just how strong is plywood

[frasertheking]

due to the bad winters we have had the last few years i am thinking of over wintering my pond fish in a small pond in my greenhouse.

my thinking so far is a tank 50 cm tall , 100cm wide, 150cm long


but here is were my confusing comes in.

most pond on here use large heavy frames made from 2 by 4s to support there ponds, but when i look into it on other forums building plywood tanks they seem to get by on just 1 inch ply and carbon fiber.

so could i build this thing out of say 12 or 18 mm hardwood plywood, and use glue and long screws every inch or so, then line with a pond liner.
no window will be used.


so any wood experts that would like to help me out, are we overbuilding are ponds or is my design not up to scratch ?

 

[frasertheking]

just for all you Americanos


that rounds to 20 inches tall 40 wide and 60 inches long


12mm ply is 1/2 inch

18 is 1/7 inch

i hope that helps

 

[shellies215]

The 18mm ply and pond liner would probably be enough, but for another 30 "Americano dollars" you could put a 2x4 frame on it to be safe

 

[frasertheking]

The 18mm ply and pond liner would probably be enough, but for another 30 "Americano dollars" you could put a 2x4 frame on it to be safe

yeah i would do but my problem is my greenhouse door is only about 23 inches wide which limits me. plus limited space in the greenhouse so im trying to save as much space as possible


do you think i could add bands around the frame like a barrel ?

although not being round im not sure it would help with the bowing

thanks for the input i need to be sure my plan will not give way and destroy a greenhouse. i can just see warm water hitting glass with snow on it

 

[sashimimaster]

With carbon fiber you can build a 1000 gallon tank with no bracing and have no problems.

 

[frasertheking]

Anyone with woodworking experince able to jump in ?

 

[Oughtsix]

I am not a plywood tank builder but I have built a lot of things with plywood.

I would go for the 18mm (3/4") over the 12mm (1/2") plywood. Plywood does come in a lot of different flavors and strengths so use good quality plywood with a lot of plys and no voids (Baltic birch?). I would build metric 2" x 4" frames for the ends. i.e. a 20" by 40" rectangle at each end and use these for screwing the sides together putting the 20" x 40" plywood ends inside the 2 x 4 frames. You will loose 4" (more like 3" in reality) of length on the tank but it should be plenty sturdy.

I would then use 2" x 2" strips along the inside bottom front and back as screwing strips to hold the front and back to the bottom. Screwing into the end grain of plywood isn't very strong and will weaken the plywood itself. Screwing into the 2 x 4 frame should be plenty sturdy. I always use glue and screws in projects like this for extra insurance and to add rigidity (Titebond III?).

I would expect a lot of bowing to the tank. If the seams can withstand the bowing the plywood should be plenty strong enough to not burst. I would strongly consider a center support in the middle of the tank to control the bowing even though it will be a nuisance for working in the tank.

I would be a little concerned about the long term effects of the plywood bottom sitting on the ground / cement and retaining a puddle of water between the plywood and the ground. A sheet of fiberglass on the bottom (or all around) would probably go a long way in ensuring the tank life.

 

[frasertheking]

I am not a plywood tank builder but I have built a lot of things with plywood.

I would go for the 18mm (3/4") over the 12mm (1/2") plywood. Plywood does come in a lot of different flavors and strengths so use good quality plywood with a lot of plys and no voids (Baltic birch?). I would build metric 2" x 4" frames for the ends. i.e. a 20" by 40" rectangle at each end and use these for screwing the sides together putting the 20" x 40" plywood ends inside the 2 x 4 frames. You will loose 4" (more like 3" in reality) of length on the tank but it should be plenty sturdy.

I would then use 2" x 2" strips along the inside bottom front and back as screwing strips to hold the front and back to the bottom. Screwing into the end grain of plywood isn't very strong and will weaken the plywood itself. Screwing into the 2 x 4 frame should be plenty sturdy. I always use glue and screws in projects like this for extra insurance and to add rigidity (Titebond III?).

I would expect a lot of bowing to the tank. If the seams can withstand the bowing the plywood should be plenty strong enough to not burst. I would strongly consider a center support in the middle of the tank to control the bowing even though it will be a nuisance for working in the tank.

I would be a little concerned about the long term effects of the plywood bottom sitting on the ground / cement and retaining a puddle of water between the plywood and the ground. A sheet of fiberglass on the bottom (or all around) would probably go a long way in ensuring the tank life.

thanks mate great info

that makes alot of sense i never even considered what splitting the layers would do.
that should solve my fear of the joints splitting.

thanks alot for the help guys

 

[Oughtsix]