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I am planning on building a bookshelf as shown below. Each shelf (maple plywood) will only be supported via a dado cut into the 2x4 (SPF pine), and glued. I am currently planning on cutting 3/4" deep dados.

How do I go about figuring out how much weight each shelf can support? I am hoping to use it as an home entertainment unit, so it needs to hold up to 30 lbs per shelf for consoles and audit receivers.

I tried the Sagulator, but it apparently is for shelves supported on both ends.

Also, what are some better ways then dado + glue? I am thinking one or all of the following:

  1. Screws from the back of the spine into the back side of the shelfs
  2. adding L brackets underneath each shelf (I am hoping not to do this for aesthetic reasons)

Would these measures be overkill?

Spine Bookshelf Sketch

Edit: On a separate note, does anyone has any ideas how can I mount this to the wall as clean as possible? I mean clean as showing little hardware, and flush to the wall. I was thinking doing French cleat cutout on the spine, but I don't know if I can given I only have a circular saw.

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    Hi, welcome to StackExchange and thanks for the well-crafted Question and the SketchUp drawing (we often get only the barest description in words, without even a pencil sketch to go on). There's no reason this couldn't work, but with caveats. The key factor is what species you plan on using, it's the one thing you neglected to mention in your Question :-) I am presuming SPF give the mention of a 2x4. Even assuming pine this may be OK — poss some slight sag, but towards the front and not across the width as sag generally happens. It's the second that's much more easily seen by the naked eye.
    – Graphus
    Dec 17 '20 at 9:14
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    There are various ways you can reinforce to reduce or completely remove any tendency to sag, including both your 1 and 2 (with obviously 2 going further towards preventing sag entirely). Even though screws wouldn't be enough to prevent sag — this is inherent to the material of the shelf in the design as drawn — they would be a good idea to add anyway if other options aren't selected. This would be for security, so you never have to worry about a shelf coming loose just by itself, or from receiving a bump, with disastrous consequences.
    – Graphus
    Dec 17 '20 at 9:23
  • Thank you, and I appreciate your detailed response! You can post this as an answer. For the material, you are right about Pine SPF for the spine. For the shelves, I am planning on using 3/4" maple plywood with some edge banding. Would this be weaker or stronger than slicing and gluing together 2x4s?
    – Vance
    Dec 17 '20 at 16:32
  • In general ply would be noticeably less resistant to sag or bending compared to solid wood (even solid wood of a lesser species). You can take a look at some sample numbers for an imaginary shelf on the Sagulator to get an idea, but real-world the problem can be worse than these textbook/idealised figures because of the known variability of so much modern plywood, except for the really primo stuff. Anyway, this isn't a reason not to use ply, I'll Answer accordingly.
    – Graphus
    Dec 17 '20 at 17:57
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    Oh BTW, your drawing does show 4" x 2" for the upright so in case you don't know, the dimensions of 2x material are now nominal. If the past when you bought wood at the lumber yard or home centre it was the stated dimensions, but now a 2x4 would typically be 1 1/2" x 3 1/2", give or take a little margin.
    – Graphus
    Dec 17 '20 at 18:29
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How do I go about figuring out how much weight each shelf can support?

Based on what follows this becomes sort of irrelevant, but if you did want actual numbers the only way I can think of realistically is to make sample shelves (note, not just one) and test them, see how much weight they can take before A) bending too much to be acceptable, B) fracturing or C) somehow falling out of the dado. Empirical numbers are the only real way to know for sure, especially with plywood.


Also, what are some better ways then dado + glue? I am thinking one or all of the following:

Both your 1 and 2 would add strength and security here, with obviously 2 being the superior option in a couple of ways. But like you say, brackets under the shelves spoil the aesthetics here and it would be nice to maintain the clean, uncluttered look.

So, based on this, other design details and given the material of the shelves is intended to be plywood, I would recommend using either two long bolts or lag screws per shelf, approximately 3" apart. Or even better, steel rod/threaded rod since either might be cheaper if you want to go, say, a full 6" into the edge of each shelf (although then drilling the holes becomes more of a technical hurdle).

This mimics to some extent the way that floating shelves have long been installed so it's a sort of proven technology, and what's more it may then be possible to make this a knockdown piece so if/when you move you don't have to try to transport it as one awkwardly shaped item.

