Blizzard of 2016 Dropped 26.4 Inches on Harrisburg: Snow Load Lessons

What a two-foot event actually weighs

It depends entirely on water content. Two feet of light, dry snow might load a roof at something in the region of six to ten pounds per square foot. Two feet of dense, wet snow can be several times that.

The dangerous variable is what happens afterward. Snow that partially melts and refreezes gains density. Snow that absorbs subsequent rain gains a great deal of weight — for reference, water is about 5.2 pounds per square foot per inch of depth, and a snowpack that has taken on rain behaves accordingly.

So the event to worry about is not necessarily the biggest snowfall. It is the substantial snowfall followed by a warm rain and then a hard freeze.

Which roofs struggled

• Flat and low-slope roofs — they hold everything; a pitched roof sheds a meaningful portion
• Roofs with drift-prone geometry — valleys, areas behind dormers, and lower roofs sitting beneath a taller wall, where wind deposits several times the flat-ground depth
• Multi-layer roofs — every accumulated layer is permanent dead load using up capacity the snow needs
• Lightly framed additions — porches, sunrooms, carports and enclosed patios, frequently built to lighter standards than the main house
• Older agricultural buildings — large spans with aged framing
• Roofs with existing deflection — a structure already sagging has already told you its capacity is reduced

The drift problem specifically

Uniform snow depth across a roof is the easy case. Drifting is where the real loads occur.

Wind moves snow and deposits it wherever the airflow slows: in valleys, against dormers and chimneys, behind parapets, and particularly on a lower roof where it meets a taller wall. A rowhome rear section beneath a three-story party wall can accumulate several times what the open portion of the roof carries.

That is also where the load is least anticipated, because nobody sizes framing for a drift they cannot predict.

Warning signs during a heavy event

If you see several of these together, get people out and call a professional. This is not a wait-and-see situation.

1. 1

   Doors and windows binding

   Frames deflecting under load will bind doors that worked fine yesterday, particularly upstairs.

2. 2

   New cracks in drywall or plaster

   Especially above doorways and at ceiling-to-wall junctions.

3. 3

   Visible ceiling sag

   A ceiling line that is no longer straight. Take this seriously.

4. 4

   Unfamiliar creaking or popping

   Framing under unusual load is audible. A house that suddenly becomes noisy under snow deserves attention.

5. 5

   Sprinkler heads or fixtures moving

   In commercial buildings, deflection frequently shows first at ceiling-mounted equipment.

Safe removal

For a pitched roof: a roof rake used from the ground, clearing the lower three to four feet. That reduces eave load and removes the fuel for ice dams. Do not get on the roof — a snow-covered pitch is one of the most dangerous surfaces a homeowner encounters.

For a flat or low-slope roof genuinely carrying too much: professional removal. It is done with plastic tools, leaving a protective layer of snow rather than scraping to the membrane, and working symmetrically so load is not shifted onto one side of the structure.

Never use metal shovels on any roof, never use a torch or heat gun anywhere, and never salt a roof.

What to do before the next one

Know your roof. If you have a flat roof, a multi-layer roof, an addition of uncertain framing, or any visible deflection, get an assessment before winter rather than during a storm.

Clear gutters and drains in late autumn so meltwater has somewhere to go. On flat roofs, make sure overflow scuppers are clear — they exist precisely for the event where the primary drain cannot cope.

And address any known deficiency in autumn. Everything is more expensive, more dangerous and more urgent in February. Our roof repair service books faster in October than it does in January, and it costs less.