Which size plywood? - The Planted Tank Forum
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post #1 of 26 (permalink) Old 05-18-2020, 03:10 AM Thread Starter
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Which size plywood?

I am building a stand for my 40 breeder and need help choosing the thickness of the plywood. Reason being, it will hold a 40 but was actually built for a 75/90/120 gallon for my next tank when I upgrade (wife doesnt know that part lol). So for now it wont sit on the ends but in the middle or front and because of that I did include a center brace.

I know 1/2 would work but would 1/4 work? It will be hardwood plywood in case I stain it instead of painting it. Heres a pic on its side but so you can see the construction type. Click image for larger version

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1/4 is 1/2 the price of 1/2 and Id like it to be as thin as possible for the finished look.


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post #2 of 26 (permalink) Old 05-18-2020, 05:32 AM
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Acrylic or Glass?

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post #3 of 26 (permalink) Old 05-18-2020, 06:03 AM Thread Starter
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Originally Posted by Johntheskier View Post
Acrylic or Glass?

Glass so the weight will sit on its edges


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post #4 of 26 (permalink) Old 05-18-2020, 10:39 PM
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I'm assuming in that pic that it's standing on end? If so it looks like the 2x4s should provide all the strength you need to put a tank on it. I built a similar one for my 29g and skinned it with 1/4" hardwood and it's done fine for quite a while now. Structurally it should help with the racking, but I don't think you'll necessarily need it.


Disclaimer: I'm not a great carpenter, there's a LOT of other people here with much more experience than me. Hopefully some of them will come along and help educate both of us as I'm still working on getting better.
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post #5 of 26 (permalink) Old 05-18-2020, 10:44 PM Thread Starter
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Yes the stand is currently on its side I just posted a picture like that so yall could see how the supports are constructed and all of that. Im just so used to building stands that are the exact width dimensions of the aquarium itself which will be the case long-term but just not right now as the tank that will sit on it will be 6 inches shorter on either side


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post #6 of 26 (permalink) Old 05-18-2020, 11:52 PM
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1/4 inch should be fine . If you are worried at all about racking just glue and nail/screw it to the frame and it aint going nowhere...lol
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post #7 of 26 (permalink) Old 05-19-2020, 09:14 PM Thread Starter
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1/4 inch should be fine . If you are worried at all about racking just glue and nail/screw it to the frame and it aint going nowhere...lol

Thanks for the reply!

I added an extra support to the middle. Now I feel much safer about the tank being more in the middle of the stand. There will be 6s on either side of the tank.
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post #8 of 26 (permalink) Old 05-19-2020, 10:22 PM
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Stupid question , but are you going to put plywood on the top ? I would use more than 1/4 for that .
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post #9 of 26 (permalink) Old 05-19-2020, 10:58 PM Thread Starter
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Which size plywood?

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Stupid question , but are you going to put plywood on the top ? I would use more than 1/4 for that .

Lol yeah. Top, sides and doors in the front. Inside of the stand will also be plywood reaching all the way front, back and side to side and Ill cut the corners so it will be all one piece. The 40 to the right of the stand will sit in there with a canister filter and two co2 tanks and then there will be another 40 on top which will eventually get bigger. You thinking 1/2 or greater for the top?


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post #10 of 26 (permalink) Old 05-19-2020, 11:08 PM
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Yea at least 1/2 because you will spill water on it and thicker won't warp as easy . I overbuild everything though so take that in consideration...lol Also if it won't be sitting on the edges you want thicker to hold the weight over the spans . I fact if it were me I would put a brace between the end and middle on each side . In the center of the space .
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post #11 of 26 (permalink) Old 05-19-2020, 11:28 PM Thread Starter
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Yea at least 1/2 because you will spill water on it and thicker won't warp as easy . I overbuild everything though so take that in consideration...lol Also if it won't be sitting on the edges you want thicker to hold the weight over the spans . I fact if it were me I would put a brace between the end and middle on each side . In the center of the space .

Yeah I might as well do that. Just one more board to buy. Ill get that when I go get the plywood.


