I am planning to build a bookcase/shelving unit on top of my existing unit, however I'm trying to get my head around how I can best adapt/scribe this to the walls on all 3 sides...

The left wall appears to be plumb, however the right wall has a slight lean to it (represented in the image below). I have not yet checked the back wall for how true that is both vertically and horixontally.

The plan was to not use a backing board as the Mrs has painted the wall behind as to show through and contrast with the wood (emulsion for the wall that "wouldn't" work on a backing board apparently).

I have a few general questions but any other advice is welcomed.

  1. How much of a gap should there be between the walls and the bookcase for adding a filler piece? If any at all.
  2. How would I best go about scribing to the back wall if it is off? Each vertical and horizontal would need scribing and that surely would be a lot of work? It's going to be birch ply, so would caulking and painting be out of the question?

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  • 2
    It appears you are going with fixed shelves, instead of adjustable. if so, let the side follow the wall and cut the shelves accordingly. I would imagine you did not know the wall was leaning until you checked it. If the shelves are adjustable it will be a different matter.
    – Jack
    Jan 2, 2023 at 18:41
  • 1
    For books, or similar sized items, there is no need whatever for the shelves to touch the wall. A gap of an inch or 25mm matters not at all, to a book, or most anything else larger than an inch that you put on a shelf.
    – Ecnerwal
    Feb 1, 2023 at 21:33

2 Answers 2


Another classic solution: build a bookcase which is square, then apply trim which is scribed to match the shape of the opening That would retain the ability to use adjustable shelves, and would look 'built in", but someone who is Really Picky might complain about the fact that the trim's width varies.

(Note that this is what you would do if one of the sides was a stone wall or fireplace, where straight lines simply do not exist.)

Basic assumption: Nothing in a house is completely square, level, or plumb the day after it is built, and that just continues to drift over time. Build to what's actually there, or hide the deviations, or both.


I agree with Jack's comment 100% - build the shelving unit with the right side just off vertical to match the slope of the wall. The bottom shelves will be slightly wider than the top ones.

After all, you've lived there for how long now and didn't realize that the wall wasn't plumb until you started measuring it up. Nobody else will realize it either.

If you want to get really fancy, you could measure the angle of the wall from vertical and cut a slight bevel on the right hand edge of each row of shelves to make the joint nice and tight. Or, if it's a pretty shallow angle (as I suspect), just leave a bit of gap at the bottom, then, when you're gluing & screwing (or whatever fastening method you choose), fill the gap with some wood glue mixed with sawdust generated by your cutting & sanding. This is an excellent homemade wood filler that's guaranteed to match in color to the wood you're using, since you're coloring it with that wood.

  • It's a new build so finally getting around to building some extra storage :D I might have to try and find the angle to check just how drastic it will be, rather than gauging by eye with my spirit level. At what point would you add some sort of packing and a filler piece?
    – physicsboy
    Jan 2, 2023 at 20:55
  • Cut the outer box pieces. Square up the left side (just clamp it temporarily), then put the right side on. If the gap is noticeable to you and bothers you, you'll want to bevel the edges of your shelves or fill the gap. If it looks ok to you, then just run with it as is. It's up to you. I would probably bevel the edges of the shelves, but I've got a table saw that will make it easier and I'm just a bit picky (ask my wife). OTOH, the wall could be at a 15° angle and my wife would be happy to just run the shelves in and not bother with filling the gap, either.
    – FreeMan
    Jan 2, 2023 at 23:27

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