View Full Version : 2000 gal plywood sump

09/18/2017, 07:23 PM
ok. I'll show the picture first:

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it's two tanks: 600 and 1400 gals.

the water level is 20"

the structure is 3/4" plywood with 2x12 frame (yes, I realize that it's supposed to go the other way, but I'm tight on space).

The large tank is frame by the wood structure on one side and the sunroom steel frame on the other. So it's squeezed into shape.

I also added two 2x4s in the middle to brace the center.

They're covered in 1/4" of 2 part epoxy.

will it hold?

09/18/2017, 07:39 PM
Can you do a eurobrace? I would feel better with a brace. But I think you'd be fine without it.

09/18/2017, 07:57 PM
It's hard to epoxy when there are too many joining interfaces. I considered running PVC pipe through the ends to create cross spans. I might do that instead of the 2x4 braces.

09/19/2017, 05:57 PM
The joinery will be your biggest difficulty. How are you planning to actually build the tanks?

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09/19/2017, 06:25 PM
I've built plywood tanks before. The joints are just wood glue and screws.
The acrylic soaks into the wood and forms a contiguous plastic surface.

09/19/2017, 07:25 PM
I decided to use concrete block epoxied to the concrete floor to act as a permanent stop against the outside edges.

I also added a 1" stainless steel square tube all around the rim.

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I put a model (my size) to scale

09/19/2017, 08:33 PM
When I built a large plywood tank years ago, I made and epoxied the main box with dual thicknesses of 3/4 ply. I then made a single 3/4 thick eurobrace top (think the top on an acrylic tank), epoxied it on all surfaces then screwed it down into the box and ran a bead of silicone in the top joint.

09/19/2017, 08:59 PM
Was the top eurobrace plywood also? Or acrylic?

How wide was it?

In my case, I would have to make it of multiple sections... would it still be effective? Maybe use 2x4s to get the continuity of a single block or sandwitch two offset 1/2" plywood sheet sections to avoid the butt joint stress.

09/19/2017, 09:03 PM
And.. is it really necessary given that the perimeter is supported by external walls and structures?

09/20/2017, 12:08 AM
If you're going to epoxy bricks to the floor why not do a cement or cinderblock frame?

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09/20/2017, 06:17 AM
To do it right, I'd need to use rebar and create a wood frame to pour concrete... since I'm making a wood frame, I might as well just epoxy that. :D

09/20/2017, 06:57 AM
I would think with the 1" square tubing, if welded at the corners would be sufficient...especially if you went with 3/16" wall. Alternatively, you could do 2" angle iron, 3/16" thickness. Lay it over the top edge of the plywood like eurobrace would be. Then weld in flat stock across the span. 3/16" should have more then enough rigidity though either way you go

09/20/2017, 07:00 AM
Also, what is this for? Those are huge sumps. <--(captain obvious)

09/20/2017, 07:24 AM
Was the top eurobrace plywood also? Or acrylic?

How wide was it?

In my case, I would have to make it of multiple sections... would it still be effective? Maybe use 2x4s to get the continuity of a single block or sandwitch two offset 1/2" plywood sheet sections to avoid the butt joint stress.

Yes, eurobrace was a full sheet of plywood with cutouts. Footprint of the tank was essentially 4x8. Unless you're freakishly tall, the pictures suggest you'd be able to use a coupe of sheets with the 8' length across the sumps. Perhaps not necessary if you are going to reinforce the sides with blocks/cement. 2x12 will bow laterally though.

on the spot
09/20/2017, 07:31 AM
Also, what is this for? Those are huge sumps. <--(captain obvious)

I've been curious about that too.

and why you wouldn't use closed poly tanks for some of that volume...

09/20/2017, 08:42 AM
Cost. The tanks would cost ~ $2000
The DT is ~ 1500gal and the system is closer to 4000gal


09/20/2017, 08:50 AM
Yes, eurobrace was a full sheet of plywood with cutouts. Footprint of the tank was essentially 4x8. Unless you're freakishly tall, the pictures suggest you'd be able to use a coupe of sheets with the 8' length across the sumps. Perhaps not necessary if you are going to reinforce the sides with blocks/cement. 2x12 will bow laterally though.

The big tank is 13' x 8' and made of 4 plywood sheets on the bottom.

Yes, the 2x12s are used in the wrong direction... but they're sandwiched against the framework of the room and the concrete block against the floor.

I'm expecting the 1" square tube brace to mitigate the bowing.

