A roof in Waukegan is built to hold a lot of snow, but not an unlimited amount. The number that matters is the design snow load, measured in pounds per square foot (psf), and for most homes in Illinois it is at least 20 psf. That is the minimum flat-roof snow load in ASCE 7, the engineering standard behind the building code, and it is the number the roof snow load calculator uses as its floor. Twenty pounds per square foot sounds abstract until you turn it into snow: it is about 4 feet of fresh, light snow, or about 1 foot of wet, packed snow, or about 4.2 inches of solid ice.
The 20 Psf Floor: Where the Number Comes From
The 20 psf figure is not a guess. ASCE 7 sets a minimum roof snow load of 20 psf (for an ordinary home, importance factor 1.0) on low-slope roofs, and the flat-roof formula, pf = 0.7 times Ce times Ct times Is times pg, gives 17.5 psf for a typical Chicago-area home with a ground snow load of 25 psf, so the 20 psf minimum governs. On a 1,500 square foot roof, 20 psf is 30,000 pounds, or 15 tons, of snow. That is the weight of about ten cars sitting on the roof. A roof built to code handles that load every winter without complaint. The trouble starts when the load goes past the design number, which happens with drift, with rain on snow, and on roofs that were built before the current code or have been weakened by rot, leaks, or age.
Fresh Snow vs Wet Snow vs Ice
| Snow type | Weight per foot of depth | Depth that reaches 20 psf | What it looks like |
|---|---|---|---|
| Fresh, light snow | About 5 psf per foot | 4.0 feet | Dry powder that blows off a shovel |
| Wet, packed snow | About 20 psf per foot | 1.0 foot | Heavy slush that sticks to the shovel |
| Ice | About 57 psf per foot | 4.2 inches | A solid sheet after a freeze and thaw |
That table is the reason roofers ask how the snow feels, not just how deep it is. A Chicago-area winter usually delivers a mix: light lake-effect powder early in a storm, then heavier wet snow as the temperature rises, then a freeze that turns the whole layer into a crust, then rain that soaks into it and adds weight fast. A roof with 2 feet of fresh snow on it is carrying about 10 psf, half its design load. The same roof with 2 feet of wet snow is carrying about 40 psf, twice its design load. The depth did not change, but the weight doubled. If the snow on your roof is wet enough to make a snowball easily, treat every foot as 20 psf, and if the total is anywhere near the design load, clear it or call someone who can.
What Makes One Roof Weaker Than Another
Age is the first factor. A roof framed in the 1960s or earlier may have been built to a lower snow load than the current code, and sixty years of small leaks can rot the deck and the rafters from the inside, cutting the real strength far below the paper number. Pitch is the second factor: a flat or low-slope roof holds the full load and gets no help from gravity, while a steep roof sheds snow and is designed for less, about 15.9 psf on an 8/12 pitch against 20 psf on a flat roof of the same size. Drift is the third factor, and it is the one homeowners miss: wind blows snow off the open roof and piles it against a parapet, a chimney, or the wall of a taller section, and the drifted load there can be double the balanced load. A roof that is fine in the middle can be overloaded at the drift line.
How to Check Your Own Roof
Start from the ground. Look at the roofline from the street: a straight ridge and straight eaves are good, and a dip or a sag in the middle is a warning. Then look at the snow itself: is it deeper on one side than the other, piled against the chimney or the taller wall, or deeper over the garage than over the house? Those are drift lines, and they are where the load is highest. Then go inside and look at the ceilings under the roof: new cracks in the drywall, a ceiling that bows down in the middle of a room, doors that suddenly stick in their frames, and popping or creaking sounds during a storm are all signs the structure is moving under load. If you see two or more of those signs together, do not wait for the thaw: clear the snow or call a roofer, and keep people out of the rooms underneath until it is checked. Run your own numbers through the roof snow load calculator to see what your roof size and pitch are designed for.
When to Call a Professional
Call a roofer when the snow is deeper than about 2 feet and wet, when it has been rained on, when you can see a sag in the roofline, or when the ceiling inside shows any of the warning signs above. A professional crew clears the roof evenly with plastic shovels that do not damage the shingles. The cost of a clearing is a few hundred dollars for a typical single-family home in Waukegan, and the final quote comes after an inspection of the roof size, pitch, and access. That is a small price against a collapsed roof, and a collapsed roof is a small price against an injury. If the snow load is the question, the answer is to measure it, not to guess at it, and the guide on how to calculate roof snow load shows the math step by step.
Related reading: roof snow load calculator, plus our guides on how to calculate roof snow load and signs your roof is overloaded with snow, and the roofing calculators hub.
Worried About the Snow on Your Roof?
Call now for a free roof inspection in Waukegan and Lake County. We will check the snow depth, look for sagging and leaks, and tell you honestly whether the roof needs clearing.
Frequently Asked Questions
How much snow can a typical roof hold?
Most homes in Illinois are designed for a roof snow load of at least 20 pounds per square foot, which is the minimum flat-roof snow load in ASCE 7. That is about 4 feet of fresh, light snow, or about 1 foot of wet, packed snow, or about 4.2 inches of solid ice on the same roof. The exact limit depends on the age of the roof, the pitch, and the local code it was built to, so 20 psf is a floor, not a guarantee for every house.
How many inches of snow is too much for a roof?
There is no single inch number, because snow weight depends on how wet it is. A foot of fresh, fluffy snow weighs about 5 pounds per square foot, while a foot of wet, packed snow weighs about 20 pounds per square foot, and a foot of ice weighs about 57 pounds per square foot. At the 20 psf design limit, that means 4 feet of fresh snow, 1 foot of wet snow, or 4.2 inches of ice. If you have more than 2 feet of wet snow on the roof, or the snow has been rained on, it is time to check it seriously.
Can snow collapse a roof in Illinois?
Yes, it happens, usually to older roofs, flat or low-slope roofs, and roofs that already have a problem. A roof collapses when the load passes what the structure was built for: deep wet snow, snow drifted against a parapet or a taller wall, or rain falling on snow and adding weight fast. The warning signs are a sagging ceiling, new cracks in the drywall, doors that suddenly stick, creaking or popping sounds, and leaks that appear during a thaw. If you see several of those together, get out of the rooms underneath and call a roofer.
Does a steep roof hold less snow than a flat roof?
A steep roof holds less snow, but it is also designed for less. The ASCE 7 slope factor cuts the design load as the pitch rises: a 6/12 gable (about 27 degrees) still carries the full flat-roof load, while an 8/12 pitch (about 34 degrees) carries about 15.9 psf against 20 psf on a flat roof of the same size. The snow slides off a steep roof sooner, but what stays is denser and icier, and it can slide off in a slab, so the eaves and walkways below a steep roof need care after a storm.
Should I clear snow off my roof myself?
For most homeowners, no, not from the roof itself. Climbing onto a snow-covered roof is one of the most dangerous jobs in winter roofing: the surface is slick, the edge is hidden under snow, and a fall from a two-story roof is often fatal. What you can do safely from the ground is use a roof rake to pull snow off the lower few feet of the eaves, which is also where ice dams start. For anything above the eaves, or for a full clearing, call a professional with the right equipment. Our snow removal service covers Waukegan and Lake County, and the final quote comes after an inspection.
Zayn Robert is a content specialist focused on roofing, construction, and home improvement topics. He creates easy-to-understand guides that help homeowners learn about roof measurements, materials, maintenance, and repair decisions. His work combines detailed research with practical insights from roofing professionals at Zuniga Roofing Inc.