Snow and Frost on a Roof Before Anyone Climbs It
Why this matters
An even blanket of snow makes every roof look the same: smooth, white and featureless. That is exactly the problem. Snow covers the skylight that will not hold a person, the roof edge that looks like more roof, the low parapet that becomes a trip line, and the ice underneath that turns a careful step into a slide. Frost on a clear morning does the opposite trick, leaving a roof that looks dry and is slick. Most winter roof falls are not caused by the weather that day being severe. They are caused by somebody walking onto a surface whose layout they read from memory instead of from evidence. The checks that prevent that happen on the ground, before anyone is on a ladder.
Roof load from snow is a separate question, covered in Snow Loads and Roof-Mounted Equipment. This card is about what snow and frost hide from the person walking on it.
What snow hides
Openings. Skylights, smoke vents, open hatches, unguarded roof penetrations and plastic domes sit only a few inches above the deck. A plastic skylight dome is not walkable and many will not hold a person who steps on it, let alone one who falls onto it. Under several inches of snow they vanish. Skylights are holes for fall-protection purposes: 29 CFR 1910.28 requires holes, including skylights, to be guarded or covered for general-industry work, and 29 CFR 1926.501 does the same for construction work.
Edges. On a roof with no parapet, snow erases the line between roof and air. Worse, wind piles snow into a cornice that overhangs the downwind edge, so the snow surface you can see extends beyond the roof it sits on. The apparent edge is outboard of the real one, and the overhang will not hold weight.
Low parapets. A parapet that comes to your shin is a trip hazard in summer. With snow drifted against it, it can disappear entirely, and the snow surface runs smoothly from the roof to the top of the wall and past it. A low parapet is not fall protection in any weather; under snow, it is not even visible.
Ice beneath. Snow on a heated building melts from below, runs down and refreezes at colder spots, especially at eaves, around drains, at scuppers and on the north side of equipment. The snow on top gives a boot something to grip and then shears off the ice beneath it. On sloped roofs this shows up as ice dams along the eaves, which is also where a ladder lands.
Sliding snow. Metal roofs and steep slopes shed snow in sheets. The zone under the eave of such a roof, which is where a ladder usually goes, can take a slab of snow and ice without warning.
What frost does
On a clear, calm night, a roof surface radiates heat to the sky and can cool below the air temperature, the dew point and freezing while the thermometer in the yard reads above 32 F. The physics is in Radiational Frost: Why a Surface Freezes Above 32. The field consequence is a thin, nearly invisible film of frost on membrane, metal, shingles and ladder rungs at first light, often on a roof that looks dry from the ground. The frost goes as the sun reaches each slope, so north faces, shaded areas and low-slope roofs with insulation below them (which keeps heat from the building away from the surface) stay slick the longest.
Checks from the ground, before access
- Ask whether anyone needs to go up. Many winter calls can be diagnosed from inside, at the unit's disconnect or from the ground. Some can wait for the sun to clear frost or for a thaw. The safest roof trip is the one that was not needed.
- Get the roof layout from something other than memory. Building drawings, the shop's own photos from a previous visit, the customer's roof plan or aerial imagery. Count the openings: skylights, hatches, vents and drains. Write the count down.
- Count skylights from inside. On a commercial building, skylights show as light wells in the ceiling. Walking the space below and counting them takes minutes and confirms the drawing.
- Walk the perimeter. Note which edges have parapets and how tall they are, where snow has drifted, where cornices overhang, where icicles and ice dams have formed, and where snow could slide off. Look at the ladder location in particular.
- Read the snow. Depth at the ground near the building, whether it is dry powder or wet and heavy, and whether there was rain or a thaw before the last freeze, which means ice beneath.
- Check the frost. On a clear morning, look at the ladder rungs, the truck roof and the shaded slopes. If the truck roof is white, the roof deck probably is too.
Worked example: a rooftop unit call after a snowfall
A commercial HVAC tech gets a no-heat call at a single-story retail building the morning after 8 inches of snow. The tech has been on this roof twice in summer. The plan below comes from the shop's photos and the building drawings, with the snow conditions found that morning.