09/20/2017, 09:01 AM
You're fine.
Could just do some 2x6" crossbraces if worried but you'll be good with the metal around it

My 860g plywood tank I eurobraced with 2x4's on sides and back and 2x6 on front(since I lean on it to get to bottom of tank) and the cross braced it with 2x6 and 2x4s on top of those as well. No deflection at all

09/20/2017, 09:31 AM
As an engineer and not having done any actual calculations my gut says you're probably okay. My worry are the butt joints you've drawn in with the 2x12s. Use a full length one.
Better do a steel 'exoskeleton' like mentioned before with only the plywood.
I'd consider going that then just a large pond liner.
Not sure what would be more expensive, the liner or epoxy.

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09/20/2017, 09:50 AM
Hmmm.. steel tube is more expensive than 2x12s, but maybe I can use less.

But since the plywood sheets would be butt-jointed, I would need twice as much plywood without the 2x12 frame. I think the wood with a single steel "brace" at the top and concrete "brakes" at the bottom is just cheaper.

Two part epoxy is not cheap, but I'll be buying it in 5 gallon drums for this project, so - probably have enough.

on the spot
09/20/2017, 10:36 AM
closed poly tanks for some of that volume...

Cost. The tanks would cost ~ $2000...

maybe. IBC's with new poly tanks are $200 for 275 gallon capacity. rinsed sugar tanks about $80 or so there around Dallas.

but you have a big door? drawn on the greenhouse - 1500 gallon tanks are $600 bucks.

right not the thing to look at, but still an economical option.

regardless of how it goes, it is fun to watch.

GL moving forward.

09/20/2017, 10:36 AM
My 360g wood tank had a double layer of 3/4 plywood eurobrace about 4" wide. I measured for deflection and there was literally zero. I would have been happy with "some."

09/20/2017, 11:12 AM
There's a garage door in that back section of the sunroom. But the 1500 gal tank costs $900 and the small one is $600 (before shipping)... so ~$2000 shipped


IBCs are commodity and used ones sell for peanuts but I can't make a giant open sump with them.

09/20/2017, 11:16 AM
Which is stiffer over a 13' span:

1" square stainless tube?
4" x 1.5" plywood eurobrace (1.5" ~ 2 x 3/4" overlapping joints)

I think it's the plywood but by how much?

on the spot
09/20/2017, 06:38 PM
...I can't make a giant open sump with them.

There, that part about the giant and the open.

What's it for?

I've seen most of your threads on the build but must have missed that post on the why you want so much open capacity in the sumps.

That's also why I said 600 for the 1500 gallons tank. I was thinking the giant closed water tanks, not the open trough.

any way it comes together I'm still fascinated.

Best of luck.

09/20/2017, 08:05 PM
Ah... my tank has multiple systems that can surge/flood. My experience is that the sump needs to be large enough to accommodate a complete tank drain with these kinds of systems. The reality is that this is an acquired rule of thumb, not an absolute measure.

Second, I need a very slow flow with the widest possible cross-section to allow the water to fully clear of bubbles. The configuration is such that the "travel" is 8' and the width of the tank is 13'. That means that the water in the sump is moving at 1/3 the highest flow in the DT at 3x the width.

This might seem excessive but this "reciprocal rule of thumb" allows me to do just about anything I want with the water flow and still have a reasonably clear tank.

Sump (1300gal) ~ DT (1600gal) in volume
Sump W (13') ~ DT L (12')
Sump L (8') ~ DT W (8')

I'm also using a large settling filter to extract particulates for re-injection as food...

So you're thinking ...??!?!

These mechanisms don't change the bubbles and particulates in my system. They allow me to control when and where they appear in my DT without compromising two key variables: food (~particulates) and flow (~bubbles).

09/20/2017, 08:21 PM
So if I were you Id reshape your tanks with the max length being 8 simply because thats the length of plywood. Then you dont have to worry about joints besides those at the corners.

With the way you have it 3/4 plywood with a plywood eurobrace will Work with caution.

What epoxy are you planning on using?

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09/20/2017, 10:02 PM
I've used US Composites 635 for years... great stuff


15 gallons is ~ $600

The tank is 8' long... but 13' wide.

09/20/2017, 10:04 PM
also, my DT is 8' x 12' with a plywood floor.

But that's made with 4 layers = 3" thick of overlapping plywood seams. check out the video that walks through the build.

I've done this kind of thing on a smaller scale before:


I've built a cube 24x24x24 and a plywood sump 72x24x24

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09/21/2017, 08:05 PM
From an engineering standpoint, whatever you did that works for a 24 depth tank will Work for the 18 depth tank in theory regardless of length and width

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09/23/2017, 08:38 AM
Ok... so wild idea...

