Question & Answers

1. When do I need to provide blocking between floor joists?

2. What is the minimum size of noggings in wall frames?

3. If I use straps to tie top and bottom plates to studs for tie-down, how long do the straps need to be?

4. The AS1684 Code covers many different wind classifications. How do I find out which one applies to the house I am to build?

5. How do you connect the top of internal bracing walls to wall and roof framing to transfer the lateral loads (racking forces) to the floor? (this is a downloadable pdf file) 50 kb

Question 1. When do I need to provide blocking between floor joist?

Answer: Where the joist depth is equal to or exceeds four times the breadth, the joists should be restrained at their supports. This restraint can be provided by fixing a continuous trimming joist to the ends of the joists above bearers or wall plates or by solid blocking or herringbone strutting between end pairs of joists and between intermediate pairs at not more than 1800 centres. Refer Figure 4.5 on page 45.

For deep unseasoned joists, where span exceeds 3.0 m and there is no ceiling, additional herringbone strutting or blocking shall be provided between all joists, in evenly spaced rows not exceeding 1800 centres measured along the joists.

Question 2. What is the minimum size of noggings in wall frames?

Answer: Size is governed by the depth of the common studs, and noggings shall be not less than the stud depth less 25 mm by not less than 25 mm thick. Refer Figure 6.6 on page 61. Nogging does not need to be stress graded.

Question 3. If I use straps to tie top and bottom plates to studs for tie-down, how long do the straps need to be?

Answer: In most cases, the straps will need to extend not less than 100 mm along each edge of the stud. This length may need to be increased to prevent splitting of the stud depending on the number of nails required to fix each end of the strap.

For jamb studs, the minimum length along each edge shall be 250 mm.

Refer Tables 9.19(a) and 9.2 a) and b) on pages 188 and 189.

Question 4. The new Code covers many different wind classifications. How do I find out which one applies to the house I am to build?

Answer: You can use AS 4055 or AS 1170.2 to determine the wind classification for any particular building site. Refer Clause 1.6.2 on page 8. However, advice may be available from local councils, building inspectors or local engineers. In the non-cyclonic area, houses in built-up areas would generally be N1 or N2 wind classification, but in more exposed situations, N3 or even N4 may be appropriate.