light-fixture
I have lap siding with 1/2" depth and 6-5/8" to the weather and need a mounting block for a replacement porch light. There's a 3" electrical mounting box behind the siding. Where can I get a mounting block? Everything I can find online is for vinyl siding. This is 30 year old masonite.
Strictly speaking, that round metal mounting bracket on the back of the fixture is supposed to be secured to an electrical box.
It can be a shallow box, but it's supposed to be a box. You must protect the wires. You want that new garage to still be there 10 years from now.
I can't really tell from the photo, but the right shallow metal box should fit within that recess on the back of the fixture without you needing to cut a hole in the siding. But if you have to cut a hole, then you have to cut a hole (if you do, rent, borrow or buy a holesaw for your drill big enough for the box).
You could attach the box through to the OSB with screws, although personally, I'd lay a 2x4 flat side against the OSB inside the garage, toenail it into the studs on both sides, and screw the box directly through the OSB into that 2x4. So a stiff breeze or errant basketball doesn't knock your light fixture off the wall. :-)
Drill a hole through the OSB and the 2x4 big enough to feed the cable and to accommodate the clamp that holds the cable in the box. The cable has to be clamped into the box.
Then attach the round mounting bracket that came with the fixture to that box, wire everything up, attach the light fixture to the bracket that you attached to the box. That's the way it's designed.
If it's a metal box, it has to be grounded (screwing the grounded mounting bracket to it will accomplish this).
You also have to secure the cable inside the garage as dictated by your local electrical code.
Finally, I'd use a dose of waterproof silicone or caulk around the box, and then again around the fixture itself after it's mounted, to make sure water doesn't seep inside.
Good luck!
I realize this is a little late, but for those reading this for answers:
Make a 4” round hole in the center. The existing hole in the siding box is around 1-1/8”, so if you have a 1-1/8” hole saw, Nest it in the 4” to stabilize. If you want to save money and not buy a hole saw, trace your octagon box (keep reading) and use tin snips to cut out the hole.
Get yourself an octagon box that has a depth close to, but less than the depth of the siding box. Install an NM clamp (plastic or metal, plastic will work better). Space the box off the back of the box so it is flush to the front, screw it down, and install the light.
Tip: Screws can be your spacer. Make a triangle out of three screws that prevent the box from setting too far back. The secure the box with 2 screws.
Best Answer
The common solution to this problem, at least in my neck of the woods, is that a block of cedar wood is fitted against the sheathing with the siding abutting the sides of the block and caulked.
In your case an oscillating saw can be used to cut a notch into the existing siding so that the cedar block can be slid into position. You would also need to drill out a round hole at the position of the electrical box. To keep with electrical code you would likely need to fit an appropriate box extension to bring the box surface edge out flush with the face of the cedar block. The hole bored through the block may need to be adjusted for size to accommodate the box extension.
The thickness of the cedar block wants to exceed the maximum amount that the lap siding sticks out from the sheathing. I've seen full 1 1/2" thick cedar plank used but in cases where I've done replacement or new installation I often plane this thickness down to 1" or 1 1/8".
Attachment of the block can be done with quality construction adhesive or nailed with stainless steel ring shank siding nails.