According to Sections 4.8 of ASCE-07-22 [1] and 1.3.4.2 of MBSM [2], the minimum uniformly distributed roof live loads, are permitted to be reduced. Table 1.3.4 of MBSM [2] summarize this sections as follows: 


As can be seen, these values depend on the tributary loaded area of the member. For example, for a roof with a slope of 2:12, the roof live load for a member with a tributary loaded area of 100 ft² cannot be reduced and shall be taken as 20 psf. However, for a member with a tributary loaded area of 500 ft², the roof live load may be reduced and taken as 14 psf, as presented in an example in the MBSM [2]. A detailed and comprehensive explanation of this subject is provided in the section titled Roof Live Load Reduction.



Since the tributary loaded area of frames is often greater than 600 ft², the roof live load is typically reduced to 12 psf for frame design. In contrast, because the tributary loaded area of purlins is often less than 200 ft², the roof live load typically shall not be reduced and should be taken as 20 psf for purlin design.


However, structural designers often apply the reduced roof live load used for frames to the design of both frames and purlins. This can significantly underestimate purlin demand, leading to substantially lighter purlins that are neither safe nor code-compliant.


References:

[1] American Society of Civil Engineers. "Minimum design loads and associated criteria for buildings and other structures." American Society of Civil Engineers, 2022.

[2] Metal Building Systems Manual, 2024 Edition.