How Much Snow Can a Flat Commercial Roof Hold? A Lake-Effect Snow Load Guide for Northeast Ohio Building Owners

When a lake-effect band parks over Cleveland, the calls from building owners and property managers sound alike: "There's a lot of snow on our roof. Do we need to get it off?"

It's a fair worry. Cleveland Hopkins averages 63.8 inches of snow a season under the National Weather Service's 1991–2020 normals, with most of it falling from December through February. The eastern suburbs and Lake County often see heavier lake-effect totals than the airport. A flat or low-slope roof doesn't shed any of it. Pitched roofs let snow slide off, but a flat roof over a warehouse, strip plaza, church, or office keeps every inch until it melts, drifts, or gets shoveled.

The honest answer is that it depends on what kind of snow is up there, where it has piled up, and what your roof was designed to carry. Here's how to think about each of those before winter shows up.

Two feet of fresh powder and two feet of old, wet, packed snow are completely different loads. The Insurance Institute for Business & Home Safety (IBHS) gives a simple rule of thumb in its commercial winter weather bulletin. Each of these adds about 5 pounds per square foot to a roof:

• 10 to 12 inches of new snow
• 3 to 5 inches of packed or old snow
• 1 inch of ice

That's why a modest-looking snowpack can be the dangerous one. Snow that has sat for a couple of weeks, been rained on, and partly refrozen can weigh several times more than the fluffy stuff that came down on top of it. IBHS notes that two feet of old snow plus two feet of new snow could reach about 60 pounds per square foot, enough to stress even a conservatively designed roof.

The National Weather Service uses the water inside the snow to get at the same idea: every inch of melted water equivalent works out to about 5.2 pounds per square foot. Rain falling on an existing snowpack doesn't run off. It soaks in and adds weight fast.

Every commercial roof has a design snow load, but it isn't one number for the whole region. It comes from the ground snow load for the location, adjusted for the building's exposure, how much heat escapes through the roof, the building's use, and the roof's shape. That calculation follows the Ohio Building Code and the ASCE 7 standard it references.

For a reference point, the City of Cleveland's published local design criteria list a ground snow load of 30 pounds per square foot. That is a starting value for design, not a promise about what your roof can hold. Older buildings may have been designed under earlier codes. Additions, new rooftop units, solar arrays, and extra layers of roofing all change the picture. If you don't know your building's design load, a structural engineer can tell you, and it's worth finding out before a big storm rather than during one.

Roofs rarely fail from an even blanket of snow. FEMA's Snow Load Safety Guide (FEMA P-957) points to uneven and concentrated loads as the usual culprits:

• Drifts at parapets and walls. Wind scours snow off the open field of the roof and dumps it against parapets, penthouse walls, and the edges of higher roofs. A drift can be several times deeper than the snow a few feet away.
• Lower roofs next to taller ones. Snow blows and slides off the upper roof onto the lower one. Additions and loading-dock canopies are classic trouble spots.
• Around rooftop units. HVAC units, exhaust fans, and other equipment act like snow fences and build drifts on the downwind side.
• Rain on snow. A January rain on top of a deep snowpack can add weight quickly.
• Blocked drains. When drains, scuppers, or gutters are frozen or clogged, meltwater collects in low spots and refreezes. On a low-slope roof, that ponding becomes a concentrated load.

FEMA lists these as common signs that a roof is overstressed during a snow event:

• Sagging ceiling tiles or boards, or ceiling boards falling out of the grid
• Sagging sprinkler lines, or sprinkler heads dropping below the ceiling
• Popping, cracking, or creaking noises
• Sagging roof members, including metal decking
• Bowing truss or joist members
• Doors or windows that suddenly won't open or close
• Cracks in walls or masonry
• Severe roof leaks
• Water collecting in places on a low-slope roof where it shouldn't

If you see any of these, FEMA's guidance is clear: get people out of the building and have the local building authority or a structural engineer inspect it. Don't send anyone up on a roof that is showing signs of distress.

When the load is getting close to what the roof can handle, or more snow is in the forecast, removal makes sense. It should be done by trained people with fall protection, not by whoever happens to be on shift. FEMA's guide and insurance loss-control guidance agree on a few basics that protect both the crew and the roof:

• Use plastic shovels, not metal, and no sharp tools or mechanical equipment on the membrane. A TPO, EPDM, or PVC membrane punctured during snow removal becomes a leak in the spring.
• Leave at least 2 inches of snow on the roof surface. Scraping down to the membrane is how roofs get cut.
• Clear drifts first, at parapets, roof steps, and around equipment, then work on the rest.
• Remove snow in balanced strips so you don't create a new uneven load, and don't pile it up somewhere else on the roof.
• Keep the ground below clear of people and vehicles, and keep drains and scuppers open so meltwater can leave the roof.

After the snow is gone, have the roof inspected for punctures, open seams, and damaged flashing. Small damage from shovels is much cheaper to fix in March than after it has leaked through April rains.

The best time to deal with snow load is in the fall, while the roof is clear and you can see it. Before winter, we recommend that commercial building owners:

• Clear roof drains, scuppers, gutters, and downspouts, and confirm they flow
• Look for low spots and ponding areas that hold water after rain
• Check flashing and seams at parapets and around rooftop units, where drifts sit longest
• Know where your building's high-risk drift areas are, so you know where to look after a storm
• Find out your building's design snow load, or get a structural engineer to tell you
• Line up a qualified contractor for snow removal and post-storm inspection before you need one

Our Commercial Roof Maintenance Plan covers fall and spring inspections like these, so drains, seams, and flashing get checked before winter and again after it.

We inspect and repair flat and low-slope commercial roofs of every common type, including TPO, EPDM, PVC, modified bitumen, built-up roofs, and silicone coatings. We serve building owners throughout Cuyahoga, Lorain, Medina, and Lake counties, including Cleveland, Brook Park, Bedford Heights, Solon, Elyria, Mentor, and Painesville. If you're comparing membranes for an upcoming replacement, our guide to TPO vs. EPDM for Cleveland commercial buildings is a good place to start.

Pring Roofing has served Northeast Ohio since 1936. We're an Owens Corning Preferred Contractor, an Elevate (formerly Firestone) Licensed Applicator, and a Carlisle authorized applicator. We hold a BBB A+ rating and are fully insured.

Want your flat roof checked before the snow flies? We offer free quotes and inspections. Call 216-941-6298 (7am–10pm, 7 days a week), email contact@pringroofing.com, or request your free inspection online.

• National Weather Service Cleveland, Cleveland 1991–2020 climate normals
• FEMA, Snow Load Safety Guide (FEMA P-957) and Snow Load Safety Guidance flyer
• Insurance Institute for Business & Home Safety, Commercial Risk Engineering Bulletin: Winter Weather
• National Weather Service, Snow Loads
• City of Cleveland Building & Housing, Climatic and Geographic Design Criteria