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Snow loads on Roofs

Updated: Dec 24, 2023

With winter storm warning into effect for our region, it is important to bring to your attention the potential snow loads impact on roof structures.

snow on roof
Snow on Roof

Snow load on roof
Snow load on roof

Snow loads on roofs can vary by state, location (depends on topographic factors), elevations, ground snow load, insulation, roofing materials, pitch and many other directly or indirectly related factors like drifting, wind, etc.

Pictures below from FEMA document “snow load Safety Guide”

Explanation of terminology
Figure 1a

More explanation
Figure 1b

It is important for homeowners and/or property managers to develop a routine maintenance plan to inspect their roof structure after a heavy snow fall especially for older structures or If your structure was renovated or you know that it had structural elements upgraded, replaced and or changed with no permits or stamped drawings to document that the modification was reviewed by a licensed engineer.

Potential issue:

We have seen a fair share of modification to roof structure to convert a conventional roof structure (rafters and joists) to a cathedral ceiling style with no proper reinforcement to reduce the thrust on side walls. Heavy snow fall will put these kinds of modification to test when roof will experience heavier load, hence some cracks may start developing as a result of the improper modification to the roof structure.

Local Snow Requirement for City of Pittsburgh:

City of Pittsburgh modified/amended code requirement for ground snow load to be 30 pound per square foot.

When calculating flat roof snow load following ASCE7-10 formulas, tables to select exposure factor Ce, thermal factor Ct, importance factor Is:

Pf=0.7*Ce*Ct*Is*Pg= 0.7*1*1*1*30=21 psf (pound per square foot)

Snow load on sloped roof, Ps, will be a factor of the flat roof snow: Ps=Cs*Pf

Roof slope factor, Cs, values can be determined from Sections 7.4.1-7.4.4 (ASCE7-10) for warm, cold, curved and multiple roofs.

It is important to bring to your attention Partial Loading, unbalanced roof snow loads, drifts on lower roofs, roof projections and parapets, rain on snow surcharge as other effects as explained in ASCE7-10

Please consult with your structural engineer for more information and clarification. This document is not intended to summarize the section of the code/standard (IBC2015/ASCE7-10) nor it is sufficient to use to complete your own snow load calculations without referring to the referenced documents and having it reviewed a licensed engineer.

Pittsburgh Design & Engineering Services LLC can assist you with your snow load calculations or any other structural engineering needs at your request.

Written by Firas Abdelahad, P.E.

Firas Abdelahad has been a practicing structural engineer since 2005, collaborating with a diverse range of professionals, including consultants, architects, investors, homeowners, contractors, and subcontractors. Together, they tackle the various challenges that can arise during the design and construction phases of projects.

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