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post #12 of 26 (permalink) Old 05-20-2020, 12:41 AM
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Structural engineer here, I recommend you look up plywood span tables. Lumber manufacturers publish tables that tell you the load carrying capacity of a sheet of plywood based on the thickness and the span (the distance between supports).

Example is Page 4 of this document: http://www.pacificwoodlaminates.com/...SpanTables.pdf

Essentially, the tables published will give you a maximum weight per square foot that the plywood is rated for depending on how far you space the supports.

A 40 breeder is 40 gallons (roughly 340 lbs) over a 36inx18in area (about 4.5 sqft) which means you're looking at a psf value of 76psf.

You need to look up the available plywood type that you have (in terms of grain pattern) and from that you can decide if the 1/4" plywood is capable of carrying the 76 psf based on the minimum dimension you have between supporting members.

You also want to go L/360, as that is deflection that will occur in the panel over that span, since you don't want the bottom of your tank bowing, L/360 is the best criteria for you.

1/4" plywood isn't really a structural plywood, so not much data is published. But if you were to go a 3/8" sheet for example, you'd need supports at every 16" at a minimum (which can take 79 psf before it becomes worrisome with regards to having too much bending).

Last edited by stlhokie; 05-20-2020 at 12:55 AM. Reason: Typos
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post #13 of 26 (permalink) Old 05-20-2020, 02:39 AM Thread Starter
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Which size plywood?

Quote:
Originally Posted by stlhokie View Post
Structural engineer here, I recommend you look up plywood span tables. Lumber manufacturers publish tables that tell you the load carrying capacity of a sheet of plywood based on the thickness and the span (the distance between supports).

Example is Page 4 of this document: http://www.pacificwoodlaminates.com/...SpanTables.pdf

Essentially, the tables published will give you a maximum weight per square foot that the plywood is rated for depending on how far you space the supports.

A 40 breeder is 40 gallons (roughly 340 lbs) over a 36inx18in area (about 4.5 sqft) which means you're looking at a psf value of 76psf.

You need to look up the available plywood type that you have (in terms of grain pattern) and from that you can decide if the 1/4" plywood is capable of carrying the 76 psf based on the minimum dimension you have between supporting members.

You also want to go L/360, as that is deflection that will occur in the panel over that span, since you don't want the bottom of your tank bowing, L/360 is the best criteria for you.

1/4" plywood isn't really a structural plywood, so not much data is published. But if you were to go a 3/8" sheet for example, you'd need supports at every 16" at a minimum (which can take 79 psf before it becomes worrisome with regards to having too much bending).

Thanks very much for the link and explanation!


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post #14 of 26 (permalink) Old 05-20-2020, 02:52 AM
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If this is a glass tank, none of the load will really be carried by the plywood though, right?

I usually use 1/2" to 3/4" plywood on tops and bottoms, and 1/4" on the sides and fronts. If you are using plywood on the top, there really isn't any need for additional cross braces there in my opinion. They'll only serve to make the stand heavier.

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post #15 of 26 (permalink) Old 05-20-2020, 03:45 AM Thread Starter
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If this is a glass tank, none of the load will really be carried by the plywood though, right?

I usually use 1/2" to 3/4" plywood on tops and bottoms, and 1/4" on the sides and fronts. If you are using plywood on the top, there really isn't any need for additional cross braces there in my opinion. They'll only serve to make the stand heavier.

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The stand is being made to eventually hold a 75/90/120 but will holding a 40 breeder for probably 6 months or more. So it can sit on one major end support but not more so I need to make sure the middle can handle the weight on its own or at least part of the weight.

I was looking at hardwood plywood before when originally thinking about this but maybe Ill go with a 23/32 sanded plywood (so I can prep it easier to stain) in addition to adding the two extra supports. I think I will also do a rear center vertical brace connecting to the top braces in the back of the stand. So that should be two more 2x4s (96) and two pieces of plywood at 23/32. Its annoying that the length is 48.5 so I need to buy the second sheet. Ill have scraps to do the sides and doors at least but Ill still need to buy a trim board for the doors ugh. And stain lol


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