A carpenter is laying out floor joists for a 4.0 m wide room. The joists are 38 mm x 184 mm SPF No. 2, and the design calls for a joist spacing of 400 mm on centre. The first joist is intended to be flush with the outside edge of the end rim joist. What is the distance from the outside edge of the end rim joist to the centreline of the third joist?

Question: A carpenter is laying out floor joists for a 4.0 m wide room. The joists are 38 mm x 184 mm SPF No. 2, and the design calls for a joist spacing of 400 mm on centre. The first joist is intended to be flush with the outside edge of the end rim joist. What is the distance from the outside edge of the end rim joist to the centreline of the third joist?

Answer options:

• 800 mm
• 838 mm ✅ 1200 mm
• 1238 mm

Correct answer: 1200 mm

Explanation: With a joist spacing of 400 mm on centre, the centreline of the third joist will be 2 x 400 mm from the centreline of the first joist. Since the first joist's centreline is 19 mm (half of 38 mm) from the outside edge of the rim joist, the third joist's centreline will be 800 mm from the centreline of the first joist, which is 800 mm from the reference point.