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I need to mount a Generac automatic transfer switch to my house beneath the meter base. The construction of the wall I need to mount to is (from outside inwards):

  1. 30 yr old vinyl siding (for which I cannot buy new siding to match)
  2. R7 foam-board, 3/4(?) thick (a regrettable decision in retrospect--wish I had used 5/8 plywood for all walls)
  3. 2x4s 16" o.c.
  4. 1/2" drywall

I think I have basically two options:

  1. cut and remove all material that'd be behind the automatic transfer switch, mount 1 or 2 layers of 3/4" plywood to studs on either side

or

  1. mount the plywood directly over the vinyl siding--making sure it is large enough to fasten to two (or more) studs. I would use long lag bolts and washers to ensure that they didn't pull thru the plywood. I estimate lags will be 3/8x3". The thing I dislike about this option is maintaining/painting the plywood and preventing the need to ever have to replace it.

Option 2 would be easiest and I am not convinced that Option 1 is sufficiently more structurally sound than option 2. Is it worth the risk of damaging the irreplaceable siding?

Are there better options I have not considered?

and are there any electrical related concerns (with either approach)?

The wall is east-facing and sees little direct precipitation although lots of direct sun.

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  • 1
    option 3 is to cut through the drywall and install plywood or blocking between the studs, not that I'd recommend this approach unless you're really worried about how it looks from outside. obviously a non-starter if the drywall is inaccessible or covered with irreplaceable mural or wallpaper.
    – Jasen
    Aug 6, 2020 at 21:01

1 Answer 1

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The first option that comes to mind is U-strut. (Channel strut or C-channel)

enter image description here Photo from fireequip.com.au

The first two struts would be mounted vertically on studs with at least three points. Use double ended lag screws, (wood threads on one side and machine threads on the other)

enter image description here

Pilot drill through the siding/foam board and into a stud. Drive the appropriate length lag in so that a nut on it will be proud of the siding. Then on each lag goes a nut, big flat washer, U-strut, washer nut. Now you have two solid vertical rails to work with.

You can mount horizontal U-strut to these or a board, You have many options.

A YouTube search will give many sources of info on U-strut.

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  • I love this site--for this reason! Great answer--and one I would never have thought of. Thank you!!
    – peinal
    Aug 6, 2020 at 23:48
  • This worked out great. I had difficulty finding large double-ended lag screws; otherwise, it was a perfect solution. Thanks Alaska man!
    – peinal
    Aug 14, 2020 at 2:00

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