How Much Snow Can My Roof Support?

Posted by in Roof, Snow Loads, Winter Concerns, Wood | March 02, 2015

After a succession of heavy snowfalls, we are often asked the question, “How much snow can a roof support?” or more specifically, “How much snow can MY roof support?” and “Should I shovel the snow off my roof?”

Most who ask are anticipating an answer measured in inches or feet of depth.  Unfortunately, the density of snow is not consistent.  That is, a foot of snow in one storm may be much heavier (more dense) than a foot of snow in another storm.  We have all noticed this phenomenon when we shovel: a foot of snow in March is often much heavier than a foot of snow in January.  The difference in weight is due to the different densities.  Density is proportional to the amount of moisture (water) in the snow or ice.

A one foot depth of freshly fallen snow can weigh as little as 3 pounds per square foot (psf).  Studies have shown that a one foot depth of snow that has compacted by thawing and re-freezing, the weight of overlying snow accumulation, or wetting with rainfall can weigh up to 30 psf.  A foot of ice weighs approximately 60 psf.

Most rafter or wood truss framed dwellings in Northeast Ohio have been designed or constructed for snow loads that range from 20 to 35 pounds per square foot (PSF).  These load capacities assume that the roof framing has been constructed properly, has been maintained, and hasn’t been damaged by previous overload, or weakened by decay, aging, modifications, etc.

Because of the wide range of possible snow densities, the range of design/construction load capacities, and the unknown condition of the framing we answer these questions, “It depends.”

But that is no help to you, so here are some considerations for Northeast Ohio.  The weight of snow and ice on a roof may BECOME a concern when there are significant accumulations of snow on the roof and any of the following are observed:

  1. There is more than approximately 18 inches of snow accumulation on the roof.
  2. There is new visible sagging of the rafters, the trusses, or the ceiling.
  3. There is new cracking in any of the ceilings, the headers over openings, or the top of the walls.
  4. There have been creaking, cracking, popping, snapping, or breaking sounds emanating from the roof.
  5. There is a forecast of significant rainfall (more than one inch) when there is still a substantial accumulation of snow on the roof (approximately greater than one foot).

These observations don’t necessarily mean there IS a structural issue, but these conditions are often evidences or precursors to a structural issue.

If you notice any of these and you are handy and physically able, it may be beneficial to reduce the weight of ice and snow on the roof.  One of the easiest methods to reduce the weight is the removal of the snow using a roof rake.  A roof rake has a long handle that enables one to pull the snow off the roof while standing on the ground. See our article Chalk Up Another Victory Over Ice Damming for more about this.

All the snow doesn’t need to be removed, just a good portion of it.  Also, be careful not to remove snow from one slope of a gable or hip, and not removing it from the opposite side.  This creates an unbalanced condition that may stress the roof framing more than the full relatively uniform load.  Many contractors or handymen can perform this service for a fee.  (Please be careful with the roof rake around electric power lines.  Don’t get electrocuted!)

If the weight of ice and snow cannot be reduced, the roof framing may require temporary shoring.  Unless you are experienced in construction, this will require the services of a general contractor.  If you think that the situation is an emergency, contact your local building department or fire department for additional help in assessing the situation.  If you need professional help, contract the services of a local Professional Engineer practicing in the structural engineering discipline.

Lastly, don’t forget about garage roofs. Garage framing is often of a lower capacity and less substantial than the framing over the dwelling because older building codes were less stringent with regard to garage construction.  Also, since garages are usually unheated there is less chance of snow and ice melt which results in the draining of roof runoff and the reduction of the weight.

Bottom line: if your intuition is causing you to be concerned, reduce the weight on the roof by having portions of the snow safely removed or get help to evaluate the situation as noted previously.  Your intuition is telling you something is wrong, and it very well may be right.

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