# Sonotube depth for footings



## ront02769

Can't tell where you. Are....but if in cold climate your tubes don't need to be sunk 2' into the ground, they need to be sunk to below the frost line whatever that is...and in my town that is four feet. As to gravel, you can or not, your choice. And for concrete stuff, compacted grqvel is most often considered as "soil contact" ron


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## Jacey

Sorry, I'm in central Texas, no real frost issue...this will be to support an eight foot high second story deck. So is gravel advised then?


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## Canarywood1

It all depends on the type of soil in your area, you need to have a soil test done.


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## Jacey

It's deep river bottom, some clay at the lower level, maybe four feet deep. Can't imagine needing a soil test.


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## Canarywood1

Some types of clay turn to mush when they get wet, extremely unstable.


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## Msradell

With this being a second story deck you are talking a bout a good bit of weight bearing on the footers. Depending on soil type and number of footers it's quite possible you're going to need larger diameter than what you are talking about or at least you're going to need to put bell bottoms on them to get more contact surface.

Your local code officials could probably give you some better advice. They normally are quite good to work with for situations like this and whatever you do will have to pass their inspections.


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## Jacey

No inspectors, rural off the grid thing. But I take your point. Perhaps I can find info out on it locally, but not sure how accurate that will be. The deck is only 8 feet wide, ledgered to standing building with good foundation, with 8' pier spacing. Deck will wrap in L equally.


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## de-nagorg

Did I dream it or didn't this get solved just a couple of weeks ago?

ED 



You are over thinking this, put the footers deep enough that they will be stable.


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## Jacey

uh, no, maybe you think all aspects were answered, but I had a question about depth that no one answered on my last post. Since you will not be affected for the foreseeable future and I will, I decided to narrow the scope to those with experience with sonotubes. Their own site doesn't address these questions, so relying on someone with experience and appreciate the input about various issues. I'm a measure twice, cut once type, so sorry if you are tired of the subject, not your problem. Maybe someone else will not mind inputting on the subject.


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## Bondo

Ayuh,.... Sonotubes are nothin' but cardboard concrete forms, which really changes nothing,...

The only Tip I've got is, put the sonotubes in the ground _Just_ before ya pour,...

Wet cardboard ain't so strong,...


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## concretemasonry

Unfortunately, you are too far off the "grid" and cannot use a building inspector to give you some help, especially with local conditions and soils.

In the end, the diameter of a Sonotube is controlled by the load (yours is not too high) and the soil type, which is not really known since you never had a soil boring. Some clays can be great for construction and strength, while other can be absolutely worthless and cost a lot of money when you have to deal with them.

you are just relying on rh upper levels of the soil for strength and that can be highly variable. - This is especially true with clays that are very difficult to compact when used to level a site after construction has rooted around in a hap-hazard procedure. Usually, is is just leveled enough to put in sod or plant seed.

Stone in the bottom of a hole is clay does not provide any real drainage. It does make for a cleaner hole for concrete placement in the Sonotube.

Dick


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## Daniel Holzman

Concretemasonry is correct in his comments. The Sonotube itself is simply there to make it easy to place the concrete, and is very useful when you want the post to extend above ground. As previously commented, the tube itself makes no difference in support of the post.

To determine if your sonotube is big enough, you can run the following rough calculations. First, determine how much load each post is supporting. To do this, figure out the area of the deck. Approximately half the deck area is supported by the ledger (if you have one), so the other half is supported by posts. If you have no ledger, take the full area of the deck. Normally the building code will tell you how many pounds per square foot for your deck, but if you don't have a building code, you can use 50 pounds per square foot. Multiply the area of the deck by 50, divide by the number of posts, this will give you the load per post (remember what I said about the ledger if you have one).

Then calculate the area of the post, and divide the load in pounds by the area in square feet. This will give you the load in pounds per square feet that your soil needs to support. Here are some representative soil strengths.

Dense sand or gravel, up to 20,000 lbs/sf. Loose sand or silt, up to 5,000 psf. Hard clay, up to 5,000 psf. Soft clay, as little as 400 psf. Since none of us are there, we cannot tell you the strength of your soil, but this should at least give you some idea what your are dealing with.