What if I used the concrete slab floor and plywood sides?

The floor will already have epoxy on it.

I would use epoxy to glue the sides. The rest stays the same.

The wood to wood butt joint would be replaced with an epoxy (on wood) to epoxy (on slab) joint.

Once the two are attached, I would apply an epoxy filet and a couple more layers over that.

The plywood bottom wasn't really doing anything. It would save ~ $300 in wood and about $100 in epoxy given that I was planning on costing the bottom floor side of the bottom plywood to be fully sealed.

I am giving up flexibility in redesign, but I can probably redesign around that sump:

Also, I am expecting to have to walk into the sump to access the far wall... and even using a fiberglass ladder in it to access the raised fuge... concrete + epoxy directly should be more durable?

09/23/2017, 06:28 PM
Makes sense to me. With that route Id consider doing block walls so you can sit, step or put stuff on them.


The only issue I see with this would be cracks. If cracks develop in the concrete floor it will go through to the foundation. Honestly not really a problem, more of something to be aware of.

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09/23/2017, 07:13 PM
The wood has more continuous support and should be cheaper.

I've vacillated between concrete block, rebar and concrete, and wood...

Block is heavy and has weaknesses in the joints.
Rebar wall would be very messy and a lot of work.
Wood should be cheap, light and relatively easy.

09/23/2017, 07:17 PM
The floor is 18" triple rebar construction... that's why it's so expensive.

With drylok and epoxy, it should be near impregnable?

09/23/2017, 09:43 PM
I've vacillated between concrete block, rebar and concrete, and wood...

Re-bar & concrete got my attention. Have you priced making the entire sump walls with poured concrete & re-bar supports? The re-bar can be epox'd into the existing slab as upright supports & re-wire used to add lateral . You can build the form & pour the concrete in small stages, so you're not purchasing a bunch of plywood for forms, to then be trashed after a single pour. You are also not limited by the lumber dimensions & won't have to employ structural walls as added security. Just wanted to give you another avenue. Best of luck with your build

09/23/2017, 10:06 PM
Actually, that's why I went to plywood. If I'm using the wood forms to pour concrete, I can just leave the wood and coat it in epoxy. It's just less work to get the desired result.

I could make smaller pours but concrete already takes a long time to cure and building it continuously should provide the greatest strength, I believe. It's also a lot of perimeter to pour in small sections.

I was never a big fan of wood really. But I have to admit that its strength to weight ratio does make it a very convenient building material. It won't stop a truck - concrete will. But for 99% of the time, it's a lower cost, lighter weight, strong and convenient building material.

Its weakness is its longevity. While rebar and concrete could last 100 years (more), wood would eventually rot, mold, flake, burn, dry, be eaten ... or otherwise disintegrate. Epoxy changes the math though. Fully epoxy encapsulated plywood has the best of both worlds.

It could survive a flood, hurricane- not sure about tornados, earthquakes and fire though :D

09/23/2017, 10:42 PM
I could make smaller pours but concrete already takes a long time to cure and building it continuously should provide the greatest strength, I believe.

I understand your choice for all the reasons you stated, but just wondered if you had crunched the numbers. The form could be constructed with 1/4" ply or even hardboard sheet, so long as it is braced.

Concrete can be poured in stages with no loss in strength. The strength comes from the type of concrete & how dry the pour is (drier is stronger)

If you build with ply, have you considered constructing a center brace/ cross braces ( really a tie or ties) to help prevent the side walls bowing out ? A length of small angle iron, a 2x2 or even some PVC pipe would work. HTH

09/23/2017, 10:52 PM
I have considered both kinds of bracing actually (see above), but I really need an open space.

I am planning on a 1" square stainless steel tube around the outside perimeter. While it is possible to deflect a 13' stretch of 1" square tubing, I don't believe the force exerted by 20" of water would be sufficient.

On one side!!!
The support is 3/4" plywood + 2x12 beams + 1" square ss + 1/4" epoxy

And in the other corner!!!
Pressure of 20" of water applied over 13' span


Would a eurobrace top make it even more solid? Absolutely! Do I need it? I don't think so.

I can always brace is once I fill it and measure actual deflection...

09/23/2017, 10:56 PM
Confession: working with concrete is also much dirtier and might require renting equipment. I also gravitate towards my comfort zone, especially if the cost differential is low.