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  • If you're going to drill threaded rod 6" into the shelf, go all the way through and run the rod right out the front end! Drill a larger recess there to put a nut in, and when you put the hardwood edge band on to hide the edge of the plywood (you're doing that with plywood furniture, right?), you'll also hide the nut. Put a nut and washer on the back side of the pole and tighten. This is what they do when building with "post-tensioned" concrete and it works there, can't see why it wouldn't work here, too.
    – FreeMan
    Dec 18 '20 at 18:43
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    I like the idea of steel rod reinforcement. I think I can go with a dado/channel underneath the plywood shelf instead of drilling a long hole into the side, to maximize the amount of material for support and ease of installation.
    – Vance
    Dec 18 '20 at 19:17
  • Oh that's great, it is much easier to do and it offers advantages as you say. I didn't want to suggest it as I assumed (you know what they say about assumptions LOL) you would find the open grooves unacceptable. I don't know how the costs are going to break down for you but after you answered I had a quick look at lag screws and I was pleasantly surprised to see 6" ones of sufficient girth I would think, 1/4", were inexpensively obtainable on Amazon for example. But using channels beefier and longer reinforcements are easily doable.
    – Graphus
    Dec 19 '20 at 8:43
  • @FreeMan, 12" drilled hole without a wander is a tricky proposition to say the least! I don't know about you but I'm not set up to do that. 6" in from each side is of course another option, but that comes with its own challenges (especially 12 times!) unless the OP has a drill press with at minimum 18" capacity. While of course tensioned rod would be a superior option it's overkill, the requirement is only 30lb per shelf. I'd trust that to only 2" of projecting rod, assuming sag of the shelf material were not an issue naturally.
    – Graphus
    Dec 19 '20 at 8:52
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    @Vance please feel free to ask that as a whole new question, since it is. It's a fairly simple one, but it's a whole new one.
    – FreeMan
    Dec 19 '20 at 14:01
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Your design raises some interesting issues and requires additional consideration for connections. Each shelf is supported at only one location and the actual load on each shelf is not the only consideration. The only way I can thin k of to evaluate the design is to actual test the assembly. You may want to make a mockup of a single shelf and test it before proceeding with the entire assembly.

Here are several things to consider as you work out the design:

  1. The shelf will have a tendency to rotate out of its connection point. Imagine pulling down at the center front. The shelf will want to rotate up at the vertical connection. The joint between shelf and column must be tight to prevent this rotation. In addition, if the depth of the shelf extending into the column is too shallow, then it will be easier for the shelf to rotate out.
  2. The two side edges of the shelf are unsupported and the entire shelf will want to curve down at the sides from loads. (a similar tendency will exist at the front edge) For this reason plywood is better than wood solid wood panels since it can better resist this curving to both the sides and the front where wood planks will be weaker across the grain than with it.
  3. Your design is a cantilever and the further the weight is from the bearing point the more it will tend to curve the flat shelf profiles. This affect will be more noticeable over time.
  4. You could reinforce the shelf by using a pair of long screws (say 1/4" x 6") countersunk into the 2x4 from the rear. In that case I would minimize the dado depth and maximize the depth of solid column wood the screws engage before entering the shelf panel. Make the holes in the column very snug to prevent any rotational movement of the screws.
  5. Steel 'L' brackets might work if the steel is thick enough (1/8" thick or more). I would mount the vertical leg of the angle to the rear side of the column and pass the horizontal through a slot opening . You could cut a rabbet in the bottom of the shelf to make the underside of the steel and shelf flush. I would make the legs on the steel as long as possible.
  6. My instincts tell me that a denser hardwood would do better on the column than pine.

Whatever design you finally work out I would love if you would follow up with a post after it is completed and tested to show us what you actually did and how well it works.

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  • #5 was an initial thought of mine, too! Cut a shallow dado for the steel to sit in, then cover the bottom with veneer to hide it.
    – FreeMan
    Dec 18 '20 at 18:45
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Your question mentions that you only have a circular saw. This makes me question what other tools you might have access to. Be aware, though, that you can make jigs to help you with some tasks.

This suggestion would require the following tools:

  • A hammer
  • A bench vise
  • A drill
  • A looooong drill bit
  • A hacksaw or angle grinder with cutting wheel

I would suggest you make some internal reinforcement. Several people have suggested long screws or threaded rods. Have you considered installing a "hidden" support?

Imagine you purchase some 1/4" steel round bar. Now cut it to length and bend a right angle into it, making an ell shape of rounded rod. Make two of these per shelf, times however-many shelves.

Now, using your circ saw (and a narrow chisel, if you have one), make two grooves in the back (wall-facing side) of the spine. The grooves should be 1/2-3/4 inch apart, centered left-to-right. The grooves should be the width of the rod, and the depth of the rod. They should run from the bottom of the spine to the height of the top shelf.

At each shelf, drill a hole inside each groove. The hole should be just slightly bigger than the rod. The hole should extend through the spine, into the shelves as far as you can possible make it reach. (For 12 inch deep shelves, see if you can make a 10" or more deep hole- they sell long drill bits.) The holes should be parallel to the shelves (obviously! otherwise the holes will come out the top/bottom) and at an appropriate angle to point at the front corners of the shelf. I believe this is about 55 degrees from the centerline of the shelf.

The idea is to insert the bent ell shapes into the groove in the spine, and into the hole in the shelf. You can then secure the bottom end of the ell shapes using epoxy or just screws and metal tabs or just screws carefully placed. The ell shapes are totally invisible from every direction except when the shelf is pulled away from the wall. They're customizable to the specifics of your shelf, and they should be something you can make with a few tools.

My local orange box sells 1/4" x 72 round rod for $5.87 (USD). Your drawing suggest a small distance between shelves - maybe 6"? So 18" of rod would bend into a 12" leg and a 6" leg nicely. Thus, you could get 4 ells, or 2 shelves, per unit. 5 shelves would cost < $20 to reinforce, plus any screws, drill bits, and other tools.

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