I just built two bookcases to line the wall of a room in my house and apparently didn't build them square enough so, when I put them next to each other, I get this giant gap. (I think the one on the right is pretty square but the one on the left appears to lean to the left at the top.)

Any suggestions on how I can make this look better? The shelves are going to be sanded, spackled, caulked, and painted. One thought I had would be to remove the vertical strips of molding where the two bookcases meet (there is 3/4" square molding on all the edges) and make one custom piece that attaches to both. This piece would be wider at the top and taper at the bottom. Maybe that would look bad--I don't know. It also wouldn't address the gap on the top horizontal surface.

enter image description here

Any suggestions would be appreciated!

  • 30
    Is it possible that some portion of the gap is due to variation of the floor?
    – Michael Karas
    Commented Apr 7, 2019 at 14:15
  • 19
    You should get a tool called a framing square so that you can check squareness rather than guess. Maybe it is possible for you to remove the back of one unit and spring the shelves into square and reattach the back to hold it in the square shape. It is not quite clear how the backs are fabricated and the worst could be that you would have to make a new back.
    – Michael Karas
    Commented Apr 7, 2019 at 14:19
  • 3
    @MichaelKaras -- Your comments would make a good answer.
    – Jasper
    Commented Apr 8, 2019 at 0:45
  • 2
    Now that you've fixed it, please post an updated picture so we can all sleep at night.
    – DavidS
    Commented Apr 9, 2019 at 16:58
  • 1
    Note that you don't need a square tool to check for squareness - if you measure the diagonals, and they are the same, then it is square.
    – AnoE
    Commented Apr 10, 2019 at 14:59

6 Answers 6


Swap their positions.

Place the bookcases so that they lean into each other instead of away from each other. This will eliminate the gap between the tops.

Bookcase contents are usually quite heavy. Once the cases are filled, you will probably find that you can force the bottoms together and the weight of the contents will distort the shape of the cases so the gap disappears.

If an unsightly gap remains visible then you will need to resort to a decorative molding.

  • 31
    Another benefit to swapping them is you can see if the problem is actually caused by an uneven floor.
    – bitsmack
    Commented Apr 8, 2019 at 6:39
  • 1
    It's funny that the best solution (in my opinion) is a quick bodge. Commented Apr 8, 2019 at 18:03
  • 2
    Or flip them upside down, they look like they would go either way
    – undefined
    Commented Apr 9, 2019 at 9:09
  • 1
    @LukeMcGregor even if they go either way, flipping them will either point the opening against the wall or rotate them so that the long diagonal is in the same position it was before. Right now the long diagonals look like this: [\][/] Flip them 180 degrees and they look exactly the same. Commented Apr 9, 2019 at 17:38

Thanks everyone; I figured it out and came back here to update my post and noticed that Michael Karas said the same thing I realized (although it was a comment, so I couldn't mark it as the correct answer): I had to pop the back off the left unit (which was actually quite easy because it was just 1/4" plywood attached with brads and no glue), re-rack the unit into square, and then reattach the back. Thanks for the input!

  • 1
    Yes - this is certainly the correct answer. It is not a huge gap and the force required to push the backless bookcase into a cuboid is unlikely to be too great to be held by a few panel pins once you put the back back on. They look like nice bookcases, and good luck!
    – user99234
    Commented Apr 7, 2019 at 23:51
  • 4
    Still, it is often the case in older houses (and sometimes in new ones) that the floor is not flat. You need to level them with a spirit level and some kind of shims so they are perfectly level. Then you could consider screwing them together top and bottom.
    – RedSonja
    Commented Apr 8, 2019 at 7:00
  • 1
    So you had frame square or went and bought one?
    – Solar Mike
    Commented Apr 8, 2019 at 7:49
  • 1
    I didn't buy a framing square (although it's on my list--I used to have one but it's disappeared). But I used my smallish rafter square on the corner of the left bookcase and it looked out of square. I then made an ad-hoc plumb line and used that to verify that the left bookcase was indeed racked to the left. The right one was square.
    – jevron1984
    Commented Apr 8, 2019 at 20:52

You don't have to buy or get a framing square : just measure the diagonals... That will tell you if either or both units are out of square.

If they are both ok, then look to the floor - small change in the floor will make a large gap at the top... Then you need some adjustable feet of some sort.

  • 3
    Well, yes they won't. That's my point. Neither for a parallelogram, a trapezoid, ... The diagonals will be equal, like for a square.
    – WoJ
    Commented Apr 8, 2019 at 13:24
  • 5
    @WoJ a parallogram does not have equal diagonals, but they do bisect each other... So, the method I suggested is still valid. You may find this helpful : mathplanet.com/education/geometry/quadrilaterals/…
    – Solar Mike
    Commented Apr 8, 2019 at 13:31
  • 1
    (a parallelogram having equal length diagonals ... pfff ... the shame ...) :)
    – WoJ
    Commented Apr 8, 2019 at 13:37
  • 1
    @WoJ I think we can assume that the top and bottom are the same length, and the left and right sides are the same length, so it is a parallelogram and the trick with the diagonals works. Commented Apr 8, 2019 at 14:55
  • 3
    @WoJ - "a parallelogram having equal length diagonals", yeah, I've heard of them, they are called rectangles.
    – Glen Yates
    Commented Apr 8, 2019 at 21:57

I have used interscrews (e.g. from screwfix) in the past to join units together to make them line up nice and tight.

  • Thanks--those look like they would be helpful.
    – jevron1984
    Commented Apr 8, 2019 at 20:54

If the shelves are sturdy, just get some molding and cover the junction of the two units. Nail the molding to only one unit so they can be easily separated if you want to move them.


I would turn the left bookcase upside down. If the other side of the left bookcase is more square, or slants to the right, this will fix your problem.

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