Spacing supports drainage and movement, but the required value depends on material, moisture, temperature, board profile, fastening system, layout, and ventilation.
01 / Formula
How this is calculated
Installed board module = actual board width + required gap02 / Assumptions
What the estimate assumes
- ✓The product and fastening system are known.
- ✓Actual jobsite temperature and board condition are considered.
- ✓Drainage and ventilation details are planned separately.
03 / Worked example
See the math in context
A 5.5-in actual board with a verified 0.125-in gap creates a 5.625-in planning module, but the verified gap may change with the product or conditions.
04 / Guidance
Wood changes with moisture
Measure actual boards and follow species, treatment, moisture, and supplier guidance. A wet board and a dry board may not be installed to the same starting gap.
05 / Guidance
Composite instructions are product-specific
Follow the current installation guide for side-to-side and end-to-end spacing, butt joints, fixed points, temperatures, hidden fasteners, and ventilation.
Questions
Frequently asked
Are the calculator defaults building-code requirements?+
No. They are editable planning inputs. Verify current local code, approved plans, span tables, product evaluation reports, and manufacturer instructions before construction.
Does the estimate include the complete deck structure?+
Only if every component is explicitly listed. Footings, posts, beams, ledgers, flashing, lateral connections, guards, stairs, hardware, fascia, permits, delivery, and waste often require separate takeoffs.
How should I measure a non-rectangular deck?+
Divide the surface into rectangles and other measurable shapes, calculate each separately, then account for board direction, seams, breaker boards, borders, stairs, and reusable offcuts.
Sources