Structural engineer pittsburgh | Structural Engineer 4/4

Snow loads on Roofs

Firas Abdelahad
Dec 16, 2020
2 min read

Updated: Sep 17, 2024

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 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”

https://www.fema.gov/media-library-data/7d8c55d1c4f815edf3d7e7d1c120383f/FEMA957_Snowload_508.pdf

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.

#structuralengineer #snowload #roofstructure #realestate #pittsburgh

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.

The information and statements in this document are for information purposes only and do not comprise the professional advice of the author or create a professional relationship between reader and author.

Please note that as code changes some information may not be current on this post.

Structural Deficiencies (I)

Firas Abdelahad
Dec 9, 2020
2 min read

Updated: Sep 17, 2024

This post is intended to show some of the deficiencies that I either experienced first hand or noticed on the social media when posted by others.

Improper splice & support:

In this picture below, unfortunately many elements were either not installed correctly or modified improperly; beam was spliced inches away from the support, also the post isn’t supported properly at its base. A post needs to be supported by a footer that is sized to its load and designed to meet code requirement.

Post/Temporary post supported on Slab:

In the pictures below, a perfect example of what would/could potentially happens when a post is supported on concrete slab without distributing the stress properly and/or when it isn't resting/supported by a footer that is designed for its load. I am certain that many of us have seen few of these posts (steel jack/telescopic posts) in their basement, a family or a friend basement at some point. They tend to be very tempting as quick solution for a floor sag but unfortunately many of these posts are not installed correctly also not supported correctly as you can see.

The high concentration in loads will cause the post base to punch through the Slab.

Improperly reinforced foundation block wall:

In the picture below you will see how the I beam improperly attached at its upper end. which will lead to a question isn’t properly attached at the base ? The I beam is leaning as the wall is still pushing inward. a reinforcement that the home owner paid to do and unfortunately will have to pay again to correct.

Consulting with the right qualified structural engineer can save you a lot of money and more importantly time.

I guess a garage door track was more important than the structural integrity of the steel beam and concrete floor:

In the picture below you will see what rust can do when left untreated, you will also see how an I beam was cut for a garage door track be installed.

the 2 pictures below show how an improperly supported beam can react and essentially fail under design load. Load bearing beams need to be supported properly at splices also properly for their size, load and span.

Written by Firas Abdelahad, P.E.