# 8" deck pier?



## darren (Nov 25, 2005)

Is 10" in a written contract, if so tell him to get out there and put the right size tubes in.


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## Daniel Holzman (Mar 10, 2009)

The minimum required pier size is usually a code driven issue. Consider that concrete has a compressive strength typically of a minimum of 3000 psi (too early in the morning to convert to metric units). The area of an 8 inch pier is about 50 square inches, so the concrete can support in theory 150,000 lbs, which greatly exceeds any load you will put on it. So the soil strength always controls required pier diameter.

Soil strength is much more difficult to estimate, a strong soil may have strength up to 5,000 lbs/square foot, while a weak soil may be good for less than 500 lbs/sq ft. The allowable strength of the soil per square foot also goes up the deeper the footer is embedded. Because of the mostly unknown properties of the soil at your location, code generally specifies a minimum footing diameter, and a minimum depth, which is typically below frost, so 4 feet sounds about right for you.

I don't know what Canadian code requires for minimum diameter, where I live in Massachusetts we run on the International Residential Code for decks. My town uses the 2006 code. Table 4 presents minimum footer sizes, which vary with the beam and joist span. This table assumes minimum 1,500 psf soil bearing capacity. The table is not really applicable to sonotube installations, so in my town the code enforcement official generally sits down with each applicant before construction and approves the size of sonotube to be used. In my case, I presented 10 inch tubes, and that is what I used.

Where I live, if you want to deviate from an approved plan, you need an engineer's letter stating the reason for the deviation, and stating that in the opinion of the engineer the deviation is safe and acceptable. Assuming you had an approved plan showing 10 inch footers, but the contractor failed to read the plans and deviated, perhaps the contractor can obtain an engineer's certification that the 8 inch posts are OK. If they cannot, and your building official does not approve the deviation, the contractor really should replace them with the agreed upon size at no cost to you.


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## AtlanticWBConst. (May 12, 2006)

FWIW - In my region, 12" dia. is the code required norm in many towns/cities. As stated, you should check with you local building dept, among the other good suggestions made.


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## CoconutPete (Jan 22, 2010)

One more vote for calling the town and see what they want. Technically my plan called for 10" tubes, but the local buidling dept wanted 12".


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## concretemasonry (Oct 10, 2006)

In Winnipeg, you may need than 4' depth. Your soils there can be highly variable and I think a quick shallow sampling of the soil will tell you about the soil area under a Sonotube should be.

Dick


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## cortell (Nov 27, 2010)

quincy said:


> Hi everyone, need your opinion, please.
> Hire the contractor to put 8 piers, 4' in the ground, 10" pier agreed on. Low and behold, he only put 8" sonotube down.


Every box store in my area (there's one every square mile in this city, BTW) carries 8" and 12" sonotubes. 10" requires special ordering. That has always bugged me as 8" is a little too tight for a 4x4 post, and 12" is excessive.

Anyway, I wonder if your deck builder didn't hit the same availability issue. Seems to me, though, he should have explained it to you and made a case for using 8", given that your agreement with him stated 10". 

I would, as has been suggested by other posters, give him the option to get approval for 8" from the local building code or from an engineer. Both will require legwork and the latter will additionaly incur a non-trivial cost. You can give him a second option, though: remove the existing piers and put in 10" ones. It's doubtful he'll like any of those options, but he put himself in this situation. It's unfortunate to see a red flag so early in a project. I hope the rest of the build goes smoother.


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## darren (Nov 25, 2005)

concretemasonry said:


> In Winnipeg, you may need than 4' depth. Your soils there can be highly variable and I think a quick shallow sampling of the soil will tell you about the soil area under a Sonotube should be.
> 
> Dick


I believe they tell us to go 6 feet deep for sono tubes, but I always thought that was for frost.


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## quincy (Oct 9, 2010)

Talked to the concrete contractor, this is how he did it: drilled with 10" auger 4' deep, fill the hole with concrete to the ground surface, put 8" sonotube around 8" -12" long into the concrete with about 6" above the ground, fill the tube with concrete then place the metal bracket for the 6x6 post on top of the concrete, the tubes and brackets are align perfectly.
So, technically it is 10" hole/pier.
What is your opinion on this method?

I am still waiting for the inspector.

Thank-you all for advises so far.


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## cortell (Nov 27, 2010)

quincy said:


> Talked to the concrete contractor, this is how he did it: drilled with 10" auger 4' deep, fill the hole with concrete to the ground surface, put 8" sonotube around 8" -12" long into the concrete with about 6" above the ground, fill the tube with concrete then place the metal bracket for the 6x6 post on top of the concrete, the tubes and brackets are align perfectly.
> So, technically it is 10" hole/pier.
> What is your opinion on this method?


This sounds acceptable to me as long as it was a monolithic pour--not done in two phases.


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