James Hardie Fiber Cement Siding Specifications
Fiber cement lap planks are manufactured in 12-foot lengths with strict manufacturer exposure requirements.
| Plank Nominal Width | Lap Overlap | Exposed Reveal | Sq Ft per 12-ft Plank | Planks per Square (100 sq ft) |
|---|---|---|---|---|
| 5.25 inches | 1.25" | 4.0 inches | 4.0 sq ft | 25.0 planks |
| 6.25 inches | 1.25" | 5.0 inches | 5.0 sq ft | 20.0 planks |
| 7.25 inches | 1.25" | 6.0 inches | 6.0 sq ft | 16.67 planks |
| 8.25 inches (Standard) | 1.25" | 7.0 inches | 7.0 sq ft | 14.29 planks |
| 9.25 inches | 1.25" | 8.0 inches | 8.0 sq ft | 12.5 planks |
HardiePlank Quantity Formula
Planks Needed = ⌈(Net Siding Area sq ft × (1 + Waste%)) / (12 ft × (Exposure Width in / 12))⌉Variables & Units:
- 12 ft
- Standard factory manufactured plank length
- Exposure Width
- Plank width minus 1.25-inch required manufacturer lap
- Blind Nails
- 1 nail per stud (16" o.c. = 9 nails/plank; 24" o.c. = 6 nails/plank)
Worked Example: 1,500 sq ft Wall in Hardie 8.25-inch Planks
Scenario: Ordering 8.25-inch HardiePlank (7-inch reveal) for a 1,500 sq ft net wall area with 10% waste buffer and 16-inch on-center stud framing.
1,500 sq ft × 1.10 = 1,650 sq ft (16.5 Siding Squares)
12 ft plank × (7 inches ÷ 12 inches/ft) = 12 × 0.5833
1,650 sq ft ÷ 7.0 sq ft per plank = 235.71 planks (round up)
236 planks × 9 nails/plank = 2,124 nails (+5% buffer = 2,230 nails) + 240 joint flashing tabs
Takeaway: Order 236 pieces of 12-ft HardiePlank 8.25-inch siding, 240 joint flashing tabs, and 1 box of corrosion-resistant siding nails.
Frequently Asked Questions
How many 8.25" Hardie planks are in a square?
HardiePlank 8.25-inch lap siding has a 7.0-inch exposure. Each standard 12-foot plank covers 7.0 square feet of wall area (12 ft × 0.5833 ft). To cover one siding square (100 sq ft), you need exactly 14.29 planks (typically rounded to 15 planks to allow for overlap and end cuts).
What is the required overlap for James Hardie lap siding?
James Hardie installation specifications require a minimum 1-1/4 inch (1.25") overlap for all horizontal HardiePlank lap siding products. For example, an 8.25-inch wide plank minus the 1.25-inch lap leaves a 7.0-inch exposed face reveal.
Do you need joint flashing behind Hardie siding butt joints?
Yes. James Hardie technical instructions require joint flashing (metal flashing tabs or factory joint tape) installed behind every butt joint to direct water over the course below, rather than relying solely on caulking.
What nails should be used for fiber cement siding?
Use hot-dipped galvanized or stainless steel siding nails (minimum 0.089" to 0.091" shank diameter and 1-1/4" to 2" length) driven flush into wall studs spaced 16 inches or 24 inches on center. Avoid over-driving nails into the plank.
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