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## Jacey

:not worthy: NOW we're talkin!!


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## Mort

Is there any way to just sticky that post? I bet if people saw it, there would be a lot fewer threads asking the same question.


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## cibula11

Not sure if the OP meant this or not, but as long as your hole/footing was dug to the proper depth, isn't it possible to use the tubes in the upper half or so once the hole is dug, just to help the appearance of the pier. The concrete will still fall below the depth of the tube and below the frost line.


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## jomama45

cibula11 said:


> Not sure if the OP meant this or not, but as long as your hole/footing was dug to the proper depth, isn't it possible to use the tubes in the upper half or so once the hole is dug, just to help the appearance of the pier. The concrete will still fall below the depth of the tube and below the frost line.


That gets brought up quite a bit here, and IMPO, it's not best practice, or even acceptable where frost is an issue. The piers get un-even and provide a ton of crevices for frost to pick at and eventually heave the whole pier.......


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## concretemasonry

Keep in mind that a lot of the frost heaving (especially on a light unloaded deck that is too light to have any resistance to heaving from the bottom of the footing), because the excavation is because of the making it possible to pull a foundation up, since a hand dug foundation is usually rough and much wider at the upper portions. The ground freezes from the top down and any rough dug surface give the freezing ground to "grab" a foundation and pull it up a the ice lenses in the upper layers cause expansion and lifting. Having the bottom of a hand dug well below the frost depth is no guarantee to eliminate frost heaving.

Usually that is not a problem with most free-standing decks, but can be a problem with a ledger attached from a home that does not move. Fortunately, a heaved post foundation will return to its original location or at least close to it.

Sonotubes are smoother and make it impossible to the frost action to raise a foundation that is done properly. The little bit of a Sonotube at the top for cosmetic purposes does little to help frost heaving.

The Sonotube uses less concrete and the amount needed can be can calculated accurately and the tube can be "plumbed" and supported by a granular fill. Augered holes minimizes the unknowns and the amount of material you have to move in and out.

The mythical frost depth in the codes is really for major structures (buildings, houses, ety.) that can occur during a long period. Free-standing light, liber decks can stand some heaving and still be very usable.

Dick


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## Jacey

Daniel Holzman said:


> Concretemasonry is correct in his comments. The Sonotube itself is simply there to make it easy to place the concrete, and is very useful when you want the post to extend above ground. As previously commented, the tube itself makes no difference in support of the post.
> 
> To determine if your sonotube is big enough, you can run the following rough calculations. First, determine how much load each post is supporting. To do this, figure out the area of the deck. Approximately half the deck area is supported by the ledger (if you have one), so the other half is supported by posts. If you have no ledger, take the full area of the deck. Normally the building code will tell you how many pounds per square foot for your deck, but if you don't have a building code, you can use 50 pounds per square foot. Multiply the area of the deck by 50, divide by the number of posts, this will give you the load per post (remember what I said about the ledger if you have one).
> 
> Then calculate the area of the post, and divide the load in pounds by the area in square feet. This will give you the load in pounds per square feet that your soil needs to support. Here are some representative soil strengths.
> 
> Dense sand or gravel, up to 20,000 lbs/sf. Loose sand or silt, up to 5,000 psf. Hard clay, up to 5,000 psf. Soft clay, as little as 400 psf. Since none of us are there, we cannot tell you the strength of your soil, but this should at least give you some idea what your are dealing with.


Just when i thought all I'd have to do was the math on this, I hit a snag. Wasn't sure if you were referring to 12" support or what...so came across a pdf for sonotube installation where the company sites 

"Sandy & gravel soils have a bearing resisitance of approx. 3,000 to 5,000 lbs/sf. Clay soils vary from 1,500 to 2,000 lbs/sf."

