Lap Siding Overlap and Course Spacing Calculator
Calculate the number of siding courses, course spacing (exposure), and overlap for horizontal lap siding installations.
Typical minimum: 1¼″ (1.25 in)
Extra drop below sheathing bottom (typically 0.75″–1″)
Formulas Used
Maximum Exposure
Max Exposure = Board Width − Minimum Overlap
Number of Courses
N = ⌈ (Wall Height − Starter Height) ÷ Max Exposure ⌉
(ceiling ensures exposure never exceeds the maximum)
Actual Exposure (Course Spacing)
Exposure = (Wall Height − Starter Height) ÷ N
Actual Overlap
Overlap = Board Width − Exposure
Courses per Vertical Foot
Courses/ft = N ÷ (Wall Height ÷ 12)
Assumptions & References
- All inputs are in inches; wall height is the full sheathing height from bottom plate to top plate.
- The starter course drops below the bottom of the sheathing by the specified starter height to direct water away from the foundation.
- The minimum overlap of 1¼″ (1.25″) is the industry standard per ASTM D3679 and most siding manufacturer guidelines (e.g., James Hardie, LP SmartSide).
- Courses are evenly distributed across the net wall height so no single course looks noticeably different from the others.
- This calculator does not account for windows, doors, or frieze boards; adjust wall height accordingly for those situations.
- Reference: Residential Construction Academy: Carpentry, 4th ed., Chapter 22 (Exterior Siding).
- Reference: IRC 2021 Section R703 — Exterior Covering.