Matt
There are a few things going on here. First. ASCE 7-22 now uses a load factor on Snow of 1.0, where as earlier editions used a load factor of 1.6. The reason for this, as explained in ASCE 7-22 Commentary section C7.2 is that like wind and seismic loads specified by ASCE 7, snow loads are now risk-targeted, i.e., they are set at a level that provides the reliability targets indicated in Table 1.3-1. Snow loads used in older versions of the standard had a 50-year mean recurrence interval. The newer snow loads do not in general have the same recurrence interval and also are adjusted somewhat higher in areas where historic snow height data shows large variability from year to year, to account for the associated uncertainty. Again, please refer to the commentary for Chapter 7. You might also be interested in a new ASCE publication that provides the an explanation of the reliability and risk basis for all loads and load combinations in ASCE 7.
You are correct that the 100 psf is the locally enforced loading, and is a "legal" basis for design. I would strongly warn against using the 100 psf with a load factor of 1.0 as this would result in a severely under-designed structure. Your local engineering community should probably inform and lobby your local building department to correct this problem, as they are likely unaware.
The big issue for you to ponder is whether you do what ASCE 7-22 states or the local code. In the unlikely event that you designed to the local code and a roof were to collapse in a large storm, you could be found liable for negligence, in that you were aware that the national consensus for design (ASCE 7) stated substantially higher load. Your defense would be that you complied with the applicable code. It could go either way, but who wants to put themselves at that risk?
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Ronald Hamburger, SE
Consulting Principal
Simpson Gumpertz & Heger
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Original Message:
Sent: 06-12-2025 07:00 PM
From: Matthew Melcher
Subject: Snow Load
I am working on a project where there appears to be a large discrepancy between the county prescribed "Roof" Snow Load and ASCE 7-22. Specifically the county prescribes 100 psf and ASCE 7-22 indicates a Pg of almost 500 psf. The county value has been in place since the 70's. Is there a resource to query how this value for ASCE 7-22 was determined? According to the code, the AHJ has the final say, so a project submitted to the county would be 100 and a project under another AHJ could be 500?
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Matt
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