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I'd like to build a French cleat storage system on an exterior wall in my basement shop, but am concerned about whether the cleats will have sufficient support given the construction of the wall. The wall is constructed of 3/4" drywall over lath and plaster (~1/2" of plaster over 3/8" laths), with additional 1/4" and 3/4" boards between the laths and the studs.

My current plan is to use 5/16" x 5 1/8" structural screws (rated as equivalent to a 1/2" lag bolt) to attach the cleats, two per cleat per stud, as shown in the diagram below. (I have not shown the screw details because my Sketchup skills aren't quite there yet.) The structural screws will be countersunk 3/16" since they have a washer head.

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A photo showing the actual materials (minus the stud, which is covered by insulation, and the drywall, which is not visible in the area where I have this cross-sectional access):

enter image description here

I have questions on a couple aspects of this:

1) Will the 1 13/16" depth of the screw in the stud provide sufficient withdrawal resistance?

2) Can I rely on any of the intervening material to provide any support as far as shear resistance, or do I effectively have about 2 1/2" of unsupported screw? And if the latter, can this possibly hold any significant weight? If not, do I have any options other than a) not doing the project or b) completely ripping out the drywall and lath and plaster to secure the cleats more directly to the studs?

3) Would using a toggle bolt into the lath-and-plaster between the studs be wise in addition to (or instead of) the structural screws into the studs with such a long unsupported length?

Thanks!

EDIT: Additional information requested by commenters:

  • I have had some (modest) success using a small metal detector to locate the nails in the laths. I have first located the drywall screws and then used those as a guide to locate the lath nails; they register faintly but some do register. My plan was to mark these and then drill small test holes to ensure I hit a stud at the expected depth rather than a cavity (I expect I might need to do 2-3 per stud to ensure I can find the center).
  • The studs are spaced approximately 16" o.c. in other parts of the basement where the lath and plaster has been cut away sufficiently for me to visually inspect them. I say approximately because there seems to be some variance of up to 1/2", and in at least one case the distance is 22". The nails/screws I have been able to locate in this wall are also approximately 16" o.c.
  • The studs themselves are 2x6; they are slightly larger than today's 2x6s as I believe they date from the original construction of the house in 1900. Most of the house has been remodeled or added on to, but this side of the basement is mostly original except for the addition of the drywall, which obviously is much newer.
  • I plan to use the cleats for hanging various power tools, accessories, clamps, etc. I would expect the total weight per cleat per stud will not exceed 100lbs. But more accuracy here is probably desirable.
  • Lastly, sadly, yes, Seattle is definitely in a seismic zone. My plan was to follow a suggestion I saw from someone who built French cleats in California who used some short cut-off nails inserted in an angled hole in the back of the shelves such that they protruded just below the cleat to prevent upward movement that would cause the cleat to "jump off", but could be easily removed to reposition it.
  • Wow, that is quite a wall! How do you find the studs through all that other material? What is the spacing of the studs? What are the dimensions of these studs? When was this constructed? There would of course be friction between the back of the French cleat and the surface of the wall which would provide a lot of shear force. How much weight do you plan to put on these shelves? Is Seattle considered a seismic zone? – Jim Stewart Feb 13 at 9:58
  • @JimStewart, thanks for the questions. I have updated my question to include answers. – clang Feb 14 at 3:00
  • I assume you plan to use wooden rails for the cleats. What is the recommended species of wood for this? In addition to the shear stress there is of course the pull-out force on the screws. There was a question about this on this site a few months back. The pull-out force is inversely proportional to the length of the lever arm of the back of the shelf. In a French cleat this lever arm projects downward, meaning that if you have an exceptionally heavy object you would want to place it on a shelf with a longer downward projection; heavy objects should be placed as close to the floor as possible. – Jim Stewart Feb 14 at 12:22
  • If you have an Android phone you might consider a Walabot to get info on the wall. The claim is that it can give info 4 in (10 cm) deep. amazon.com/Walabot-Imager-Android-Smartphones-Compatible/dp/… – Jim Stewart Feb 14 at 13:26
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    Yes that is the question, and your interpretation is what i was thinking. Any projection above the cleat would presumably be irrelevant because it would not be fastened to the wall (at least not usually) but the projection below the cleat would be the relevant length. The longer that projection would be, the less would be the pull-out force at the cleat. And the vertical position of the shelf on the projection would have no influence on the pull-out force at the cleat. – Jim Stewart Feb 14 at 20:06
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My guess is that the screws you want to use won't work well if the shelves are heavy. The embedment in the stud is too shallow and the screw will flex quite a bit. I think the shaft of the screw is too small for the drywall and plaster to provide much support in shear.

I'd simply strip off the drywall and screw 3/4" plywood on top of the plaster. I'd nail on a pressure treated 2x4 like a base molding so the plywood isn't in contact with the floor, and rest the 4' edge of the plywood on the 2x4 before attaching. This way the weight is carried by the plywood rather than the nails.

With that in place mounting your cleats will be very easy. The plywood will also hold up better than drywall behind the shelves. It's a bit of extra expense and effort but you'll have a very strong and durable and nice looking finished product.

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    The drywall on plaster makes an excellent fire resistant layer against fire originating in the living space. I don't think you should breach this. Of course, maybe the ceiling has exposed wooden joists so this may be irrelevant. To transfer shear resistance into the interior of the thick drywall you could drive multiple 18 ga brads or 15-ga finish nails of a length which will go through the rails and only into the drywall. These could be all along the rail or just around the structural screws (mainly above and below). – Jim Stewart Feb 13 at 15:56
  • Is the idea here? that the plywood would carry the entire load (via the 2x4), or would screws still go into the studs such that they are carrying some of the load and the plywood is primarily providing shear resistance? – clang Feb 14 at 3:04
  • @JimStewart, understood about the fire resistance. In this case, the ceiling does have drywall on it as well, although the rest of the basement beyond a nearby doorway has exposed wooden joists. There is also some exposed framing in a corner of this room, so the fire resistance may not be a large benefit to keeping the drywall (though the expense and effort certainly may be). Is the idea of having the nails go only into the drywall to avoid going into the plaster at all, so it is not exposed to the shear force at all? – clang Feb 14 at 3:05
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    @clang - the plywood would carry the load. The PT 2x4 is there so you don't have to use PT plywood. The fasteners just have to make sure the plywood doesn't pull away from the wall. I would still use long screws into the studs to hold the plywood to the wall - lots of them, maybe spaced 1' - but no need to be large diameter. – batsplatsterson Feb 14 at 9:19
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    @clang the purpose of have nails penetrate only into the drywall would be to save the plaster from damage. At some time in the future someone might want to remove the cleat rails and restore the walls. One would probably put the nails in at a downward angle (like a picture hanger) and to facilitate this they should be put in as parallel as conveniently possible. To the extent the nails would be out of parallel they would resist being pulled out and would damage the wall as a rail would be pried off. I think the drywall would resist less. – Jim Stewart Feb 14 at 11:04

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