Sonotubes/shed foundation

[dmlehmann]

Hello,

I am planning to build a shed (training for a bigger project). I do not want to use a gravel bed or set the shed on "cinder blocks". So, I am considering using Sonotubes. I live in Raleigh NC (10" frost line). The shed will be 12x12' - a permit won't be necessary. For the foundation, I am considering digging down to 18", adding 3-4" of gravel and then place the Sonotubes (leaving a few inches above grade). Do I really need to use the Sonotube bell footings? I would rather not since they am appear to be enormous (I.e. much larger hole).

Thanks


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[Ray-CA]

Since you are digging a hole, just bell out the bottom. When you set the sonotube, just leave it short of the bottom of the hole.

Ray

 

[dmlehmann]

Sounds reasonable Ray.


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[theoldwizard1]

Do I really need to use the Sonotube bell footings?

Probably not, but it depends on home many footing you are putting in.

 

[dmlehmann]

I am planning to park my riding lawn mower in the along with other lawn related tools and equipment.

I am planning to use 9 footings (10" diameter).


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[theoldwizard1]

I am planning to park my riding lawn mower in the along with other lawn related tools and equipment.

I am planning to use 9 footings (10" diameter).

That should be adequate if your using 2x8 PT for the floor joists. I would double the outer ones and the center one and use joist hanger for the crossing ones. You will have some seams in the floor that will not land on joists. Add an extra joist there for support. Use stainless nails/screw or hot dipped galvanized when going into PT.

Personally, I would go 24" deep, just because.

If you build it close to the ground, "critters" will burrow underneath. Build it so there is light and moving air underneath (bottom of the joist should be about 12" above grade).

 

[mechanix311]

The stability comes from the bell at the bottom. When you backfill, the dirt holds the shoulder of the bell in place making for a more secure foundation. If you have ever seen leaning sonotubes it is because they put the tube directly on the soil not allowing for a bell end. The sonotube needs to hover so the concrete can blow out the bottom.

 

[dmlehmann]

These sound like good suggestions.

I was thinking about doubling the joists around the perimeter and down the center.

Do you think it is necessary to put three parallel "skids" down on the Sonotubes and then run the joists perpendicular to these?

Thanks


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[Todd.Brock]

When I built a 7 x 20 deck with in city limits, the city inspector said that the holes had to be. 12" wide 30" deep. That allowed for a
6 " footer and 2 ft of pole in the ground. He said the some tubes were fine, but not necessary. Just a 6" thick footer. So I hacked up a sonotube into 6" tall rings, but 12" tube in 12" hole don't fit. I just took a stick, marked it at 6" and "checked the oil" until it was deep enough. Took a few pictures , left the stick laying out for the soil inspection and was good to go.

This may be something to consider doing so you don't have to deal with those footer tubes. I looked at them as well and they are huge.

 

[volleyball]

Are you digging or augering? If you auger, then the tubes will sit in the hole fine. You don't need to bell if the soil on bottom is undisturbed. Decks are built this way all the time.
The tilting ones seem to be from digging and therefore the disturbed side walls and the soil returned to the hole is loose so the tubes tilt.
If you dig, your hole is going to be larger than the tubes anyways.
For a free standing shed, I'd remove the grass 14 x 14, lay fabric, lay stone on top and those little pier blocks. The shed may move a bit but so what? When the ground thaws, it will return. The gravel will promote drainage and keep it dried out. Critters won't like it as much and you won't have to deal with weeds

 

[rocklobster]

The stability comes from the bell at the bottom. When you backfill, the dirt holds the shoulder of the bell in place making for a more secure foundation. If you have ever seen leaning sonotubes it is because they put the tube directly on the soil not allowing for a bell end. The sonotube needs to hover so the concrete can blow out the bottom.

You may be right about stability with the shoulder (cone) of the bell, but that would be a negligible gain, the reason to have the bell at the bottom is for spreading the bearing load over more surface area so that the pier does not tip over:

But for a shed you will be fine without the bell. A few years back I replaced the failing cinder block foundation on my shed that the previous owner had made I did not use the bell and everything is fine. The shed is 20' X 8' and on a side slope. I jacked each side of the building up dug my hole below our regional frost penetration depth (4'), placed a 12" sonotube and poured concrete. I did six tubes, four corners and middle.

The shed is solid as a rock now. Also make sure to use anchoring brackets in the top of the sonotube to attach your lumber to, don't just place a 4x4 post into the concrete or something that is hard to replace if the wood rots.

 

[dmlehmann]

Thank you all for the helpful suggestions.

I considered the precast deck blocks. I may regret it later, but I thought it would be better to construct my own Sonotube piers.

I am planning to rent an auger. As has been commented on by others, some times the tubes lean. One poster attributed the issue to not building on undisturbed soil. I figured that I would put some gravel in the bottom of each hole and tamp it down prior to filling the tubes. Hopefully, that will suffice.




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