We have had good luck with a drill & tap attachment directly thru the purlins.
I first researched thread pullout strength in steel on the www using 5/16-18 SS bolts. I copped a short section of standard gauge C-purlin from a local steel erection company and did my own simple pullout testing. I dont have the calculations at hand but I recall that the purlin was .090 thick which allowed almost two full threads in the steel. It was considerably more pullout strength than lags in wood. We would have used 5/16-24 bolts but fine thread is not available in SS in that small size according to our supplier. We rented the smallest magnetic-base drill press available and attached it to a heavy steel plate since the mag wouldnt stick to the sheet steel roof. The plate has to be large enough to get both feet on to hold it flat which assures that the drilled hole doesnt wallow out from the drill moving around. I also used a tap guide that assured the tap went in straight and the threads were clean. Etrna-Bond® tape on the bottom of a Snapnrac L foot attached directly to the roof after clearing metal chips with a can of compressed air. The thick sealant tape made an accurate torque value hard to verify, I ended up just going by feel rather than waiting for the click. We ended up with 2 attachments per 81 rail and 2 landscape modules per pair of rails. Im completely confident in the installation and will use it again if the need comes along. My 2¢ [after 18 years as a machinist and toolmaker] on self-tapping screws for a critical load like this is I would never consider it. The fit will be sloppy and the more oversize or out-of-round your hole the less thread engagement exists. I was disappointed that my bolt supplier could/would not bother to look for the pullout calcs for me but maybe yours will. The method I describe is labor intensive but it works if done properly. Jim Duncan North Texas Renewable Energy Inc www.ntrei.com <http://www.ntrei.com/> NABCEP PV 031310-57 TECL-27398 nt...@1scom.net 817.917.0527 From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Chris Mason Sent: Sunday, October 21, 2012 8:22 AM To: RE-wrenches Subject: [RE-wrenches] Steel roof attachments I have a large steel roof project to install that is giving me some problems. There are many locations so no one is the same. Most of the roofs are steel sheet over steel purloins with no insulation. The steel sheeting is lying right on top of the purloins. Can I use a self drilling self tapping screw to attach? Who makes one large enough? In some cases I get get to the underneath but working off 30' ladders to make each attachment is too labour intensive as the warehouses are full of goods.. In the case of the roofs that have foam filled panels or corrugated I will have to use EJOT fasteners but these seem overkill for steel on steel fastening, they are very expensive. Thanks for the help, -- Chris Mason President, Comet Systems Ltd www.cometenergysystems.com Cell: 264.235.5670 Skype: netconcepts
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