Steel Roofing: Screw in the Flats or the High Ribs

It was a pleasant October evening back in 1985 in Blacksburg, Virginia. My friend Dr. Frank Woeste was then a professor in the College of Agricultural and Life Sciences at Virginia Tech (officially Virginia Polytechnic Institute and State University) and he had invited me to teach one of his classes for a day, in exchange for him providing some basic engineering software which would design post frame building columns, roof purlins and wall girts.

Now back in 1985, Virginia Tech had not yet become the NCAA football powerhouse it grew into under the direction of Hokies’ head coach Frank Beamer – having now participated in post season bowl games for each of the last 23 seasons. This also long predated the April 16, 2007 tragedy in which Virginia Tech student Seung-Hui Cho fatally shot 32 faculty members and students, wounding 17 others before killing himself on campus. This shooting remains the deadliest mass shooting committed by a lone gunman in United States history.

Mid-way through the evening with Frank, after digesting a hearty meal and debating whether the hops in the liquid consumed were a fruit or a vegetable (they actually are neither – they are flower cones), we digressed into the true essence of Dr. Woeste’s research at Virginia Tech – which was post frame buildings and prefabricated metal connector plated wood trusses.

Narrowing things down, a lively discussion occurred (including some of his grad students) on whether steel roofing and siding for post frame buildings should be attached with screws through the ‘flats’ or on the tops of the high ribs.

For years steel roofing and siding had been attached with ring shanked nails (read more about this and Dr. Woeste here: The traditional location of the nails was at the crown of the high ribs – knowing not all of the nails would be identically driven through the steel into the underlying wood. The concept was rain running off the roof would never get high enough to leak around the improperly seated nails on the tops of the high ribs!

So, what would happen if screws were improperly placed in the crowns of the steel high ribs?

Properly designed post frame buildings are dependent upon the diaphragm action contributed by the skin (roofing and siding) and numerous tests have been done to confirm the shear strength of the panels as properly fastened. When screws are placed through the high ribs, there is a 5/8 to ¾ inch gap between the underside of the high rib and the framing below. The screw can flex within this space, reducing the shear load carrying capacity of the sheathing system.

Furthermore, the flexation of the screws in this gap, allows the steel panels to move slightly under wind or seismic loads, eventually contributing to slots being formed in the steel around the screw shanks, and over time, causing leaks.

The answer Frank and I came up with is the same one espoused by every steel roofing and siding roll former – screw in the flats, not on the ribs!

4 thoughts on “Steel Roofing: Screw in the Flats or the High Ribs

  1. I replaced a roof that had WW2 era metal on it. 100′ barn that was leaking terribly. As stated above, each of the nail holes had wiggled a good size hole (over 80 years). Having done several metal roofs since then I would say if you are an amateur, screw on the flat with 1 1/2″ screws. I strap all roofs with nothing less than 2×4 to help lock that roof in place. Screwing high on the rib is difficult for the amateur. Pay attention to not squishing the washer or leaving it too loose. All that being said I often think of the words of my grandfatherwho said “it’ll do me out”. Do it however you want, it’ll probably be okay

    1. I am looking at Page 3 of the Ultra Vic installation guide and it shows “structural fasteners in flat of panel”.


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