plywood bookshelves

Yes - you might sag, especially with books. Consider doing both of the following:

Upsize your shelf to 3/4 plywood - quite a bit stiffer

Trim the front with a piece of oak about 3/4" wide by 1" or 1-1/4" high. You can route or roll the edges for appearance. It will give you a little more edge stiffness.

I would strongly second RonB's comments. A 36" wide shelf that's only 1/2" thick is bound to sag, especially if not supported along the length. Upgrade to 3/4" plywood at the very least. Wouldn't be surprised to see even the

If you have access to it, Andy Rae's "The Complete Illustrated Guide to Furniture & Cabinet Construction" has lots of useful info relevant to what you're asking about.

My son has a creative punker friend who is only 17, he made a set of (adjustable) shelves from 3/4 rough slabs and barb wire and nothing else. They are suspended from a ceiling beam. Ouch! They actually look pretty good.

How about an inch wide strip of 3/4 ply supporting the shelves lengthwise both at the front and at the back of the shelf? Would this help? Vlad

yes. 35" span with books and half-inch ply will most likely sag

Depends on the books. How many copies of Lehninger's Principles of Biochemistry are on them?

I rarely go over 32" with 3/4" plywood. Dunno for 1/2". (Even 32" and 3/4" will sag some, about 1/32nd to 1/16")

Why not a nice dadoed vertical strip up the middle front? Looks nice, less work.

A 35" span will require 3/4" ply, plus a 1.5" lip on the front to prevent sagging. Rabbet the lip and secure with carpenter's glue and biscuits or finishing nails. If you want super-strong shelves, put a lip on the back as well as the front. Half inch ply is just not practical for heavy loads.

Your shelf will sag about 0.30 inch over a 35" length. Anything over .09" would be noticeable, according to the Sagulator: