The Roof Panel That Caught a Gust at the Eave

Why this matters

Every crew lead has said "twelve miles an hour, we're fine" on a morning call. On this job it was true of the number he read and false of the day, and a metal roof panel ended up on the hood of a car. Nobody was badly hurt, which is luck, not design. What makes the case worth reading is that the office had to reconstruct it from the record for an insurance claim, and the record showed both what the lead saw and what he did not look at. The fix the shop wrote afterward is a table of wind holds keyed to gusts, and it would have stopped the panel work before the truck left the yard.

What the office was handed

The day after, the owner pulled together everything the job had produced. In the order it was found:

• The lead's morning note on the job, entered at 7:10 a.m.: "Wind 12, dry, go."
• The tech's statement, written the same afternoon: he was carrying a corrugated metal panel, about 3 feet wide and 10 feet long, up an extension ladder at the eave to a helper on the roof, holding the ladder with one hand and the panel with the other against his side. Near the top, at about 11:20 a.m., "a hard gust came around the corner of the house" and swung the panel's far end out from the wall. He let go to keep both hands on the ladder, and the panel slid off sideways.
• Photos: the panel on the driveway, a dent and a cracked windshield on the homeowner's car parked alongside the ladder, and the tech's bruised forearm where the panel's edge caught it.
• The forecast. The office still had the hourly forecast from that morning on the shop's weather service, saved under the job. The lead had not saved it; the office went and found it.

The gaps were as informative as the contents. The morning note held one wind number and no gust number. No one had recorded a wind reading on site at any point. Nobody had noted that the car was parked inside the ladder's drop zone.

What the hourly said

 Saved hourly forecast, job day (wind and gust in mph)
 Hour   Temp(F)  Wind  Gust  Dir   Rain
 8am    46       8     14    W     0%
 9am    49       10    18    W     0%
 10am   52       12    24    W     5%
 11am   54       12    30    WNW   5%
 12pm   55       13    31    WNW   10%

The lead's "wind 12" is the 10 a.m. and 11 a.m. sustained figure, and he read it correctly. The column next to it says the gusts that hour were forecast to reach 30 mph, two and a half times the sustained speed, and to keep rising into the afternoon. The incident happened inside the 11 a.m. hour, at 30 forecast, and the tech's "hard gust" is that column arriving.

The lead was not careless in how he read it. Most weather apps put the sustained speed in large type at the top of the day and leave gusts to a smaller line or a second screen, and a morning call made from the headline gets the headline. The saved forecast showed the gust figure on the same row as the sustained figure the whole time. The record does not show a misread; it shows a column nobody had been told to read, which is a training gap and not a judgment failure, and the shop treated it as one.

The column that matters depends on the task. Sustained wind is an average over a couple of minutes; gusts are the bursts over a few seconds that ride on top of it, and on a sunny, mixing morning over houses and trees, a gust figure double or more the sustained figure is ordinary. A person on a ladder holding a sail does not experience the average. He experiences every gust, one at a time, with no warning. The general rule for which number a limit means is owned by Sustained Wind Versus Gusts and Which One a Limit Means; this case is what it looks like when the wrong column gets read.

Why the gust column was the one that mattered

The push of wind on a flat sheet grows with the square of the wind speed. Velocity pressure is about 0.00256 times the speed squared, in pounds per square foot with speed in mph. Run the panel through it:

• At the 12 mph sustained figure: 0.00256 times 144 is about 0.37 pounds per square foot. On a 30 square foot panel, that is about 11 pounds.
• At the 30 mph gust: 0.00256 times 900 is about 2.3 pounds per square foot. On the same 30 square feet, about 69 pounds.

The gust is two and a half times the speed and 6.25 times the push, because 30 divided by 12 is 2.5 and 2.5 squared is 6.25. And those figures are only the velocity pressure: a flat sheet held broadside to the wind takes a drag force somewhat higher than that, and the force does not arrive centered in the hand. It arrives on the far end of a 10 foot lever.

