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I'm hoping to install a 94" wide, 8" deep, 1/2" thick bookshelf made of poplar in a location surrounded by three walls.

Thanks to the three walls, it would be painless to support the shelf on boards attached to studs at both ends, and along the full length of the rear 94" edge.

Because the shelf will be in a "showy" area of my home, and because the underside of the shelf will be visible when coming up a staircase, I'm hoping to keep the installation tidy, without unnecessary brackets.

Because I'm matching an existing built-in "pedestal-top" shelf below, I want to stick with stained poplar, rather than a harder wood.

I don't have a good instinct for whether simply supporting the shelf along three edges will be enough to keep an 8"-deep poplar board from sagging over time, when fully loaded with books.

Any thoughts on whether three-edge support will be enough? Or might it call for a bracket or two for additional support?

Edit: I'm intending to securely attach the shelf to "rails" made of the same 1/2" poplar, which would in turn be securely attached to wall studs.

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  • How deep are you planning to make the support boards?
    – Alexander Gruber
    Commented Jul 1, 2019 at 6:40
  • Good question, @AlexanderGruber--I've answered in an edit, above.
    – bland328
    Commented Jul 1, 2019 at 14:16
  • Maybe show some simple isometric drawings of your intentions to help you sort out the details and help us understand the best way to answer.
    – user5572
    Commented Jul 2, 2019 at 14:27

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Is support on three sides of a long poplar bookshelf enough?

Yes, but see note.

Unless you expect to heavily load only the front edge of the shelf (seems unlikely!) it should be OK. Shelves installed this way are relatively common and while 94" is a very long shelf and poplar isn't a particularly stiff wood the rear support makes all the difference. You wouldn't get away with it if the shelf were only supported at each end....

The data in the Sagulator tells us that a normal shelf of this width supported at the ends only would deflect excessively with the load I assumed1, but with a substantial edging strip (simulates the support provided by the batten attached to the wall at the back, which can't be deflected) this becomes acceptable.

Note: this is based on the shelf being firmly attached to all three supports, not floating.

If in doubt, reinforce
Obviously if you did add a bracket or two as you mention in the Question it/they would do the job.

But since you'd prefer not to use brackets other reinforcements are possible that I presume won't mess with the aesthetics too much. This includes adding an edging strip to the front edge2, making the rear support thicker (double up by attaching a second piece of the same wood) and using thicker poplar than 1/2" for the shelf itself (going to 3/4" makes a significant difference).

1 Don't know the type or number of books you want to load on the shelf so as I typically do I went heavy to give a good safety margin. In this case I assumed a weight per foot well over double that given for paperbacks by some published sources (checked against a stack of modern paperbacks just now on my kitchen scales which were a tad over 3kgs, 7 lb). This gave a total in excess of 130 lbs for a shelf of this width.

2 You can install the shelf as planned and if it does bow more than you'd like you could add an edging strip at any time in the future, in situ if necessary.

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  • My gut says that 3/4" shelf material and nosing is necessary to keep this from sagging under books. If that turns out to be not enough, brackets on the top side can be subtly hidden among the books.
    – Aloysius Defenestrate
    Commented Jul 5, 2019 at 0:05
  • @AloysiusDefenestrate, I get that, I went into this expecting to find it wouldn't work! Obviously the actual load is the key thing here, as always, but since the shelf is only 8" deep paperbacks seem a safe assumption. IIRC the actual sag amount was 0.1X", which equates to >3mm. Even on such a long shelf this is probably visible, but as I say I greatly exaggerated the expected load — doubled the weight I got here, from atypically heavy paperbacks, and then added even more. So unless the poplar is unusually weak I would be surprised if this sagged enough for even the fussiest person to notice.
    – Graphus
    Commented Jul 5, 2019 at 6:48
  • Even with continuous, attached support along the back edge, I'd fully expect a 1/2" thick shelf to twist so that the front edge sags noticeably. A thicker shelf would be much less likely to twist, so the support along just the back edge would be fine, but I'd be amazed if such a thin shelf, unsupported over almost 8 feet, doesn't sag in front.
    – Caleb
    Commented Feb 21, 2023 at 5:05

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