DIY Stand question

[USMA36]

I’m starting my 240 gallon tank stand. I have a couple of questions. I will be using 2x6 instead of 2x4. My first question is it ok to have a 4’ section not supported by uprights? The tank dimensions are 72” long and 30” deep. I want to have a 48” open section in the back so I have full access to my sump from the rear. The front would have 2 uprights (4 if you count the corners) next how many cross beams are needed? Again I plan on one at 18” then having 48” open without any cross bracing. Once that is done both the top and bottom of the stand will have 3/4 plywood. I want to use the full height of the stand without losing space due to the cross braces. Lastly most diy stand I see have several cross supports in the bottom and they place the plywood on top of those cross supports. I’d like to conserve space and sit the stand on top of the plywood and have the plywood directly on the floor. The 48’ open section will hold the sump. The remaining 24” section will have a partition inside and will be used as a controller compartment/board I hope I painted a clear enough picture on what I’m trying to accomplish.

 

[Soren]

I assume you want to access the 24" controller portion from the front?

If I am understanding your questions correctly, it would probably be fine to use a design like I have pictured below and as shown in the attached eDrawings model (viewing software is a free download).
The boards on the inside of the legs are 2x4's to keep them from protruding past the edges of the 2x6's. All other boards are 2x6's as you stated. Cross-braces under the top surface could be either 2x4's or 2x6's, and are probably mostly for additional stability than actual support. Cross-braces are also added to give support to the middle span of the plywood, but this is not really necessary for tanks with rim frames, since all the load is on the bottom glass and transferred to the outside edges.

...that is, assuming your tank is a traditional glass tank with top and bottom framing trim. If this is the case, almost all load will be transferred through the stand legs on the 4 corners, so the uprights in the middle are just for extra security for material failure and might be able to be left out entirely (though I recommend all that are possible without obstructing access).

I do not have a lot of experience in tank stand building to give you confidence in my suggestions, but I work as an agricultural structural engineer and have some carpentry experience. I just recently built my own 90-gallon aquarium stand to allow for a 75-gallon sump, so the back of my stand has one upright in the middle and no uprights in the front to allow for placing and accessing the sump from the front. My setup is less than half the weight, though, and only 48" long, so it is not directly comparable. The main difference in my design versus your design is that I ran 2x6's between the frame top and the top plywood along the full front and back length as well as one in the middle (as shown in the model). These were screwed into the front and back boards of the top frame to make them into a sort of angle to increase stiffness and support strength without uprights. I would recommend the same for your design to ensure the boards do not bow, but this is probably over-designed and might be just fine if you use just the plywood and fasten it to all boards touching the underside of the sheet. I plan to custom-build a countertop from 3/4" plywood and laminate sheet to put over the 2x6's on top of my stand frame.

Using plywood directly on the floor would be fine structurally, assuming the floor is strong enough to support a full sump directly on it (I have little doubt this is your case, but I am no professional). The main downsides of plywood on the floor directly are that water may seep under the plywood and have no way to dry while ruining the flooring underneath, and there is no good way to level the stand (though you can do this during construction or on the top surface).

...(Edit) and I forgot to mention that I would include at least the diagonal braces on the ends to prevent frame twist. The braces should go from top corner against the wall (if possible) down to bottom opposite corner. Diagonals in at least one opening on both front and back would also help stability, but are not necessary if you use 2 or more screws vertically in each 2x6. The screws spaced apart should offer enough moment-stiffness to prevent the frame from twisting and collapsing.

Feel free to ask questions about any aspect of this!

This picture is looking from the back with the 48" opening shown.

Here is another angle showing the back from the opposite corner and showing the front braces better.

 

[Soren]

...also, screw location is fairly important for proper strength from a design like this.
Here is an image showing all visible screw heads from this angle and showing the typical concept of where the screws should be. Again, I can explain this in more detail as necessary for structural considerations.

 

[USMA36]

Thank you for the detailed answer. That’s basically the design I’d be building. The back will be open to My fish room the front and sides will be skinned with 3/4 red oak plywood and have doors. The stand is in my basement so weight won’t be an issue. The tank is euro braced on top traditional bracing on the bottom.

 

[RocketEngineer]

What are the full set of tank dimensions? I saw 72 long, but is the 30” the height or the width? Depth can mean either.

Note, you only need the larger beams in the top, the legs can stay 2X4 which gives you a bit more access.

The main purpose of a cross beam in the top is to keep the long beams from twisting. In the bottom, the plywood underneath is just fine.

 

[USMA36]

What are the full set of tank dimensions? I saw 72 long, but is the 30” the height or the width? Depth can mean either.

Note, you only need the larger beams in the top, the legs can stay 2X4 which gives you a bit more access.

The main purpose of a cross beam in the top is to keep the long beams from twisting. In the bottom, the plywood underneath is just fine.

Thanks for your help. The tank is 72x30x25 the 25 is how deep the water is. I was planning one cross brace at 24” if you think I’ll need more I’ll add another at 24” on the other side. Main reason for the 2x6 is I have it on hand already, but maybe I’ll rip a couple of the 2x6 down to 2x4. Can 2x4 be used as cross bracing with the 2x6? The top and bottom will both have 3/4 plywood I am hoping that will also prevent twisting. The plywood will be on the very bottom making somewhat of a tub that I will line with a shower liner to catch spills. The skin will be 1/2 inch hardwood plywood.

 

[Sleepydoc]

Also, I would glue in addition to screwing. Screws can loosen as the wood dries, especially in softer wood like pine. Glue takes the load and spreads it out over the entire surface rather than just a single point like a screw.

 

[Metcho]

Agreed with sleepydoc glue and screw it together

 

[USMA36]

Anyone ever have an issue with wood twisting or warping? My 2x6 are kiln dried Home Depot boards but I’m sure they still have some moisture in them. They say you should leave the boards sit in the room they’ll be used for 6 to 8 weeks. I’d rather not have to do that.

 

[Paul B]

That would be fine. My 125 gallon has the same dimensions 6' long and I built it out of Red Wood 2/4"

I did that because redwood does not warp like pine or pressure treated does.
That Home Depot, Kiln dried wood will warp. It is cheap pine and no matter what you put on it, it can warp. I would go for a little more money at least on the top rails.

I have a 6' area in the front with no verticle supports as all the weight in a tank is at the corners.

The two 45 degree pieces in the back is for lateral support. I purposely built the legs out of 4 X 4s because I didn't want anyplace where water can get in between two layers of wood. From experience I know that if water gets on a sandwiched piece of wood, it will go all the way through and never dry. The top front where there are no supports I built out of 2 2 x 4" and sealed them together with silicone.

This stand has been up about 3 years now. This is the finished stand, no plywood at all as it is built behind a movable wall. All hardware is hot dipped galvanized lag bolts or hot dipped galvanized threaded through bolts. I would not use any screws. My last tank was in the same place for 40 years and I built that stand. This is very simple, very strong construction.
It's solid as a rock.

 

[Metcho]

I built mine out of all kiln dried pine and let it sit inside my place for two weeks before building the put all together and used rubberized paint to paint it all. Kinda like a waterproof barrier. It’s been good with no problems for 4 years on my other smaller tanks and almost a year on my 225 peninsula