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Roof load and structural integrity

Question: Should I shovel the snow off my roof?

Post Date:02/26/2019 12:45 PM

Should I shovel the snow off my roof?

For those of you that are wondering “When should I shovel the snow off my roof?” Here is a quick guideline to help you determine if you may need to be concerned.

Basic calculation – (for a quick analysis only) This calculation uses a 25% moisture density which may be conservative for our current snow fall. As a general rule, saturated snow weighs about 20 pounds per cubic foot. The moisture content of snow can range from about 1% to about 33% so snow can weigh from about 1 pound per cubic foot to over 21 pounds per cubic foot.
S=Inches of snow on the roof
1.25= Weight of 1 sq ft of snow for each 1” of depth
P= Pounds per square foot

If my roof has 20” of snow, what would that equate to?
(S)(1.25)=P (20”)(1.25 lbs/sq ft) = 25 lbs/ sq ft
In this example the roof would be ok.
Note:  Any ice build-up on the roof needs to be added to this formula. Use 5.2 lbs for every inch in thickness.
You can always check your roof snow load yourself by weighing a one-foot (12”) X one-foot (12”) square of snow cut from your roof to determine if your snow load is getting close to your design roof snow load of 20-25 lbs. per square foot depending on the age of the structure. This would include any ice build-up on the roof.

1. Structures constructed in the City of Bend with building permits are designed for 20-25 lbs. /sq. ft. minimum roof snow load. Older roofs may be of concern and may not meet this minimum roof snow load.
2. Until recently, manufactured homes were designed for a roof snow load of 30 lbs/sq ft.  Look at the certification label (a metal label affixed to the manufactured home) to verify your roof snow load.
3. Flat roofs are more of a concern (Less than 3/12 pitch). Flatter roof pitches tend to hold more snow moisture.
4. If we receive excessive rainfall, the weight of the roof snow can increase rapidly.
5. If your structure is in question, it is best to consult a licensed engineer.
Please be aware of the potential dangers of shoveling or raking snow from a roof. Besides the potential damage to the roofing materials and structure, there are such factors as a person sliding off the roof, falling off a ladder, overexerting themselves, or injury from snow sliding on top of them.

In summary, the City of Bend does not make recommendations on when to remove snow from roofs. It is up to the individual property owner to consider the benefits and dangers of snow removal and decide their own course of actions. Remember that, as a general rule, saturated snow weighs about 20 pounds per cubic foot and then consider the depth and relative moisture content of your snow and the capacity of your roof structure in making your decision to remove snow or not.

If you are concerned about the integrity of a roof structure due to snow loads, please consult a licensed architect or engineer to assess the integrity.  If you feel there is an immediate danger please contact the non-emergency line, 541-693-6911.  If you have determined there is an integrity issue and wish to have the problem temporarily remedied with structural shoring, the following information will be necessary to provide to the Building Safety Division for a permit.  Permits for temporary remedy will be issued at no charge.  Permits for permanent remedy will follow the division’s standard processes, fee schedule and timeframe.  The construction and installation of the temporary shoring shall be completed by a contractor licensed in the State of Oregon.
A narrative from an Engineer or Architect licensed in the State of Oregon will be required and shall provide the following detail/statements:
• If the building can be occupied once temporary shoring is in place or if temporary shoring is only to allow for preservation of the structure and/or construction for repair (repair permits will need to be secured separately)
• If for occupancy a statement that all fire and life safety systems are intact
• If for occupancy what portion(s) of the building are occupiable (pertains to multi-tenant buildings as well as single tenant buildings)
• A timeframe for how long they are indicating the temporary shoring is to be in effect, per code cannot be longer than 6 months.
• Narrative shall bear the engineer/architect stamp/signature
• The construction and installation of the temporary shoring shall be completed by a licensed contractor

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