I'm installing shelving in my 5x5 kitchen pantry. My plan is to run 1x2 strapping on the back edge and left/right sides of each shelf, then use 1x12 boards as shelves. I'm considering poplar for the shelving materials.
I'm wondering if the side/back support will be sufficient or if I'll need a brace in the middle. We don't plan on putting too much weight on the shelves but I'd rather do the project once than have to come back to it later.
Unsupported 1x12 over a 5' span will sag (even under its own weight). As an alternative you could get some of these shelf tracks (~$2.25/track)
and some 12" brackets (~$1.30/brace)
and then lay your 1x12's on top to make the shelves. This option also allows you to easily adjust the shelves up and down, to suit your needs.
If you want to make the shelves completely from wood (as you suggest), you'll need to beef up the design and add additional supports.
I have 1x12 shelving in my pantry area in which one wall is about 7'. Each board on the longer length is braced with three of what are sold as 'utility shelf brackets' in most hardware stores. They're not as fancy as the adjustable tracks that Tester101 referenced, but they tend to sell for about $1 each in the stores, and they allow the board to go fully against the back wall (as there's no upright in the way), making it easier to put a strip along the back to deal with sagging along the length of you ever have to. You also screw or nail the bracket into the shelf, so sagging is greatly reduced compared to just setting a board across a floating support.
The only issue with fixed shelves is that you have to know what sizes of things you're going to be storing. Mine are of differing heights; the bottom could hold a 2L bottle with an inch to spare (although, I keep them on their side in little cubbies, sized so it'll 64oz juice bottles or a double-wide stack of boxes of chicken stock). The others can hold a double-stack of 28oz cans with a couple inches to spare. I keep larger stuff up above (eg, large cereal boxes, rolls of paper towels), as those tend to be lighter things.
The only problem things are really small jars, where you're tempted to stack 'em 3 high, but I'm afraid of them falling over. That might not be a concern for most people, but my pantry space is along my basement steps, so I'm always afraid I'll hit 'em when carrying the laundry up & down ... other people likely won't have that concern ... but I still find 2 high means I don't have to dig through stuff to retrieve what I want.
(Note: except for adding the cubbies and some other modifications to extend the bottom sill, this was all installed by a previous owner and they're painted, so I'm not sure what type of wood it is. To the best of my knowledge, the utility brackets and being attached to the shelf on the short length are the only support used)
I am a designer building a pantry at home and what I realized in the planning stage is you don't want all 12" shelves. Those are for large machines like Crock Pots or Woks. For food, you want to use 6-8" shelves so that all your canned goods or packaged mixes are visible for easy grocery list-making. All your food items should be in plain sight, not stuffed into the back of a deep shelf. I dislike shelves with brackets that impede items on the shelf below. I like the IKEA sort of design with holes all along and little metal pegs to hold the shelf sides making it adjustable. Or I'd use 1x2's to brace the sides and backs but screwed in so they can be moved fairly easily. One major thing to avoid is metal shelves. Nothing sits flat on them and they will drive you crazy. Needs change, your pantry should be adaptable. If you plan it carefully and have narrow food shelves spaced close together you can avoid overloading and make the pantry more functional.
IF worried about sagging, use the Sagulator (sagging calculator) as found online... Consider it makes a large difference whether your shelves are loose-laid on top of something bearing them, or screwed fixed (with pocket screws, naturally). [The aim of the sagulator is to get you optically flat shelves, less distorted than visible.] Screwed fixed is great if you know long-term you're right; loose is great if you need to reorder to different heights.
The idea that you need shelves of various depths and widths is something to keep in mind. My corner pantry (cobbled together from three hanging IKEA corner units, heights 2x 60cm 1x 80cm = 200cm seamlessly fitting with the rest, just add the same legs) so there'd be 3 "full" shelves. Then I've bought extra shelves, collected what contents I'd expect to have in there, made cardboard templates and positioned them at the various intended heights to see how it would practically fit: I've cut those extra shelves into a sort of U-shapes, narrow arms and wider at the 'bottom' bit of the U, with jigsaw+router, then steamed on the usual white 'formica' edging.
Different heights have different reachability and safety (dropping stuff!): Highest one is just 2.5inch wide. The middle shelf contains the coffee maker, so its surrounded by a self-made shelf for various coffees and cups and cocoas and chocolates. Above it 5--10 breakfast cereal varieties in the central part, jams/honeys/... surrounding it. Heavy lumpy things (sugars, flours, baking stuff, ... ) below it. So heavy things don't sit on the narrow self-cut shelves, but on the "full planks" below them.
[If anybody runs with this idea: Caveat is that you can insert a shelf into a finished unit if it's 80cm high; the 60cm units it cannot enter & rotate flat, you must disassemble the unit. Or cut your new shelf in two. This being modular it means it's loose-shelves-so-height-adjustable.]