My calculations were based on 8x8 section on ledger board so I'm getting 8x8=64 divided by 2...so 32 x 50lbs=1600 divided by 2 so 800 psf sounds like..I have some clay 2 feet deep, but it can't possibly fall into that 400 psf of soft clay you mentioned...sounds like that would be quicksand. So if there's still input on this, that's where I'm stuck. :huh:


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## de-nagorg

Just a suggestion: 

Dig your hole down to solid bedrock, then drill three 3/4 inch holes 6 inches deep into the bedrock, 120 degrees apart within the 12 inch diameter of the tube. Drill them at 23 degrees of off plumb angled outward. 
Then get 3 1 foot long rebar 5/8 inch diameter, epoxy them into the holes , set your tube centered over these rebar, fill the tube. 

It is now pinned to bedrock and will not move. :thumbsup:


ED


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## Jacey

de-nagorg said:


> Just a suggestion:
> 
> Dig your hole down to solid bedrock, then drill three 3/4 inch holes 6 inches deep into the bedrock, 120 degrees apart within the 12 inch diameter of the tube. Drill them at 23 degrees of off plumb angled outward.
> Then get 3 1 foot long rebar 5/8 inch diameter, epoxy them into the holes , set your tube centered over these rebar, fill the tube.
> 
> It is now pinned to bedrock and will not move. :thumbsup:
> 
> 
> ED


Pretty sure that's overkill..but thanks for the suggestion.


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## landfillwizard

Personally, if you have a doubt on the bearing grade of your holes, I would dig a hole 3' deep and 18" square. I would pound the bottom of the hole with a hand tamp and add 4" of #57 gravel then pound the pi$$ out of it, Keep adding layers till you have a solid 1' of the gravel in the bottom of the hole. I would set up batter boards to find the exact center of the piers you will be putting in. I would get 4' #5 rebar and drive them in the exact center of the pier down to ~2'' below final grade. 

I would center a 12" sonna tube that the top is at final grade over the hole and backfill with the material you removed from the excavation. As I backfilled I would keep the sonna tube centered and compact the backfill material. Pour your concrete slowly to keep the sonna tubes centered. Finish off the top of the concrete and you are done. Wait at least 2 weeks to start construction.


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## Jacey

landfillwizard said:


> Personally, if you have a doubt on the bearing grade of your holes, I would dig a hole 3' deep and 18" square. I would pound the bottom of the hole with a hand tamp and add 4" of #57 gravel then pound the pi$$ out of it, Keep adding layers till you have a solid 1' of the gravel in the bottom of the hole. I would set up batter boards to find the exact center of the piers you will be putting in. I would get 4' #5 rebar and drive them in the exact center of the pier down to ~2'' below final grade.
> 
> I would center a 12" sonna tube that the top is at final grade over the hole and backfill with the material you removed from the excavation. As I backfilled I would keep the sonna tube centered and compact the backfill material. Pour your concrete slowly to keep the sonna tubes centered. Finish off the top of the concrete and you are done. Wait at least 2 weeks to start construction.


Sounds good...not sure what #57 gravel is. Have access to road base, which packs really well, not round like pea gravel, sharp edges that lock together. Will that work in the bottom? Also, should the rebar go 2 inches below the gravel, or just into the gravel that's a foot deep?


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## jogr

onjtrainee said:


> Just when i thought all I'd have to do was the math on this, I hit a snag. Wasn't sure if you were referring to 12" support or what...so came across a pdf for sonotube installation where the company sites
> 
> "Sandy & gravel soils have a bearing resisitance of approx. 3,000 to 5,000 lbs/sf. Clay soils vary from 1,500 to 2,000 lbs/sf."
> 
> My calculations were based on 8x8 section on ledger board so I'm getting 8x8=64 divided by 2...so 32 x 50lbs=1600 divided by 2 so 800 psf sounds like..I have some clay 2 feet deep, but it can't possibly fall into that 400 psf of soft clay you mentioned...sounds like that would be quicksand. So if there's still input on this, that's where I'm stuck. :huh:


Your calculation is close but wrong units. The load you calculated per post is 800 pounds. It is not 800 psf. So if your clay has a bearing capacity of 400 psf you need a 2 square foot footing for the post. A 20" diameter sonotube would do that. Or you could use a 12" sonotube with a bigfoot type bell on the bottom that flares out to 20" diameter.

