View Full Version : slab thickness

01/13/2017, 07:20 PM
I'm putting a tank in the backyard. It'll be surrounded with a greenhouse structure.

How thick does a concrete slab need to be to support a 6' x 8' x 3.5' tank?

~ 1250gal ~ 10500lbs ~ 5.2 tons

But it's only 1.5 psi in pressure (due to the large surface area). That's less than a person standing.


4" porch slab
6" slab with rebar reinforcement
12" with integral footers and bottom rebar reinforcement

01/14/2017, 01:28 AM
What is the likelihood of ground frost/freezing in your area?


01/14/2017, 08:59 AM
Glad to see that you're going to do it! Good luck.

01/14/2017, 09:15 AM
It rarely freeze. Maybe one week out of the year. Why?

01/14/2017, 10:39 AM
Because frost will heave the ground up and then when the frost melts the earth will subside. Buried things tend to get pitched over sideways unless you dig down deep below the frost line and install proper footings and support.


01/14/2017, 11:16 AM
I would like to have a covered walkway (or even a direct doorway) to the house, so the slab can't really float away from the house. In that case, sounds like I need a solid footing and potentially integrated rebar)...

01/16/2017, 11:50 AM
other than freezing, no feedback?

01/16/2017, 12:27 PM
I don't have any experience in building a slab for an aquarium, but I do have experience with concrete slabs and kennels. One of the things I learned from building a structure with a concrete slab is you need to make sure there is a way to drain off water so that it does not stand. If you are in a humid climate it will not need to rain in order for water to collect either. Where I lived it did snow and ice a few weeks out of the year and a 16 inch deep slab was more than enough to keep things stable (it supported a 30x35 foot concrete block building). We did have rebar laid out flat on the underside but nothing other than temporary frame for the edges until it dried. I think though, if I were going to build an actual out door aquarium spot (and I have thought about it) I would not pour a slab. Slabs tend to hold the smells of the water, they stain (and there is just about nothing that can stand up to dog urine might be different for other stressors but why chance it if there is a better alternative) and with a concrete foundation, you'd have just about no chance of saving fish that jump out and hit the ground or if you drop coral. A solid base for the aquarium would be great, but the ground surrounding it might do better to be pea gravel or some other type of filtering ground cover that will drain water, get air circulation, and be enjoyable to stand on for an extended amount of time. Not to mention if you drop something maybe it won't "shatter" on impact. Concrete also tends to hold temperatures and will sweat. Back to the humidity and standing water issue.

I might totally change my mind if you could provide a mock up of your potential design. However, if it were me, and I wanted a "floor" I would do something similar to the inside of a house like tile and have a drain to remove spills. Personally for an outside aquarium I'd probably want more of a garden atmosphere and want a more natural ground cover.

01/16/2017, 04:00 PM
other than freezing, no feedback?

Your 1.5 psi calculation assumes that the aquarium is sitting directly on the slab. Even if it is possible to get the slab sufficiently level, temperature changes in the slab will directly impact your tank. If you're using some sort of stand, you'll have to redo that psi calculation to the actual load bearing surface area.

- Ivan

01/16/2017, 05:02 PM
The current support structure is a series of 2x4s and 3/4" plywood. The bottom is also plywood. It is very short (no sump or storage space). I've considered using concrete blocks too. I'll take more pics of this.

As far as drainage - yes! I'm planning on a slope to run the water to the outside door with a gravel or other covered gutter. We might add ceramic tile but the most likely surface treatment will be stamping or coloring with an epoxy surface coat - like a garage.

To go from concrete to the aquarium base, I was going to frame out the base and poor more concrete to level that section. Maybe I'll just continue wil concrete blocks and pour more concrete to make a contiguous concrete slab/floor/base.

Solid but very stiff. In case of an earthquake, it would have no give whatsoever... :)
Then again, any give at this scale is likely just as catastrophic.

This won't really be an open space either. It's a ... glass room, sunroom, greenhouse, conservatory. It just has a really large door/wall.

Here's a link to the main thread. The first few pages have the drawings and links to the "root-threads" that fed it:


<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/1_zpszqwqzacd.png.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/1_zpszqwqzacd.png" border="0" alt=" photo 1_zpszqwqzacd.png"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/0_zpshfv77rgv.png.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/0_zpshfv77rgv.png" border="0" alt=" photo 0_zpshfv77rgv.png"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/2_zps0e17wld4.png.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/2_zps0e17wld4.png" border="0" alt=" photo 2_zps0e17wld4.png"/></a>

<a href="http://s1062.photobucket.com/user/karimwassef/media/Designs/3_zpsloptmily.png.html" target="_blank"><img src="http://i1062.photobucket.com/albums/t496/karimwassef/Designs/3_zpsloptmily.png" border="0" alt=" photo 3_zpsloptmily.png"/></a>

It's a dropoff tank, but built inside of a full-size tank to create a large cave region under the reef. More to come.

