Why Do Spacing Requirements Significantly Increase for Fasteners Installed into Douglas Fir?

Douglas fir has specific spacing and edge distance geometry requirements that are larger than timber species with similar densities. This is reflected in both the CCMC and ICC-ES approvals of the ASSY self-tapping fasteners.

This is because in Douglas Fir the fibers are more prone to splitting compared to other softwood lumber species such as SPF or Southern Yellow Pine (SYP). Therefore, larger spacing requirements must be used when using self-tapping fasteners, even in glulam. As an exception, smaller spacing requirements may be used in CLT as the cross-lamination reinforces the panel against splitting.

A solution to avoid large spacing requirements would be to pre-drill the wood member prior to the installation of the fastener. The designer must keep in mind that pre-drilling is more labor intensive.

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