Help with mounting light on metal building

I planned to use a motion detector with the two flood lights centered above my garage door. I started cutting thinking I would put a plastic
box behind the metal siding inside the garage. However, being centered it is over a raised rib where two panels meet and it looks like I made a mess. It was hard to cut and I just stopped. Any ideas on how to properly mount this light and make it look ok and not leak?
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Probably there needs to be a box outside, where the lights are? So, find a place where a hole can be drilled big enough for 1/2" plastic conduit (the gray stuff is outdoor-rated), and run it to your box. Either a straight run into the back of the box, or a right-angle (with one of those panel-over-the-bend fittings, so you can run the wire by hand-feeding) will be weather-proofable. Just use a bit of flashing and a bead of caulk.
Most flood fixtures use a box-cover-size mounting plate, match the box size and shape to the fixture. I sometimes cut wood blocks (cedar for weather resistance) to mate the vertical back of a box to the irregular exterior surfaces; put your mount screws through the wood into the steel, of course
I like to drill a weep hole or two (small, 1/8" will do) in the bottom of any box that lives outdoors. Don't know if it's allowed by code, but it soothes my moisture-corrosion hackles. Insects like it, too!
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On Mon, 28 Sep 2009 05:13:14 -0700, stryped wrote:

How thick is this "raised rib"? If it's not too high (say, 3/8"), I'd just shim it up, install the light and box normally, and caulk it liberally.
Or finish cutting the recess, mount it as usual, and caulk.
If it's like corrugated metal with 1" high ridges, I'd ask a pro. ISTR seeing some stuff designed for that; it's got a profile that matches the corrugated, and acts like the shim above.
Good Luck! Rich
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Or cut a piece of wood to fit the profile closely, paint it all over, then clamp it on with some silicone rubber as a sealant. Mount the light to the wood.
Mark Rand RTFM
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Those fixtures barely cover a box, letalone overlap siding for a weathertite seal. I'd not try to recess a box.
If one of these will sufficiently cover the hole you started... http://www.rabweb.com/product_line_detail.php?prodline=RNDBOX Mount one over your hole and enter the wire from the back with some duct seal or silicone. Use a couple bits of aluminum or PVC tubing the height of the ribs to act as spacers behind the mounting ears. Run the screws through the ears, tubing spacers and into the steel. The siding can be patched with a small scrap if necessary, hopefully it would be a small patch hidden behind the box.
Aim for slightly offcenter and not on a rib next time?
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Well, I think I messed it up too much. I am going to try to replace the two pieces of metal. I have to full length pieces left over from when it was built.
Can I cut these pieces with my jig saw or is there somethign else that will work better?
I also bought one of those wet location boxes with the two ears to mount on top of the metal as everyone here said. My plan is to run pvc conduit to the openign in the back of the fixture. Do I have to use a screw in fitting into the threaded portion of the back of this outside box? The reason I ask is I intend to drill a hole through the metal as close to the outside diamter of the conduit as I can. If I have a threaded fitting, wont that outside diameter be different that the pvc pipe attached to it. it is hard to explain but I hope you understand.
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A circular saw with one of the metalcutting abrasive blades is a good solution. Sparks will fly, don't cut near flammables or dry grass. You'll need to put a board atop the ribs to give the saw a flat guide surface, and extend the blade by the board thickness plus rib height plus a bit.

Most such boxes I've seen have threaded fittings; you can use a short PVC molded pipe nipple threaded on both ends, OR the PVC conduit with a threaded fitting (and the irritating diameter change). Never glue the threads.
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Why never glue the threads?
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On Tue, 29 Sep 2009 14:12:11 -0700, stryped wrote:

Because if you ever need to repair/replace anything, you'd have to cut out the whole joint and do the whole schmear all over again.
Hope This Helps! Rich
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The outside of the fitting (terminal adaptor) will be bigger than the conduit. My local code doesn't allow plastic male threads into a threaded box hub. We have to use a steel close nipple then a female thread adapter glued on the PVC conduit. The reason being the male threads are fairly weak and tend to snap off if the assembly is stressed. Your local accepted practice may vary.
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I went to Lowes and could find no threases "nipple" in pvc, just a "male fitting".
Got to thinking, the metal siding is not that thick. Can I drill a hole in the metal the size of just the 1/2 inch threaded portion. (would that be 7/8 inch or so diameter?) and place the larger diameter "nut" side inside the building with just the threads going through, then screw the threads into the back of the aluminum box? Then of course a very short piece of pvc conduit into the back of the male fitting? Siliconing the threads and everythign?
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responding to http://www.polytechforum.com/metalworking/help-with-mounting-light-on-metal-building-201144-.htm andrew22 wrote: i think, its always better to search the forums or ask the friends before making a mess. But as every question has an answer so does every problem has a solution too.
stryped wrote:

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http://www.polytechforum.com/metalworking/help-with-mounting-light-on-metal-building-201144-.htm
Checking on the first page of the rittercnc link and not seeing this mentions, plus deducing you have a corrugated metal building, let me suggest a look at the profile moldings used to match greenhouse corrugated plastic as possible elements in resolving the difficulty. Separate moldings are available to go along a rib and across ribs. These often come in wood but are sold in plastic.
Regards,
Edward Hennessey
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