BTW, I think you are right that 400 psf clay would be pretty obvious. Have someone who weighs about 200 pounds stand on 1 foot on your soil. That's probably slightly more than 400 psf on their shoe. Do they sink? I doubt it. Your clay is likely upwards of 1000 psf, probably close to 2000. Isn't there anyone (builder/inspector) in your area that can tell you what is typical of the clays in your area?


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## landfillwizard

#57 stone is a crusher run gravel with stone sizes ranging from -1" (stone that passes through a 1" screen) to dust size particles. The crusher leaves the stone with many angular sides which along with the other size particles bind them selves into a very dense material when compacted. Adding a small amount of moisture (~5% for crusher run limestone) will create a product that will set up almost like concrete. However the stone needs to be placed in thin layers when using a hand tamp. The gravel will "bridge" low compresssion clays or sand for a structural sound base.


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## Daniel Holzman

Clay is a bit tricky to get the allowable load correct. Dry clay can be very strong, many thousands of pounds per square foot. The same clay when it is saturated can be quite weak, only a few hundred pounds per square foot. Of course you want to size your footing for the worst case, which is when the clay is wet, so if you happen to run a test during a dry part of the year, you can get badly wrong results. Another thing to think about is that most people cannot tell the difference between clay and silt, this is even a problem for professionals.

Clay strength is typically very dependent on how wet it is, silt not so much (silt is like really fine sand). So deciding what size footing you need is not as simple as standing on the ground and seeing if you fall in. Usually best is to see what your neighbor did on a similar project. If it worked out well, you probably have similar soil. Or your local building inspector, who has experience with your type of soil, can often tell you how large a footer you need. On a personal note, I built my deck using 8 inch sonotubes 3-4 feet deep. Many of them stopped on rocks (I have rocky soil). They are bombproof, because the glacial till in my area is extremely strong (up to 20,000 pounds per square foot), and when you get down to 3-4 feet, the soil is so hard it feels like rock.


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## Jacey

landfillwizard said:


> #57 stone is a crusher run gravel with stone sizes ranging from -1" (stone that passes through a 1" screen) to dust size particles. The crusher leaves the stone with many angular sides which along with the other size particles bind them selves into a very dense material when compacted. Adding a small amount of moisture (~5% for crusher run limestone) will create a product that will set up almost like concrete. However the stone needs to be placed in thin layers when using a hand tamp. The gravel will "bridge" low compresssion clays or sand for a structural sound base.


Sounds like what we call road base here, has the same attributes and is about the same size...is dolomite. I talked to a concrete artist, high end polished stuff who uses it for all kinds of things. He said I'd be good to go with the 12" diameter sonotube a foot deep, with a foot of roadbase 24" wide beneath it. He though I could go less deep with the roadbase, but max at 1 foot.


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## Jacey

Daniel Holzman said:


> Clay is a bit tricky to get the allowable load correct. Dry clay can be very strong, many thousands of pounds per square foot. The same clay when it is saturated can be quite weak, only a few hundred pounds per square foot. Of course you want to size your footing for the worst case, which is when the clay is wet, so if you happen to run a test during a dry part of the year, you can get badly wrong results. Another thing to think about is that most people cannot tell the difference between clay and silt, this is even a problem for professionals.
> 
> Clay strength is typically very dependent on how wet it is, silt not so much (silt is like really fine sand). So deciding what size footing you need is not as simple as standing on the ground and seeing if you fall in. Usually best is to see what your neighbor did on a similar project. If it worked out well, you probably have similar soil. Or your local building inspector, who has experience with your type of soil, can often tell you how large a footer you need. On a personal note, I built my deck using 8 inch sonotubes 3-4 feet deep. Many of them stopped on rocks (I have rocky soil). They are bombproof, because the glacial till in my area is extremely strong (up to 20,000 pounds per square foot), and when you get down to 3-4 feet, the soil is so hard it feels like rock.