So the man on the ladder planned for roughly the effort of holding a sack of groceries away from his body, and got a sideways pull of close to 70 pounds or more at the end of a long sheet, while standing on a rung with one hand free. He did the right thing in the moment, which was to let the panel go and keep his grip on the ladder. The decision that put him there was made at 7:10, when "12" went in the note.

The corner of the house added to it. The tech said the gust "came around the corner", and that is typical: wind accelerates around corners and over eaves, so the air at the top of a ladder set near a corner can be stronger than the forecast for open ground. That is one reason a forecast gust is the floor for a sheet-carrying decision, not the ceiling.

The other two failures in the record

Two things in the record are not about forecasting, and the shop fixed them alongside the wind rule.

The panel should not have gone up that way at all on that day. Carrying a large sheet up a ladder by hand leaves one hand on the ladder and gives the wind a lever. OSHA's construction ladder standard, 29 CFR 1926.1053, says in substance that a worker on a ladder is not to carry an object or load that could make them lose balance and fall; it binds the employer for its own employees on construction work, and roofing replacement is construction. A 30 square foot panel on a gusty day is that load. The better methods (staging panels from a lift, a hoist or a hand line from the roof with a tag line held below, or carrying them up a roof ladder system built for it) are the subject of Wind on a Ladder and the Sheet You Carry Up It.

The car was in the drop zone. Anything at the eave of a roof being worked can fall, and a panel caught by wind can fall well out from the wall. The homeowner's car had been parked beside the ladder when the crew arrived, and nobody asked to move it. The shop now sets a cone line at the start of any roof job, roughly as far out from the wall as the eave is high, and asks for vehicles inside it to be moved before materials go up.

The claim went through the shop's general liability coverage for the car, and the tech's injury was reported under the shop's workers' compensation coverage the same day, with the forecast screenshot, the photos and the statements attached. How either claim is handled depends on the policy and on state rules, so the owner called the agent before anyone spoke to the homeowner about who was paying for what. This is not legal advice; it is the order that kept the shop from saying anything it later had to take back.

The wind-hold table

The shop's answer was a table, keyed to the gust column for any hour inside the work window, with an on-site check as well. The numbers are the shop's own starting points, set from its crews' tasks and tuned since; they are not a standard and they are not a manufacturer's rating.

Three rules travel with it:

• Any hour in the work window counts. If the 11 a.m. hour crosses a line, the task stops before 11 a.m., not at it.
• The site can be worse than the forecast, never better. The lead carries a handheld anemometer and takes a reading at the top of the ladder position before sheet work starts. A measured gust over the line holds the task even if the forecast says it is clear; a calm reading does not release a task the forecast holds.
• A hold moves the task, not the job. Most days with a sheet-goods hold still have work in them: tear-off by hand, underlayment fastened as it is rolled, flashing details, or sheet work in the first calm hour.

Applied to this job's morning: the 8 a.m. hour showed gusts of 14, under every line. The 9 a.m. hour, at 18, was under the 20 mph sheet line but inside the 5 mph margin the SOP asks of a forecast, so sheet work that hour needed a site reading at the eave first. From 10 a.m., at 24, carrying sheets up the ladder and handling them on the roof were both held, and by noon the forecast was at 31 and still rising. The panel work fit into the early hours, if the panels had been staged on the roof from a hoist by 10 a.m., or it moved to another day. Everything else on the job could have gone ahead under the hands-free line until the 11 a.m. hour, when the forecast 30 crossed its 25 mph line.

The lead's morning call now reads the gust column out loud, and his job note records both numbers and the hold that applies, for example "W 12 G 30, sheet goods held from 10, hands-free OK until 11." Reading the hourly for the work rather than the day is covered in Reading a Forecast for the Work, Not the Weekend, and the procedure the table now sits inside is The Wind Hold for Ladders, Roofs and Spraying SOP.

References

• 29 CFR 1926.1053, OSHA construction ladder requirements, including the duty not to carry a load that could cause a loss of balance; binds the employer for employees on construction work.
• See related: Sustained Wind Versus Gusts and Which One a Limit Means
• See related: Wind on a Ladder and the Sheet You Carry Up It
• See related: The Wind Hold for Ladders, Roofs and Spraying SOP
• See related: Reading a Forecast for the Work, Not the Weekend