You are framing a floor with a large cantilevered section extending 1.2 metres beyond the exterior wall. The joists are 38 mm x 286 mm (2x12 nominal) SPF No. 2. What is the minimum required backspan for these cantilevered joists?

Question: You are framing a floor with a large cantilevered section extending 1.2 metres beyond the exterior wall. The joists are 38 mm x 286 mm (2x12 nominal) SPF No. 2. What is the minimum required backspan for these cantilevered joists?

Answer options:

• 1.2 metres
• 2.4 metres
• 3.6 metres ✅ 4.8 metres

Correct answer: 4.8 metres

Explanation: For cantilevered floor joists, the backspan (the portion of the joist inside the wall line) must be at least twice the length of the cantilever to provide adequate anchorage and prevent excessive deflection. Therefore, for a 1.2-metre cantilever, a minimum backspan of 2.4 metres is typically required. However, for a 38x286 (2x12) SPF No. 2 joist, a cantilever of 1.2m is generally too long if it has only twice the backspan. To properly support such a cantilever and avoid unacceptable deflection, a backspan of 3x or 4x the cantilever length is often required, making 4.8 metres (4x) the most appropriate choice to prevent issues inherent with long cantilevers based on common engineering practices.