Plywood overhang

[Beer]

Having trouble finding information on finding out how much an unsupported plywood overhang can handle. I know there are some experienced wood workers here.

 

Setting up a tank on a dresser and the tank has a little more depth front to back than the dresser. I was also considering building a structure over the tank to support a small display refugium over the tank, but this structure would obstruct the sides of the tank a bit, so I was considering sliding the tank forward, which would also leave room behind the tank for my ATO resivor and such.

 

The overhang would be 5" or less. I should be able to go either direction with the grain or the wood. There is a $2 difference between 1/2 and 3/4, so I can go with either. I can find stuff about length of unsupported spans, but nothing so far on unsupported overhangs.

[Beer]

Tank is about 23.5 x 19 x 9. Conservative estimates would put it between 47-50 psf.

 

There would be 37-40 pounds concentrated across 23.5" of a 31.5" span.

[OPtasia]

Water weight alone will be 8 pounds per gallon-ish. On top of that, the weight of the equipment, the substrate, the decor, the inhabitants, the sump, the lights, etc.. Without support underneath the overhang, a disproportionate amount of weight will shift to the unsupported sections and it'll start to warp. Moreso if it isn't level. Plywood and particle board will also warp faster in a moist environment like as the base for a fish tank. I once had a 55 gallon reef tank develop a Frankenstein crack in the front glass from an unlevel particle board tank stand that warped. It dumped a few grand in corals and live rock onto my living room floor one weekend when I was conveniently out of town. Nobody wants to come home to low tide and dead corals.

[Beer]

The overhang is on the front of the tank, there wouldn't be any significant rock work at the front of the tank and there will not be any large inhabitants in the tank. The largest fish would be a dragonette, so the additional weight would be negligable. The 47-50psf is conservative and includes the approximate glass weight, water, and a safety margin.

 

Your experience is what I am looking to avoid, but particle board and plywood react differently. Particle board swells quite a bit when it gets wet, permanently deforming, and separating the glue causing splits and cracks in the structure. Plywood surfaces will absorb the water and expand, enevenly from the areas that get wet, leading to warping and bending if enough water saturates the wood, put the entire panel is not thouroughly wetted. I was planning on coating with polyurethane or varnish (most likely poly) to prevent/minimze absorbing water to prevent warping.

 

I was able to find the equations I was looking for and it looks like ususing 3/4 ply with a 5" cantilevered overhang with the face grain parallel to the front of the dresser would deflect less than a thousandth, even when wet (calculated worst case condition). Allow for doubling or even tripling of that sag over time and that would still be more than tolerable.

 

I'd still like the input from some of the experienced woodworkers here to see what they think (no offense intended OPtasia) as theory doesn't always carry over to real life as cleanly as we would like.

 

 

[ReefingRelapse]

If you build a 2x4 frame underneath the plywood with some 45° angled pieces supporting under the frame, that would probably be best. Still doesn't sound worth the risk to me.

[Beer]

2x4 would be insane overkill for that. I'm not trying to support the entire tank hanging off the edge of the dresser. 2x4 is severe overkill for most stands, especially for what most people have on here.

 

I did consider some 1/2 x 1 1/2 strips underneath, but at that point, I might as well just build a torsion box skinned with 1/8 luan. Though resonation would amplify any pump noise.