Flat Use Factor Explained:
A wood structure with what appears to be a flatwise roof
Normal wood will be used about it's strong axis. For example, if a 2"x4" member is being used, the force will be applied on the 2" side (the strong axis is considered the axis that has the stronger section modulus?, and vice versa with the weak axis). When the load is applied to the longer face (in the above example this would be the 4" side) aka bent about it's minor or weak axis, then the flat use factor is used. Similar procedures are used in Steel Design when a member is bent about it's weak axis or strong axis.
In general, the flat use factor (Cfu) will increase the strength of a member, therefore it is conservative to take it as Unity (1.0). The flat use factor can be found in Tables 4A, 4B, 4C, and 4F of the NDS Supplement [1]. If the wood is used normally, and not being bent about it's minor axis then Cfu can be ignored, or assumed to be unity (1.0).
Note: Weak axis bending for wood can also be known as the lumber being used flatwise (load applied to the wide face)
References:
- American Forest and Paper Association, "National Design Specification for Wood Construction", 2005