BUILT-UP MEMBERS

Sometimes you need to make up a specified timber size from narrower timber – read up on the rules below

This article focuses on using built-up members in place of solid timber. This is becoming more and more common with fabricated products, but it may pay to know that it can be done on site too.

This article provides some basic guidance on the subject, and you’ll be able to find more detailed information in NZS 3604:2011.

Jack studs, and bottom and top plates must always be solid timber members but Timber-framed buildings NZS 3604:2011 does allow other solid timber members to be replaced by built-up members. 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 built-up member.

For example, a built-up member consisting of two 90 x 35mm and one 90 x 20mm components must have:

• Nailing spacings at 120mm centres maximum (6 x 20mm = 120mm where 20mm is the thinnest component member).
• Nails that are at least 82mm long (35mm + 20mm + (0.75 x 35mm) = 82mm) (Figure 1).

When members are 140mm or more in width, at least two rows of nails are required but the other criteria remain the same (Figure 2).

 

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).

 

This article first appeared in Codewords – Issue 87