Roof Snow Load: How Much Snow Can a Roof Hold?

The number your roof was built to

Ward County's ground snow load is 35 pounds per square foot. Roofs are engineered against that figure with a safety factor, so a properly built modern roof here handles a normal winter without drama. The failures happen in three situations: old or under-built structures, drifting that concentrates load in one area, and rain falling on an existing snow pack.

Working out what is on your roof

Depth alone tells you nothing, because snow density varies enormously. Rough figures:

Snow type	Weight per inch, per sq ft	Inches to reach 35 psf
Fresh light powder	~0.4 lb	~85 in
Settled snow	~0.8 lb	~44 in
Packed, wind-blown	~1.5 lb	~23 in
Wet or partly thawed	~2.5 lb	~14 in
Ice	~5 lb	~7 in

Swipe the table sideways to see every column.

Which is the real point: fourteen inches of wet snow weighs more than six feet of powder. A February thaw followed by a freeze can double the load on a roof without a single new flake falling.

A better estimate than guessing

Push a ruler down into the snow on a flat surface, then melt a known-area sample. Simpler version: a 1 inch depth of water equals about 5.2 psf. So if your snow pack would melt down to 4 inches of water, you are carrying roughly 21 psf.

Drifting is what actually breaks roofs

Average load across the whole roof is rarely the problem. Wind moves snow into specific places and the load in that spot can be two to three times the average:

• Against a parapet or a taller wall on a low-slope roof
• In valleys, where two planes funnel snow together
• On the lee side of a ridge, away from the prevailing wind
• Below an upper roof that sheds onto a lower one, which is the classic porch and addition failure
• Behind rooftop units on commercial buildings

This is why a roof can fail in one corner while the rest is fine.

Warning signs, in order of urgency

Get out and call someone now

• Cracking, popping or creaking noises from the structure
• A visible sag or dip in the ceiling or roof line that was not there before
• Doors or windows that suddenly stick or will not close, meaning the frame is being loaded
• New cracks in drywall, particularly above door frames
• Sprinkler heads or light fixtures dropping below the ceiling line

Address soon

• Heavy drifting against a wall or in a valley
• Large ice dams at the eaves
• Interior doors on an upper floor rubbing their frames

Removing snow without wrecking the roof

• Use a roof rake from the ground. Clear the lower three or four feet, which also prevents ice dams. This is the whole job for most homes.
• Leave two inches on. Scraping to bare shingle takes granules with it.
• Work in horizontal strips from the edge upward, pulling snow straight down.
• Never get on a snow-covered roof. You cannot see what you are standing on and you will not hear the structure until it goes.
• Do not use a shovel or an axe on the roof surface. Cold shingles are brittle.
• Watch for power lines before extending an aluminum rake.

Metal roofs and snow

Metal sheds snow rather than holding it, which is a genuine structural advantage here and one of the better arguments for steel in this climate. The trade is that it comes off in slabs, so snow guards above doorways, walkways, the driveway and the gas meter are part of the job rather than an upgrade. See metal roof vs shingles.

If you are worried about a specific building

Commercial low-slope roofs, older farm buildings and additions built onto a house are the three we get called about most. A structural engineer is the right call when there is a visible sag. For everything else, an inspection before winter costs nothing and tells you whether drifting is building where it should not.

Questions we get asked