0

I’m opening a wall to add some 3/4 plywood blocking. Something similar to this picture.

What screw should I use considering I’m going from metal to wood? Something that is rust resistant and strong enough since the idea here is to hang a small network rack that may be over 100lbs (after the wall is closed).

enter image description here

Some additional details for those wondering what and why - this is the rack:

https://www.penn-elcom.com/us/17-3-4in-deep-9u-white-wall-mount-rack-with-10-32-threaded-rails-and-poly-front-door-r6409w-1032

enter image description here

According to the documentation, the rack alone is 55 lbs, I’m assuming another 50 for the equipment.

Dimensions: 20 H x 23 1/2 W
The wall mount brackets are 20 1/2 W

This is a rough idea of what I’m planning:

enter image description here

And this close up should give a better idea of how the plywood board is attached to the studs:

enter image description here

Image extracted from: https://frtw.com/?page_id=144

7
  • Do the rack's mount holes miss the studs? By "small" do you mean narrower than the stud spacing. If it's wider and the mount holes miss, can you possibly add holes to the rack? Maybe a couple of aluminum bar beams (or painted steel ones) can join it to the studs and still look nice?
    – popham
    Jan 7 at 20:30
  • This is an odd place - corner. There are 3 studs, which means two wood blocks. The first 2 studs are 24 inches apart, and the third one at 16 inches from the first. The rack is less than 24” wide but to have it exactly where I want, it will have to be attached to both wood blocks, missing the first and third stud. I hope it makes sense. What kind of aluminum bar are you referring to? Would it support a bit over 100 lbs? I initially thought about opening the wall and add a large (maybe 3 feet high) plywood blocking because I thought it was too much weight.
    – Ebrito
    Jan 7 at 23:39
  • Something like alcobrametals.com/aluminum/structural-aluminum-angle-equal-leg or alcobrametals.com/aluminum/…. That particular grade of aluminum has similar strength to mild steel (A36 steel). I would have to see the rack and have a better understanding of the dimensions involved (wall and rack) to prescribe something specific. Eccentricity of the rack's weight from the wall is relevant also. Better: Studs are at 3", 11", and 27" from the corner. Rack goes at 5" and 22"...or maybe a drawing or image w/ dimensions.
    – popham
    Jan 7 at 23:55
  • I add some info the post. But this bar is an L shape, I thought you meant something flat.
    – Ebrito
    Jan 8 at 6:16
  • the second link is bar, but bar is weak if it's getting pulled away from the wall. The L shape has better strength pulling away from the wall, where your eccentric loading would could be handled at probably 20% or so of the cost using angle instead of bar.
    – popham
    Jan 8 at 7:08

2 Answers 2

2

For a network rack ~100 lbs I think this is overkill. Not the plywood - that's perfect. But opening up the wall is just not necessary, and it means a lot of extra work to patch the drywall.

In addition, you may open up the wall and find that the metal studs are mounted in a way that you can't actually install wood through them as shown in the picture.

  • Find the studs.
  • Place the plywood over the existing drywall, which is over the studs. Use some screws around the plywood to temporarily hold it in place.
  • Drill pilot holes in the plywood.
  • Drive screws through plywood and drywall into the studs.

Any screw designed for wood + steel will do the job. A quick search found this page from Simpson but there are plenty of other options.

The plywood can be painted before mounting so that it matches the wall and equipment rack.

6
  • Thanks for the suggestion, I thought about it but this will be in my laundry room and wife thinks the exposed plywood will make it look weird :-) I had to expend some extra money on a white rack just so it can match the other cabinets so… you get the point :-)
    – Ebrito
    Jan 7 at 18:10
  • 4
    Two coats of white paint on the plywood before you mount it will be a lot easier than a full patch/paint of the drywall. Jan 7 at 18:13
  • 5
    Heat and humidity and lint and electronics! Good combo if you sell electronics, less good if you buy them. ;^)
    – Ecnerwal
    Jan 8 at 0:05
  • @Ecnerwal True. But if the laundry room is reasonably well ventilated, and in particular if the dryer is properly vented to the outside, then this really shouldn't be a problem. Jan 8 at 0:06
  • 2
    When I clean a laundry room with a dryer vent going outside, somehow there's always plenty of lint around anyway...
    – Ecnerwal
    Jan 8 at 0:08
1

The screws should be calibrated to the material at the back of the connection, wood in this case. They should be long enough to stick out the back of the plywood. There's a risk of overtightening to tear the plywood (in contrast against sawn lumber studs which won't tear with typical screw lengths). 3/4" isn't very thick in comparison to the 3-1/2" studs for which most of us have a good intuition, so I would double my 3-1/2" stud intuition's screw count.

Use exterior screws or anything hot dip galvanized if corrosion is a potential problem. Non-bearing studs typically have minimal corrosion protection, so this part of your question seems a bit silly.

3
  • That is great advice, you right, I should be looking at wood screws, makes perfect sense. It’s an inside wall so the corrosion part may be me overthinking things. You mentioned 3 1/2 sawn lumber. Do you think I should go for something stronger/thicker than plywood for this?
    – Ebrito
    Jan 7 at 23:45
  • @Ebrito, it depends on the geometry involved. As the rack's center of gravity moves away from the wall, the exact geometry of the anchorages becomes important. As that center of gravity moves away, the withdrawal load in the fasteners increases. For non-trivial withdrawal load and if the rack doesn't have an extra mount hole or two for redundancy, I would probably install tee nuts on the backside of the plywood and bolt to the plywood. But now I'm making assumptions about how exactly you intend to get the plywood in there.
    – popham
    Jan 8 at 0:09
  • I think I understand what you are saying. The rack is 17” deep and the equipment stays in the front of the rack so it may add a lot of tension on the screws. I thought of using snaptoggles to secure both, the plywood to the metal studs and the rack mounting. Brackets to the plywood. The heavy duty ones: toggler.com/products/…
    – Ebrito
    Jan 8 at 6:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.