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.

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  • Do you need to get one, or are you willing to make one that'll fit perfectly? Do you have matching paint or do you want it white?
    – isherwood
    Commented May 6 at 15:53

2 Answers 2


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.

  • it looks like I'll need to make something like you described. Nothing pre-made online except for 4" vinyl siding. I may get someone to do it who's done it a few times, unless I feel like redoing 10 times myself. I'm also going to keep looking. I can't believe home builders are custom making mounting blocks, considering they can have materials delivered down to the nail. thanks again.
    – Neill
    Commented May 7 at 21:51

Nice picture. It helps us understand what is needed.

This vertical siding mounting kit may work. It has a built in box, so finding something to fit the existing box is not needed. If still not suitable, you will have to search for something to fit your dimensions. I found the one by googling " Lap siding light fixture mounting blocks."

If you cannot find what you want and cannot or will not modify anything, you will have to make something yourself or employ a professional to make one for you.


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