How far can a 2x4 or maybe 2x6 support over a 10' span?

Question

I am creating a shelf in my garage to go over the front of the car.

The rules of the town home community is that we can't have anything that puts weight on the structure of the garage, so have created two by side supports designed to have something go horizontally go from one to the other ( it's 42" deep ).

I was thinking of 2 or 3 2x4's to go span the 10', and then using 1 x 4 slats over that for the shelf.

How much weight would this hold ?

Should I use 2x6's ? Should I use 2 or 3? Or ?

Not planning to put a lot of weight up there, maybe cumulative around 150 - 250 pounds.

Any thoughts on what I should use to span the 10' wide, 42" deep shelf space ?

Answer 1

I do not know how your side supports are constructed or how you plan to keep it from racking side to side.

Three 2" x 6" set on there side so they are like floor joists, I.E. not laying flat.

One front, one middle one back, use Simpson joist hangers to attach to side support rim joist. 1/2" Plywood decking on top of that to create a shelf.

If you want even more weight capacity overkill make it four joists.

How much weight will it hold ? There is an online sag calculator you can use https://www.woodbin.com/calcs/sagulator/

It is a garage shelf, if it sags a little over time it is not the end of the world. When it does, Turn your joists over in the joist hangers so the crown is on top and get a few more years out of them. Or use 2"x 8".

Answer 2

Given your desire to make it comparatively thin (which is bad for stiffness in general) the best bet would be a Stressed Skin Panel also known as a Torsion Box

Common construction for that as a DIY is two sheets of plywood, (one top, one bottom) and some sort of spacers plus lots of glue. It's an interesting and educational experiment to compare how two sheets of plywood separated by a foam core behave without glue when you stand on them as compared to how they behave when the core is glued in, but for a shelf you don't need foam as a core - wood will do.

What this does (compared to using several joists, unbonded) is to provide a full sheet worth of strength on the tension side of the structure - similar in principle to an I beam, where the sheets of plywood (or OSB, it IS a garage shelf...) act as the tension and compression "flanges" and the wood spacers act as the "web" that holds them apart.

However, it is STILL the case (it is ALWAYS the case...) that additional depth (space between sheets in this case) pays off hugely, because stiffness is related to the cube of the depth of a beam - so a beam twice as thick is 8 times as stiff. Or a beam merely 1.25 times as thick is nearly twice as stiff, and one 1.5 times as thick is more than 3 times as stiff as the original beam.

(A different option in the "buy a solution" direction would be to get prefabricated steel pallet racking, but I assume that if you have already built side supports you are not interested in going that direction.)

If you wish to play with loading options (this is NOT set up for stressed skins, just beams/joists): https://courses.cit.cornell.edu/arch264/calculators/example8.1/index.html

Answer 3

Like the good answers by Alaska Man and Ecnerwal indicate, you won't span 10 feet with flat lumber. Even if you made the platform entirely of 2x lumber it would be a trampoline. It need to be thicker (taller), and that height needs to be composed of a single ply, not layers that can slide with respect to one another.

I'd get three small-knot 4x4 posts (or four pairs of doubled 2x4 on edge), space them evenly front to back, and overlay them with 1/2" plywood or OSB. That's as thin as you'll get (4" total) with non-engineered wood and reliably carry that load. Be sure to crown all members up for best sag resistance.