Have I calculated the full cost including material and time? No :(
Have I "rule of thumbed it" and considered that the benefits of the alternate way will likely be less than $100 - yup :)

09/24/2017, 06:21 AM
Very good point: work with what you are more comfortable with. Best of luck & I'll be waiting to see the finished project.

09/25/2017, 03:10 AM
I like your idea of using the concrete floor and wood sides. Mind you, this is not based on any knowledge or data. :0)

Would you run pipes in the floor to cool/heat the sump?

09/25/2017, 07:59 AM
You mean a fresh water or other coolant loop in the concrete?

Could even use copper since it's encased in concrete and epoxy I guess..

I need to think about this. I didn't consider the thermal benefits or penalties of the slab's direct contact... good point.

09/25/2017, 11:23 AM
Thinking it through... the large surface allows me to use a large PEX tubing to cool or heat. I think embedding it in the concrete may be an unnecessary complication and maintenance risk? The exception would be using copper lines for a freon loop (compressor, external radiator and fan) to really cool the concrete. But still- expansion/contraction... feels so risky

I do plan on having gas so running hot water would make for a very efficienct heating loop.

Intrinsically though, the current design should keep the slab very cool... the back room is shaded constantly. The sunroom has a false wooden floor that would keep the slab there shaded as well. So with limited radiated energy and being intimately heatsunk to the shaded earth below, it should help "normalize" the temperature... cooler in the summer, warmer in the winter?

The large underground cooling loops will do this even more being at 10' deep. The air loop will cool the air & the saltwater loop will cool the water. The heat exchangers are the buried pipes directly.. no secondary heat exchangers.

09/26/2017, 07:10 PM
Having the concrete floor as part of the structure and waterproofing of the tank scares me a bit, honestly. It's putting all your eggs in one basket. You basically have no room for issues, there is a single point of failure. If it were me, I'd go with the all-wood construction and just let it sit on the concrete. At the very least, this will allow for any flex or expansion in the wood/epoxy that might be at different rates than the concrete. You're talking about a very large tank - dynamics that don't matter in typically sized tanks are going to be multiplied.

09/26/2017, 07:55 PM
True, but it's 18" of concrete with three layers of rebar coated in drylock.

It's basically as watertight as a pool... so not worried about the concrete cracking.

The wood-concrete interface.... that could exhibit different expansion/contraction or at least fatigue the joints over time. But the interface is epoxy so very hard. The weakest link would break first - epoxy or wood?

There are also two kinds of wood. Plywood inside and 2x12 outside. The 2x12s should be very strong in tension. I would expect the plywood to be strong in tension and compression.

Hard to call.

Would the sidewalls made on concrete block be less prone to failure since the expansion coeff should be the same?

09/26/2017, 07:58 PM
Is there a precedence like this problem? Don't we use wood studs and beams over concrete floors? Is there relief built in?

09/27/2017, 04:36 AM
Don't we use wood studs and beams over concrete floors?

Yes but if the wood shifts a millimeter on the concrete, there aren't thousands of gallons at stake.

When I disassembled my 360g, the weakest link was epoxy bonding to already cured epoxy, even when proper surface prep had happened. It was still strong but that was clearly the weakest part of the entire structure. Your plan revolves around that type of bond in the most critical joint, and it's between dissimilar materials and under changing pressure from all your surges etc. Hence my reservations!

09/27/2017, 05:37 AM
Way to strike FUD - fear, uncertainty and doubt


I guess if I want to take advantage of the cool slab without the hassle of forms or rebar, the only remaining option is to use a 3 layer high concrete block wall ~ 24"

09/27/2017, 06:13 AM
Something to think about. If for some reason the wood failed it would likely result in a total blowout. For example the epoxy developed a crack/hole allowing water into the wood causing rot. You might not notice until it's too late due to the epoxy coating hiding it. The epoxy might be tough but your talking about walking, setting up ladders & working in there over a period of years. Now if you went with poured concrete and it started to fail you would be much more likely to notice a crack giving you time to fix the issue before total failure. Think of it as a small swimming pool, would you rather have one made of concrete or plywood? It might not cost as much as you think especially if your having the room built at the same time. Even if it does take longer and cost a bit more, why would you go cheap on the most important part of the entire system? Your ideas/plan might turn out great & likely will. Personally I'd be waking up with nightmares playing through my head, but that's just my own paranoia & I tend to WAY overbuild everything.
Good luck with the build & I can't wait to see what you have in store for the finished system.

