Built-In Bookcase: Building Shelves to Avoid Sag

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So I’ve been putting together the shelves for my bookcases this week!

This can be easy or hard, cheap or expensive, depending on your preferences.

The key is to avoid sag.

the wood whisperer

Most materials will sag at least a tiny bit under heavy books, and your eye is able to detect a sag of just 1/32” per foot!

The coolest website to utilize to avoid this catastrophe… The SagulatorAn online calculator that takes material, load, dimensions, edging, method of attachment, you name it, into account to determine the amount of shelf sag (or deflection) you will get.  This is a seriously fun calculator to play with!

You want a sag of no more than .02” per foot.

Here are some handy numbers for you!

Hardback books (9x11): weight ~ 20 pounds per foot,
magazines of the same size weigh ~ 42 pounds per foot!

I copied this shelf weight limit chart from a Woodworking magazine for you…  The numbers on the right are the pounds per foot the shelf can handle before showing sag (for shelves 11 inches deep).

Type Length
Species Thickness
24 in.
30 in.
36 in.
42 in.
Red Oak 3/4” 49 21 9 5
w/ edging 116 47 23 12
Poplar 3/4” 42 17 8 4
w/ edging 97 39 20 10
E. White Pine 3/4” 33 14 6 3
w/ edging 76 32 14 8
Fir Plywood 3/4” 32 13 6 3
w/ edging 96 39 18 9
MDF 3/4” 9 4 2 1
w/ edging 73 30 14 7

My shelves are 19 or 24 inches wide, made from 3/4” thick MDF with hardwood edging on front and back.  I bought some 1x2s and routed a 3/4” wide rabbet for the back of each shelf.  I also bought some nice decorative top cap molding that I used as a front edge for each shelf.   

I’ve discovered that my shelves will have very little sag, even with heavy hardcover books on them. Whew!  I’m glad.
But what aspect of my shelf has the biggest impact on sag?  I checked the sagulator.

Basically, hardwoods sag less than MDF or plywood.  I would have guessed that adding hardwood edging to my shelves helped them the most, but it's not the only factor.

One thing that made a big impact was thickness of my MDF… even 1/8” thinner = much more sag!

My shorter shelf width helps them sag less too (it is best to keep shelves under 30” wide, if possible).  

So now I know, and next time I build shelves I’m checking the sagulator again!!!

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4 Responses
  1. I'm laughing at that name- the Sagulator- but wow, what a great tool. We need to build some shelves in the garage before winter- I'll show my husband the Sagulator site. Thanks!

  2. I think this is a wonderful tip, especially since next spring we (the hubby and me) were talking about trying our hand at making some nice book shelves and throwing out some very cheap ones we have that are pretty shot.

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