We are on a granite uplift, which means part of the land is exposed bedrock, while you can dig to China other places on my 5 acres. It used to rain here, so if that pattern ever returns, that is an issue. We lost a backhoe one year, and I still can see the butt end exposed vertically of a railroad tie that submerged in the ooze we used to get it out..so 9' more or less sunk in the ground. Now we rarely see those conditions, but it has effected me deeply to witness that. Maybe PTSD has set in.


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## landfillwizard

OJT,

I have been on many sites I have inspected, the geotechnical engineers have allowed foundation for building to be poured over structural fill consisting of 1' to 2' of compacted #1 crusher run limestone. The load on the structural fill would be greater with a concrete foundation than with the sonna tubes you will use. 

Driving a #5 rebar into the middle of the sonna tube will help keep the center line aligned for your posts for the deck. Just make sure you cover the rebar with ~2" of concrete to keep water from rusting.


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## Jacey

landfillwizard said:


> OJT,
> 
> I have been on many sites I have inspected, the geotechnical engineers have allowed foundation for building to be poured over structural fill consisting of 1' to 2' of compacted #1 crusher run limestone. The load on the structural fill would be greater with a concrete foundation than with the sonna tubes you will use.
> 
> Driving a #5 rebar into the middle of the sonna tube will help keep the center line aligned for your posts for the deck. Just make sure you cover the rebar with ~2" of concrete to keep water from rusting.


We are setting cedar posts, (like tree trunks, not milled) over a pin, I was thinking the rebar extended above the top of the sonotube a couple of inches. I have done this with square pads, not using rebar at all except for the pin. I think we went a foot deep and several inches over 18 inch square, with a pin extending. Still holding 25 years later on earlier deck. In this case would the pin go below the concrete into the crushed rock base? Or does it need to even go that far....or further?


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## landfillwizard

As I stated before, set up batter boards and use these to find the exact center of your piers. Then drive the rebar into the center mark on top of the base material. If you want to leave the rebar up for setting the post then do so. Center the sonna on the rebar and you are good to go.


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## de-nagorg

LFW: I believe that they are confused as to how far into the base material to put the re-bar pin.

Far enough to pin it all together, 6 inches will suffice.

ED


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## landfillwizard

Drive the pin/rebar in far enough so it is self-supporting while you pour the concrete.


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## Jacey

What about the depth of the gravel? Do you agree with only needing a foot of concrete over max foot road base 20" wide? Can I use less? (I have probably 7 -10 inches on hand..) Osha made it so I couldn't load it up in my pick up this weekend, so wasn't able to get enough extra..but will go back with a trailer, errg.. if I need to)


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## landfillwizard

What does OSHA have to to with getting concrete?


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## de-nagorg

landfillwizard said:


> What does OSHA have to to with getting concrete?


You forgot that this is in TEXAS: they do things different in TEXAS. 
It is like another country.

Just ask them. :laughing::laughing:


ED


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## Jacey

Couldn't get roadbase loaded in my PU...OSHA has messed up things where you live as well, I'm guessing.

Anyway, they did sell me some 4 inch thick 16"x16" pads at Lowe's. That and 9 bags of Quickcrete. 

Just in case I can get some help tomorrow to set this up. Do the pads sound like they will work, 2' beneath the sonotube?


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## landfillwizard

OSHA has nothing to do with putting gravel in my pickup. DOT maybe but not OSHA. If you want to read my previous post, I gave you all the information you need. If you don't want t0o follow it that is up to you!


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## Jacey

WHOA!!..didn't mean to fluff up your feathers..All I know is what the guy at the pit said and he said OSHA. Told me to bring back a trailer, 60 miles round trip. 

The rest of your advice sounded right on, and plan on using it. Just don't have gravel to put beneath, but do have a 4 inch pad and guess that will be just as good, if not better than gravel. Thought someone might weigh in on the concrete pad. If not, thanks to all!

Adios!


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## ron45

Is this going to be an 8' foot wide deck, or house on pilings.?


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