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I need to bolt a solar inverter to a stud wall for three reasons.

  1. it needs to be a few inches offset from the studs, so the bolt holes in the inverter won't line up with the studs,
  2. I'd feel better if the load were carried by 4 lag bolts (instead of 2), and
  3. the inverter needs to sit out 1-2" from the studs, to clear the siding - I intend to lag a short piece of 2x6 or 2x8 to the studs (first removing a couple of siding boards), and then bolt the inverter to the board.

The unit weighs 50 pounds (23 kilograms.)

The problem is, how can I bolt the inverter to the board? I won't be able to reach the back side of the board once it's attached to the studs. Carriage bolts could work, by installing the into the board before I attach it to the studs; but I'm not satisfied with the amount of bearing surface the head of the carriage bolt would supply. A hex bolt, with washers under the head (on the back side) and under the nut (on the front side), would work. But how can I keep the bolt from rotating as I tighten the nut on the front side?

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    how many tons does this inerter weigh that you don't like carriage bolts?
    – Jasen
    Commented Jun 7, 2020 at 2:51
  • 50# = 0.025 tons
    – RustyShackleford
    Commented Jun 7, 2020 at 4:50
  • I've yet to see a carriage bolt that would pull through non-rotten wood with 200 pounds pulling on it.
    – Jasen
    Commented Jun 7, 2020 at 4:56
  • A hole and slot like a keyhole - the slot gives sufficient support for a bolt with washer - as long as there is sufficient “meat” on the stud.
    – Solar Mike
    Commented Jun 7, 2020 at 9:08
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    In shear like that, even a single hex lag screw would carry the full weight of the inverter. People generally underestimate the strength of fasteners. I've done experiments where a single 1.5" long, #9 construction screw has held over 150lbs in shear.
    – mfarver
    Commented Jun 7, 2020 at 20:20

5 Answers 5

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maybe use some unistrut instead of wood.

enter image description here

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  • Unistrut! (Other similar brands are available) Easy to use. Rock steady. There is a thinner 'low profile' extrusion than the one shown. And you'll get too use some zebedees
    – D Duck
    Commented Jun 7, 2020 at 23:13
  • Yes, they make a half-height extrusion for such applications. It's a very standard way to mount electrical gear.
    – Harper - Reinstate Monica
    Commented Jun 8, 2020 at 1:42
  • Wonder where to buy in small quantities, as I need like 2ft or so ? Ooh, I know, the scrap metal yard (aka. thrift store for guys).
    – RustyShackleford
    Commented Jun 8, 2020 at 17:40
  • Is zebedee the little spring-loaded nut ? I guess you can use any random piece of steel for a washer against the front edges of the channel, or nothing at all depending on what the back mating surface of the inverter looks like.
    – RustyShackleford
    Commented Jun 11, 2020 at 4:39
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You could install a few of these three prong tee nuts on the back of your board before it's attached to your studs and then bolt the inverter from the front. I'm curious why you'd "feel better" with two bolts instead of four.

enter image description here

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  • Yeah, familiar with t-nuts because they're used in climbing walls, so guess I should have thought of that. I guess they have a bit more bearing diameter than carriage bolts.
    – RustyShackleford
    Commented Jun 7, 2020 at 4:53
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    Typo on 2 versus 4 lag bolts.
    – RustyShackleford
    Commented Jun 7, 2020 at 4:55
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50 lbs - lag bolt it to the 2x6.

More than adequate holding power, works easily for the situation describe, no matter how you "feel" about it.

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    Hey, why are people so insensitive to my "feelings" :-)
    – RustyShackleford
    Commented Jun 7, 2020 at 4:56
  • Yeah, I guess you're right. I got fixated on wanting to bolt it,
    – RustyShackleford
    Commented Jun 7, 2020 at 4:57
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    @RustyShackleford This is a tough group Rusty. :-)
    – JACK
    Commented Jun 7, 2020 at 12:00
  • I had an old 19" CRT TV mounted 6" out from the wall in my bedroom for years with just lag bolts into studs. That TV was easily at least 50 lbs.
    – Ogre Psalm33
    Commented Jun 9, 2020 at 20:29
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The standard method is to mount a sheet of thick plywood (marine if outside) and mount it to the plywood with wood/lag screws. It's 50 pounds not 500.

Instead of 2 nuts... Run a metal strap between 2 of the screw holes

And tap the metal strap using a thread tap, for the bolts you are using. Use material thick enough to take whatever threads you are using - 10 gauge or 1/8" is plenty for a 1/4-20 or 5/16-18 (we like to see 2+ threads engaged). Fine thread would be better, but the tap tolerances are tighter (more sensitive to mis-drilled holes or sloppy wobbly threading).

Now, the strap becomes a "non-spinning nut" for both bolts. It also spreads the weight much better than a washer will.

The simplest way is if you can reach behind and hold the strap and guide it to the upper bolt until the upper bolt's threads engage. This is 10 times more annoying than you'd expect, though. Then you only have to line up the lower bolt, and you're done (this is also more annoying than you'd expect).

The alternate way is to drill holes in the strap and use wood screws to fix it in the correct location on the backside of the plywood.

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    @RustyShackleford You gotta learn Harper language. Takes awhile.
    – JACK
    Commented Jun 7, 2020 at 23:09
  • 5
    @RustyShackleford "bog-standard" means typical, or common, or conventional.
    – Criggie
    Commented Jun 7, 2020 at 23:30
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    @JACK: "bog standard" isn't to esoteric as far as slang goes. Wiktionary even has an entry for it. It is British slang, IDK if I can remember hearing it used by anyone who wasn't from the UK or spent some time there, but I've certainly heard it used by such folks.
    – Peter Cordes
    Commented Jun 8, 2020 at 5:39
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    @PeterCordes Very standard British slang. You'll definitely hear it regularly over here - no idea whether it's a saying which has transferred to other flavours of English though.
    – Graham
    Commented Jun 8, 2020 at 8:35
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    @PeterCordes Why I love this site. You get to hear from people all over the planet.... and everyone's respectful. Thanks
    – JACK
    Commented Jun 8, 2020 at 12:15
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Blind rivets.

Depending on application and the materials you're attaching things to, it might be possible to use blind rivets to attach something to a surface that you're unable to access the back side of. It's worth noting that unlike bolts, rivets can't be non-destructively removed afterwards, and you'll need specialized equipment to attach them.

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  • How much can 2-4 rivets hold? I've never put much weight on them, they aren't that thick after all and the heads tend to bend.
    – Mast
    Commented Jun 8, 2020 at 9:34
  • They can hold plenty, if they are big enough. Don't think I've ever needed bigger than 5mm, but that would be enough for this job. Wouldn't recommend it for this though, unless its REALLY meant to be permanent.
    – MikeB
    Commented Jun 8, 2020 at 11:22
  • Rivets were used to hold skyscrapers together and they're used to hold aircraft together, so they can handle a fair bit of stress. (Yes, I know those aren't the type were talking about here, but they can be designed to take all the stress necessary.)
    – FreeMan
    Commented Jun 8, 2020 at 15:25
  • rivets can be strong - the stainless ones are stronger than the aluminium ones and the fat ones are stronger than the thin ones.
    – Jasen
    Commented Jun 8, 2020 at 23:56

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