REAR WALL: PARAPET ABOUT 12 IN, DRIFT ABOUT 18 IN
+--------------------------------------------------+
| |
| (SK1) (SK3) [ RTU-1 ] |
| |
| HATCH (SK2) [ RTU-2 ] |
| | ^ |
| +-------> marked path, clear of SK2 -+ |
| |
| (SK4) o roof drain |
+--------------------------------------------------+
FRONT EAVE: NO PARAPET, CORNICE OVERHANGS THE EDGE
(SK) = plastic dome on low curb, about 6 in tall
The count. The drawings show 4 skylights. From the hatch, the tech can see 2 faint humps in the snow, at SK1 and SK3, where wind thinned the cover. SK2 and SK4 are invisible: the domes stand about 6 inches above the deck under 8 inches of snow. Inside, the tech counts 4 light wells and confirms their positions against the drawing. Two of the four openings would have been missed by eye.
The path. The straight line from the hatch to RTU-2 runs directly over SK2. A tech heading for the unit by memory, through snow that looks uniform, walks onto the dome. The planned path drops below SK2 on the plan, toward the drain row, and the tech marks the turn and SK2 with flagged stakes probed into place from the hatch side before anyone walks the line.
The edges. The rear parapet is about 12 inches tall with an 18 inch drift against it, so the drift is 6 inches deeper than the wall is tall (18 minus 12). The top of the wall is under the snow, and from the hatch the roof appears to run straight off the back of the building. The front eave has no parapet at all and a cornice overhanging it. RTU-2 is about 8 feet from the front edge on the drawings. Neither edge gives any protection, so the work at the unit needs fall protection appropriate to its distance from the edge under 1910.28, set up before the tech works there, not after.
The ice. There was a light rain before the snow, so the tech expects ice on the membrane and confirms it with a probe at the hatch: hard ice under 8 inches of snow. Traction aids go on before the tech leaves the hatch, and the path is walked slowly and once.
The skylights during the work. SK2 sits beside the planned path, and a tech carrying a tool bag back and forth will drift toward it. It gets a cover or a guard rated for the purpose, or a barricade set at a distance from it, before the work begins, per 1910.28 and the cover and guardrail criteria in 1910.29. A flag in the snow marks it; it does not protect anyone.
The call that could have changed it. The no-heat complaint turns out to be a tripped safety at RTU-2, and the reset is at the unit. Before resetting, the tech finds why it tripped: after a snowfall the usual cause is drifted snow blocking the unit's combustion air inlet or flue outlet, and resetting into a blocked vent repeats the trip at best. The inlet and outlet are cleared and checked open before the unit is allowed to fire. Any smell of gas at the unit means the tech stops, does not operate switches or the reset, leaves the roof by the marked path and calls the gas utility from a safe distance. Had the drawing been unavailable and the tech unable to count skylights from inside, the right answer would have been to wait for the shop's second tech and a probe-first approach, or to defer until a thaw for anything that was not a no-heat emergency.
The sloped residential version
On a pitched roof the same checks move. The ladder lands at the eave, which is where ice dams form and where sliding snow comes off, so the base is cleared to bare ground or sanded and the ladder is set where no slab can come down onto it, often at a gable end rather than under a long eave. Skylights on a sloped roof sit in the field of the roof and are just as invisible under snow; count them from inside, in the rooms below. And a snow-covered steep slope is not walked at all for most service work. If the task cannot be done from the ladder, it waits, or it goes to a crew equipped to set roof anchors from a safe position.
Before the work restarts after a stop
Snow and frost change during the day. Sun softens the snow surface and then the evening freezes it hard again; frost burns off one slope and stays on another. If the tech leaves the roof for parts and comes back two hours later, the path is re-checked from the hatch before it is walked again: any marker knocked over is reset, any new drift at the parapet noted, and the fall protection at the unit confirmed still in place.
References
- 29 CFR 1910.28 and 1910.29 (general industry) and 29 CFR 1926.501 (construction), fall protection including holes and skylights
- Building drawings and the shop's own roof photos for openings and edges
- See related: Snow Loads and Roof-Mounted Equipment
- See related: Radiational Frost: Why a Surface Freezes Above 32
- See related: How to Make the Crew's Path Safe on an Icy Property