09/27/2017, 06:57 AM
Wood is very forgiving in terms of noticing and solving problems. It tends to fail very slowly. If the wood core gets wet and starts to delaminate, you will get a slowly developing bow in most cases. I've seen wood tanks that had run partially failed for months. I had a local guy who had a tank with a bad leak that damaged the wood and ended up bowing several inches. He fixed the leak and kept running it with the bow. I wouldn't have but there was no catastrophic failure. My own tank had a pinhole underwater allowing water into the eurobrace for months. I lowered the water level an inch so the pinhole was above the water line and kept running it, even with the damaged wood core. A year or so later I drained the tank and ran fiberglass over the damage. The tank ran for several years after that.

I can't speak to personal experience with concrete but I can say wood tanks are very forgiving.

09/27/2017, 12:05 PM
Actually- I think wood is "self-healing" as it rots.... basically, the water that causes it to swell can actually create a seal stopping additional leakage.

It's bizzare but I've seen it in my test builds. Over time, the swollen wood weakens and will eventually fail, but imagine a glass that self seals for reference!

Also, the wood is actually sealed before being glued to the concrete so the failure would be an epoxy-epoxy failure. The wood-epoxy bond is insanely strong... stronger even than the wood alone or epoxy alone. The wood fibers "drink up" the epoxy creating a new matrix composite medium (assuming proper surface treatment).

Concrete-epoxy is also very strong. The concrete is porous.

I don't know about concrete-drylok-epoxy. I may choose to eliminate the sealant to avoid introducing weaknesses.

09/27/2017, 06:06 PM
Some day I would like to build a tank with foam cores instead of wood. Fiberglass in the epoxy. It would be strong, light, inherently waterproof, totally inert and stable, and it would last forever. Either plain insulation foam (blue or pink) or foam honeycomb.

09/27/2017, 09:21 PM
With epoxy inside and out?

09/28/2017, 04:13 AM
Epoxy with heavy glass cloth.

09/28/2017, 06:26 AM
Ok. So it's basically using the foam as a form for a fiberglass tank.

As long as the foam was fully encased, it should be comparable to the plastic tanks. At this scale though, they all use ribs or ridges to increase strength so you'll probably have to engineer that in.

Fiberglass isn't cheap either - so I'll go with wood or blocks.

I'm really intrigued by the potential for the slab to passively cool the water. That's probably why I'm pausing on just going with the plywood floor.

09/30/2017, 07:43 AM
The strength comes from the thickness of the foam - at 2" thick you're getting an incredible advantage versus a 3/4" thick wood tank or a ribbed frp tank or just a thin fiberglass tank. Any tank wall has tension on the ouside and compression on the inside - with a thicker wall core, you're basically giving leverage to the fibers under tension. If you leave the inside and outside skins alone and just double the wall thickness, the stiffness goes through the roof. You could get even more strength with careful cloth selection. In the end this approach wouldn't make much practical or financial sense compared to a wood tank, but it would be a super fun engineering project.

With regards to passive cooling - why not just put a ground source loop in under the concrete and circulate the water through a heat exchanger? Then you're taking the concrete out of the picture - after all, ultimately, the cooling effect is coming from the earth under the slab, not from the slab itself. Could just use a coil in the air to cool the room, wouldn't have to be in the water. With that much water and that much surface area, you wouldn't really need to cool the water directly. If you concentrated on keeping the entire room at the right temp, the water would be there naturally.

09/30/2017, 08:19 AM
I have plenty of alternative cooling mechanisms in the design:

1. Geothermal air cooling loop (10' deep )
2. Geothermal saltwater cooling loop (also 10')
3. Chiller
4. Evaporator tower
5. Blower fan over the 13' length of the sump

I was looking for a freebie, especially a passive freebie.

I'm going back to all wood. In case I want to make drastic changes, this would be easiest to change. :(

You can see my cooling loops in the first few minutes of my build video

09/30/2017, 08:24 AM
<iframe width="560" height="315" src="https://www.youtube.com/embed/z00RZimQGks" frameborder="0" allowfullscreen></iframe>

09/30/2017, 08:30 AM
OK you've got cooling covered!

I would probably end up with wood as well. It's forgiving, cheap, relatively easy to DIY, and easy to change later. Do it right and it'll last as long as the concrete IMHO.

09/30/2017, 09:09 AM
I started with cooling actually. Solar radiation is 1050 W/m2 ~ 11.3KW/ft2
The sunroom is 25' x 20' so ~ 5.7MW ..... silly kind of numbers
The display tank alone is 9' x 12' ~ 1.2MW
That includes light as well as heat, of course. Still silly big numbers before the greenhouse effect.