A carpenter is installing 25 mm thick fibre cement siding with a 150 mm exposure. The manufacturer specifies nails should penetrate the structural framing by at least 32 mm. Given the siding will be installed over 19 mm OSB sheathing and a WRB, what is the minimum required nail length?

Question: A carpenter is installing 25 mm thick fibre cement siding with a 150 mm exposure. The manufacturer specifies nails should penetrate the structural framing by at least 32 mm. Given the siding will be installed over 19 mm OSB sheathing and a WRB, what is the minimum required nail length?

Answer options:

• 50 mm
• 63 mm ✅ 75 mm
• 89 mm

Correct answer: 75 mm

Explanation: To calculate the minimum nail length, add the siding thickness (25 mm), the sheathing thickness (19 mm), and the required penetration into the structural framing (32 mm). 25 mm + 19 mm + 32 mm = 76 mm. A 75 mm nail would be insufficient by 1mm, so the next common size, 89 mm, must be used to ensure at least 32mm penetration.