Sliding Snow Loads on Roofs: ASCE 7 Section 7.9 Explained

The first time it happens, it sounds like a small avalanche: a rush of snow and ice sliding off a metal roof and crashing onto the porch roof below. It is not a malfunction. Metal roofs, glazed skylights, and steep asphalt roofs are designed to shed snow, and when they do, that snow has to land somewhere. ASCE 7 section 7.9 exists because that landing spot, a lower roof, a covered walkway, a sunroom, or a row of parked cars, has to be engineered to take the hit. This guide walks through when the check applies, how the surcharge is sized, and what it means for a real building.

Why slippery roofs shed snow all at once

A rough asphalt shingle roof holds snow through friction. A smooth metal panel, a glazed atrium, or a membrane roof offers almost none. Once the roof surface warms, either from sun, from heat loss through the deck, or from a rise in air temperature, the bond between the snowpack and the roof surface breaks. On a roof with enough pitch, gravity takes over and the whole accumulated mass can let go in one event rather than melting gradually over days. The steeper the roof and the slicker the surface, the more likely this happens as a sudden slide rather than a slow retreat.

This is exactly why ASCE 7 treats slippery roofs differently from typical asphalt shingle roofs when it comes to the slope factor Cs: they lose their snow load at a lower slope threshold because they are built to. The tradeoff is that whatever they shed does not vanish, it has to be accounted for on the structure below.

When does ASCE 7 require a sliding snow check?

Section 7.9 applies whenever a slippery upper roof (smooth metal, glazed, or membrane) is adjacent to a lower roof, porch roof, or other structure that sits in its slide path. The upper roof also needs enough slope to actually shed snow rather than hold it; ASCE 7 generally treats a slippery surface steeper than 2:12 as capable of sliding. A nearly flat metal roof will not generate a meaningful slide even though it is slippery, because there is not enough pitch to move the snowpack.

The check is not optional once those two conditions are met. A lower roof, an attached carport, a covered entry, or even an unroofed area used for parking or foot traffic below a slide path all need to be evaluated. This is one of the load cases worth flagging early in design, because it can change where a covered walkway or an addition gets placed relative to an existing roof.

How the sliding surcharge is calculated

The sliding load is the weight of snow that could realistically slide off the upper roof and land on the lower one. ASCE 7 sizes it as the upper roof's flat-roof snow load, Pf, multiplied by the eave-to-ridge length of the upper roof that drains toward the lower structure (often written lu), multiplied by the width of the lower roof that actually receives the sliding snow.

That total mass of sliding snow is not applied as a point load. It is distributed as a uniform surcharge over a strip 15 feet wide, measured horizontally from the eave of the upper roof across the lower roof. If the lower roof is narrower than 15 feet, all of the sliding snow lands within that shorter distance, which concentrates the surcharge over a smaller area and raises the psf value there.

Worked example: a steep metal roof over a porch

Take a house with a steep 8:12 standing-seam metal roof over the main structure, and an attached porch roof directly below it with a shallower 3:12 slope. Site ground snow load Pg is 50 psf. For the upper roof, assume Ce = 1.0, Ct = 1.0, Is = 1.0, giving a flat-roof load Pf = 0.7 x 1.0 x 1.0 x 1.0 x 50 = 35 psf.

The upper roof section that drains toward the porch has an eave-to-ridge run, lu, of 12 feet. The porch roof below is 10 feet wide, narrower than the 15 foot distribution width, so all of the sliding snow lands on that 10 foot strip. Total sliding snow weight = Pf x lu x width = 35 x 12 x 10 = 4,200 lb, spread over the 10 foot width and the length of the porch. Divided across the 10 foot width, that adds roughly 420 lb per linear foot of porch, on top of the porch roof's own balanced snow load, which still has to be carried in full.

Does the sliding load replace the lower roof's own snow load?

No, and this is the detail that trips up quick estimates. The lower roof must be designed for its own balanced snow load, calculated the normal way from its own Pg, Ce, Ct, and Is, plus the sliding surcharge on top of it. The two loads do not average out or cancel. A porch roof in a heavy-snow region can be carrying its full balanced accumulation at the exact moment a warming trend triggers a slide from above, and the framing has to handle both at once.

Slide-path hazard scenarios and mitigations

The same physics shows up in a handful of recurring building layouts. The table below covers the common ones and what typically reduces the risk. Three layouts account for most real-world cases. A two-story addition with a steep metal roof built over a single-story garage or mudroom sends its full slide load onto that lower roof every time it warms up fast. A covered walkway or breezeway connecting a house to a detached garage, if it sits under a metal or glazed section of the main roof, catches whatever slides off during a thaw. A skylight or glazed clerestory positioned above an entry door creates the same hazard for anyone standing underneath, not just for the structure below; the mitigation there is as much about pedestrian safety as it is about load.

Designing and retrofitting for sliding snow

For new construction, the cleanest fix is geometry: keep entrances, walkways, and secondary roofs out of the slide path of any slippery upper roof, or size the lower structure from the start to carry the surcharge. Where that is not practical, ASCE 7's load case still has to be met by the framing, not avoided by hoping the snow melts gradually instead of sliding.

For existing buildings where the layout cannot change, post warning signage in the slide path during winter and consider snow guards or snow fences on the upper roof. Snow guards work by breaking the accumulated snowpack into smaller sections that melt and release gradually instead of all at once, which reduces the peak surcharge on the load below. They redistribute the shedding process, they do not eliminate it, and in a major melt event a lower roof or walkway still needs to be able to take a share of the load.

During plan review, a designer should flag every slide path explicitly rather than leaving it for the framer to notice on site. A plan reviewer checking a permit set wants to see the sliding surcharge called out on the lower roof's framing schedule, not just assumed. On a renovation or addition where an existing roof is picking up a new slippery upper section for the first time, it's worth re-checking the existing lower roof's framing against the added surcharge before assuming it still works; a roof that was adequate under its own balanced load alone may not be once a new slide path lands on it.