Rules for using built-up members in place of solid timber
A built-up member is two or more component members used to make one larger member. However, there are a few rules that must be followed when using built-up members.
The built-up members must:
- be the same grade of timber
- be in one continuous length
- match or exceed the size of the solid member required in accordance with NZS 3604:2011.
Built-up members may consist of up to six component members and must be nailed together according to the nailing requirements described in NZS 3604:2011.
Built-up members of up to three components
The nailing requirements for built-up members of up to three components are that:
- the spacing of the nails is no more than six times the thickness of the thinnest component
- all nails penetrate at least three-quarters of the last component member
- nails are driven from alternate sides of the builtup member.
For example, a built-up member consisting of two 90 x 35 mm and one 90 x 20 mm components must have:
- nailing spacings at 120 mm centres maximum (6 x 20 mm = 120 mm where 20 mm is the thinnest component member)
- nails that are at least 82 mm long (35 mm + 20 mm + (0.75 x 35 mm) = 82 mm) (Figure 1).
Figure 1: Built-up horizontal member less than 140 mm depth (for lintels and beams)
When members are 140 mm or more in width, at least two rows of nails are required but the other criteria remain the same (Figure 2).
Figure 2: Built-up horizontal member 140 mm or deeper (for lintels and beams)
Built-up members of over three components
Where the built-up member consists of more than three component members, the first three component members should be assembled in the same way as a built-up member with up to three components. Additional component members are then fixed using nails that are twice as long and spaced at six times the thickness of each component member added (Figure 3).
Figure 3: Built-up member made of more than three vertical members (usually for trimming studs)
by Tom Edhouse
BRANZ Technical Advisor