# plywood thickness



## sirdavidofdiscus (Dec 8, 2006)

How thick would plywood have to be for a 29 in deep tank without using any 2x4 bracing?


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## caldwelldaniel26 (Jun 11, 2017)

What are the rest of the dimensions? I wouldn't recommend not bracing though, that's asking for trouble...


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## sirdavidofdiscus (Dec 8, 2006)

48in long and 14 in front to back


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## sirdavidofdiscus (Dec 8, 2006)

I do plan on using a center brace but no "ribbing"


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## caldwelldaniel26 (Jun 11, 2017)

Just a suggestion but you'd have more strength if you were to frame out the bottom and run your braces short ways on 12" centers and run a "rib" longways starting from the center working out with 4" screws. It's always better to overdo it than to end up having to redo it


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## Mcdaphnia (Dec 16, 2003)

Because you are using screws into the edges of the plywood, pre-drill all the holes and glue. Paint everything with marine paint. Keep the bottom from direct contact with the floor. Porous patio block would be one way to lift the bottom.

If you are using a box design, I would suggest 3/4 inch plywood, waterproof plywood core MDO both sides if you can get it. Double the edges of the wall pieces internally with strips of the same plywood clamped, glued, and screwed. Make a full box (six plywood rectangles) and place access hatches as close to center of their panel as possible and keep any doors as small as you can.

If you use an X design, half inch will be enough. Cut two pieces the diagonal length of the aquarium and an inch shorter than the projected stand height. On the center of each, cut two vertical slots halfway down. Flip one over and push the slots together to make the X. Cut two pieces the same as the tank footprint or slightly larger. Spread the X to reach the corners of the top and bottom pieces. Screw them to the top and bottom of the X. You can add triangular shelves if you wish under the top of the stand.


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## noddy (Nov 20, 2006)

I'm confused. Are we talking about a plywood stand or a plywood tank?


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## Mcdaphnia (Dec 16, 2003)

It has to be a stand. A 29 gallon plywood aquarium would be economic nonsense. A 29 wood tank would require hours of labor and cost several times as much as a new glass or even acrylic tank that would be ready to use.


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## caldwelldaniel26 (Jun 11, 2017)

Agreed mcdaphnia... but it's not a 29 gallon tank, he posted the rest of the dimensions after I asked him in the earlier posts


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## Mcdaphnia (Dec 16, 2003)

Daniel is right. After I read it carefully. I saw "29" and "plywood", jumping to the conclusion it was about a stand. I have built a lot of stands and lots of plywood tanks, and a few all glass aquariums. Half inch plywood is OK for any plywood tank no taller than 12 inches. Higher than that I would use 3/4 inch thick plywood. I greatly prefer waterproof plywood core MDO over marine for the large size aquariums. When you are building several larger plywood tanks, using the scraps to build small plywood tanks or sumps makes sense.


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## sirdavidofdiscus (Dec 8, 2006)

It is replacing one side of a 110 gallon tank. I was given 3 gallons of unopened pond armor the tank was free Managed to snag 2 sheets of plywood that are 3/4' x 48"x 42" for 15$. So for the cost of the silicone + 15$. I'd thought I would give it a try. 
I intend on doubling up the plywood so it will be 1&1/2 inch thick


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## Mcdaphnia (Dec 16, 2003)

Not sure doubling up the plywood is a good idea. Every time I've seen it done, the result was a leaky tank. The reason may apply just to an all plywood tank, but the wider seam exaggerates the amount of flexing, popping any silicone or screw and glue bonds.


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## sirdavidofdiscus (Dec 8, 2006)

So I doubled up the plywood then put a sheet of wonder board on to of that, sealed everything with drylock, then coated the wonderboard with Flexseal. Then I siliconed it against the cracked side. The Spreadable Flexseal is suppose to be aquarium safe the spray is not. 
So far no leaks I'm going to let sit in the driveway filled for a few days. 
Total cost $63 US.


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## Mcdaphnia (Dec 16, 2003)

sirdavidofdiscus said:


> So I doubled up the plywood then put a sheet of wonder board on to of that, sealed everything with drylock, then coated the wonderboard with Flexseal. Then I siliconed it against the cracked side. The Spreadable Flexseal is suppose to be aquarium safe the spray is not.
> So far no leaks I'm going to let sit in the driveway filled for a few days.
> Total cost $63 US.


 A single 3/4" plywood panel is good to 48" of water height but I prefer not to go more than 42". Bracing outside would be needed to prevent bowing.

Each time you add another layer to your cake, you make it more likely to fail. I've repaired dozens of plywood aquariums that others built, and one very common thread causing tank failure was the impulse to add more layers of stuff to the seams. It looks like that impulse is still alive and well. Imagine a chain. Adding more links to the chain does not make it stronger. To the contrary, the chain will break at the weakest link.

The 29" height I hope will help you get lucky, but I had to repair many 24" tall plywood tanks that had "enhanced" seams. A single layer of plywood would have been more reliable.


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## sirdavidofdiscus (Dec 8, 2006)

I doubled the plywood instead of bracing not because of seams


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## Mcdaphnia (Dec 16, 2003)

sirdavidofdiscus said:


> I doubled the plywood instead of bracing not because of seams


A 29" tall 3/4" plywood aquarium does not need bracing. Seam leaks are an often encountered natural consequence of adding more layers at the site of a seam. Nature has a way of ignoring our motivations, reasoning, and intentions. No matter how sound our reasoning to touch a hot stove burner, our bare hand may still get burned. As someone who repaired lots of leaking plywood tanks back when they were much more popular than today, I would only see the failed tanks and failed repair attempts. I really don't know just how long it will take for a doubled plywood seam to fail. Many I saw failed immediately, but then those that might have worked longer I would not know about. I hope you are lucky.


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