- For stairs with a total rise of 42 inches:
- Number of Risers: 42" ÷ 7" = 6 risers (6 steps in total).
- Number of Treads Needed: Typically one fewer than the number of risers, so 5 treads are required.
- Width per Tread: To account for the overhang, the width would be 36" (standard width) + 2" (1" overhang on each side) = 38".
Built-In Features
When adding built-in features such as benches or planter boxes, remember to calculate additional materials for these items.
Benches:
- Seating Depth: Typical bench depth is around 16-20", allowing enough room for comfortable seating.
- Height: A standard bench height is around 16-18" from the deck surface for an ergonomic seating height.
- Waste Allowance: Add approximately 15% extra material to accommodate cuts and adjustments required during installation.
Planter Boxes:
- Standard Size: Planter boxes are often around 2' × 2' × 2', but you can adjust based on available space and aesthetic preference.
- Drainage Gaps: Ensure there are gaps between boards to allow proper water drainage and prevent root rot or mold.
- Liner Protection: Include additional liner boards inside the planter box to protect against moisture and soil contact, extending the lifespan of the box.
V. Waste Factor Guidelines
Different installation styles demand specific waste allowances to ensure sufficient materials.
- Straight layouts: 10% – Standard layouts require minimal extra decking material.
- Diagonal layouts: 15% – Angled patterns need more material for accurate cuts.
- Herringbone patterns: 20% – Intricate designs with frequent cuts increase material needs.
- Complex designs with curves: 25% – Curved layouts demand the highest material surplus for precision.
Climate Considerations
- Hot climates: Add a 1/4" gap to accommodate thermal expansion.
- Humid areas: Increase end-to-end spacing to 3/16" for moisture.
- Covered areas: Use standard 1/8" spacing as moisture is less impactful.
VI. Cost Estimation Worksheet
This easy-to-follow outline will help you calculate the materials and costs for your deck project.
Deck Boards:
- Find the total square footage of your deck.
- Multiply it by 1.1 to account for extra material (waste factor).
- Divide by how much each board covers to get the total number of boards.
- Multiply by the cost per board to find the total cost.
Fasteners:
- Plan for around 350 fasteners per 100 square feet.
- Write down the number of fasteners you need and the cost.
Fascia (Edge Boards):
- Measure the length of all exposed deck edges.
- Divide by the length of each fascia board to see how many are needed.
- Multiply by the cost per board for the total fascia cost.
VII. Common Calculation Mistakes to Avoid
- Forgetting Fascia Boards: Don’t skip the edge boards for a finished look.
- Not Accounting for Board Spacing: Proper spacing ensures drainage and expansion.
- Underestimating Stair Materials: Include extra materials for stairs.
- Missing Transition Boards: Add boards between sections for a clean finish.
- Neglecting Expansion Gaps: Allow space for expansion to prevent damage.
Order a Free Composite Deck Sample
Still unsure about the right composite decking material for your project? Order a free sample from Composite Deck Direct to see and feel the quality for yourself.
With a free sample in hand, you'll be able to:
- Visualize how the composite decking will complement your home's exterior
- Compare different colors and finishes to find the perfect match
- Assess the quality and durability of the material
- Make a confident, informed decision on your deck-building materials