Mike Hart
01/16/2017, 05:54 PM
I would be doing at least 6" with rebar in this area of the tank, extending the rebar further out into the main floor and I would increase the concrete PSI to 4500 or higher...its not much more of a cost

01/16/2017, 10:42 PM
The engineer for my home called out an 8" slab with rebar 2x2 that extended 12" past the perimeter. It also connected to the rebar in the 4" slab of rest of house. The tank is 10'x5' footprint 1500gal in California. . Take this at your own risk as I would recommend you hire a structural engineer to review your particular circumstances. Shouldn't cost you more than a couple hundred which is some cheap insurance.

01/17/2017, 01:22 AM
+1 on the structural engineer. Plus, if you are in an area where there are earthquakes, I don't think "unmovable" is better. I mean earth vs concrete slab. I'm pretty sure earth wins on that one and the concrete slab will roll and crack, dip, twist, whatever. A structural engineer might know of a shock absorbing substance you can put under the concrete slab that will remain as level as possible during a small quake.
I'm thinking you may need to pour the slab for the aquarium, separate from the rest of the floor. You want the slab level, and you need a slope in the actual floor for run off. Again talk to an structural engineer as someone already suggested. But I think two slabs, each one independent of the other might be the better way to go. Also, you may want to consider a concrete block stand rather than a wood one. I mean, it's not going to cost you any more in supplies and you won't have to worry about the heat and humidity warping it, or termites getting into it (being outside it's amazing what those buggers can do even in a green house).
Another thing to think about is how packed the ground is. No matter how you pour a slab there is going to be some amount of settling, so you may have to dig out a large area where the aquarium will go (another reason for two separate slabs) an pour footers along with putting in rebar for the slab.

Ridgeway Swiss
01/18/2017, 03:40 PM
We did the calculations here for the foundations for my system and with a very overly safe Swiss engineer we decided on around 40" deep reinforced concrete into the floor, then we built the concrete pillars on top of that, again reinforced. The calcs we used were based on 5 x the actual weight, only then would the builder build it and only then because the engineer had designed it, and that because the insurance will pay out if it fails.

I sleep well

01/18/2017, 07:14 PM
Yikes! I'm not made of coral!

01/19/2017, 02:22 PM
He is in Dallas, TX, so Tornados and Floods are what he has to worry about ;) His frost line is 5".. you probably don't even need rebar... but.. an engineer to do all the maths for you and peace of mind costs less than most of the parts of your tank..

but.. to answer your question, a 4" slab is all you really need. (You can find a lot of people using 10,000 lbs equipment with much higher psi than your tank will have on them).. but substrate matters... so.. flooding/erosion, clay can cause problems no matter how thick the slab... a geologist may tell you more than an engineer ;) (I just say that because my dad is a geologist and he works on foundations a lot)

01/19/2017, 08:56 PM
the land is a new lot - currently farmland - and the entire area is going to be ripped up and leveled, but I hear you. North Texas soil is clay based and has some unusual hydraulics that causes foundations to crack if not properly supported. This is why the main house slab will likewise need to be set on a re-established ground foundation.

01/19/2017, 08:58 PM
-also why basements don't exist here...

well, it's not impossible, but the engineering required is similar to putting in a deep pool the size of the house - so it's very rare.

01/20/2017, 12:04 PM
If you can park a motorhome on your driveway, especially on jacks then a tank will do just fine on a slab with similar specs as the drive way; if your tank will be on leveling screws then the concrete strength becomes more important (you arent planning on screws, but something to be cognizant of)

Im in the bay area with heavy clay, although my driveway is still level with no obvious cracks my neighors have very messed up driveways (some are 20-30+ years old) - that is the only thing I would worry about for Dallas. So whatever ground prep is done for laying the main home slab is what you want for the slab under the tank.

In of the threads (here on RC or some big tank forum) there was discussion on how the point stress (leveling screws) can be addressed - Im not finding it at the moment

Since its new construction have the slab integrated into the main slab. its easier to cut it out later if you need the land for something else. Maybe Dallas gets a lot of rain and you get into gardening or farming :D

01/23/2017, 02:22 PM
The slab in my fish room cracked and sank 5/8" on one corner. Had a foundation repair company come out and take a look. My foundation is "the most level" he had ever seen. Varied by less than 1/16" over 40+ ft. The slab moved, the foundation didn't.

He wasn't concerned at all about me putting my 600 on the opposite side of the slab. Since it is new construction I would absolutely hire someone to do the work and make sure it does not shift with time. A slab is more than likely going to shift unless you are fortunate enough to have stable soil.

Just curious but what is with all the angled lights? Seems like that is going to be painful to look at.

01/23/2017, 04:54 PM
What angled lights? It's primarily solar with two thin strips of liquid cooled UV LEDs

01/23/2017, 04:58 PM
The builder is going to have it reinforced as part of the main foundation pour.

01/23/2017, 08:47 PM
What angled lights? It's primarily solar with two thin strips of liquid cooled UV LEDs

These things.

01/24/2017, 05:31 AM
oh! those are fans. I do have angled MH against the wall, but those are backups only. I don't think I'll actually use them but just in case.

01/24/2017, 05:56 PM
Ahh, ok :)

01/25/2017, 01:47 AM
think of them as emergency lighting... in case the sun is not visible for a week or so...