I built two shelves on the back of an 8X8 shed. The shed is well constructed using treated 2X4X8's as studs, top and bottom plates, as well as a top wall tie on each corner just like a house. Walls have 3/8" cdx. The roof is a 6 degree slope from front to back using a 16" studded wall on top of the front wall and sloped to the back where the gutter is. Rafters are 2X4X10KD with a 1.5' overhang on the front and a 6" overhang to the rear. Roofing is 7/16 ply with corrugated metal. Shed is resting on 9 4X8X8 solid block.
The shelves I built are on the exterior of the rear wall 25" below the gutter. They're constructed using 2X4's treated with but joints face screwed together and 5/4 decking. They're each 19" deep and 36" wide. They are countersunk and screwed into the wall studs through the 2X4 shelf plate and 3/8" plywood with 2-1/2" exterior wood screws. The bottom is supported by two 26-7/8" long 45 degree angled legs that go back to the studs and are screwed into the wall studs through the plywood with the same screws. I supported the wall inside by adding blocking between the studs where the shelf plate sits against the exterior and where the angled legs rest.
Each shelf is supposed to support a 55 gallon rain barrel. Water is 8.33lbs per/gal so they're supporting a little over 450lbs each. Do you think the 900lbs of rotational force will be too much for the wall studs or regular screws? I may add some lag screws to the equation for peace of mind so I know it won't pull the screws out of the wall studs.
In the future I plan to add two more barrels at the same height to the left and right side walls. This will total 900lbs exterior weight on each wall (left, right, rear) 2700lbs total. As I said, the shed is built according to house specs NOT shed specs. There are top and bottom plates, studs mostly on 12-16" centers, and a wall tie all the way around the top plate staggered from the walls so they're overlapping. The wall studs were nailed together using 16d nails, but the studded walls are screwed down and to each other using 2-1/2" ext grade screws.
I'm just concerned that the weight may pull the shelf plate screws out of the wall since the legs are angled back on a 45.
Look! I come like a thief! Blessed is he that keeps watch and keeps his garments, lest he walk naked and they see his shame.
I, too, think it's a risk. You could dado the shelves in, but only glue them on the back (or front, or maybe middle) 1/4 or so of the joint. That would leave the balance of the wood free to move. If...