How deep should the footers be?

[MotorCityBear]

I am having a garage built in northern GA. The garage is approximately 1600 square feet with an upstairs man cave. Just had the site cleared and “prepped” but I have a concern.

The builder brought in 14 dump truck loads of dirt to raise and level the build site. Then about 4” of rock was added. Concrete will be poured this week.

On Saturday they dug the footers, which are only 3-4 inches below the rocks. I was under the impression the footers should be 12” deep, (below the undisturbed soil) but the contractor said they did not need to be that deep.

I know north GA usually only gets a few days of freezing weather per year but shouldn’t the footers be deeper?

One other question, the contractor said he will use 3,000 PSI concrete. I thought 3,500 PSI would be more appropriate.

Any advice will be appreciated.

 

[ford33]

What does your city building codes say for depth of footers? You don't provide a location for city or county. North GA is too vague. We cannot help you enforce a depth dimension with your contractor unless you do the research.

You don't know? Then visit your city's building code website now and find the building code section and it will state depth of footers. If no code enforcement, visit the site of another close-by city for reference.

Do you have drawings of the structure? Do you have a permit for this construction? A signed contract? Usually a building of that size with two stories of space requires some drawings and permits unless you are far away from a city.

If you have none of this, then search this site for garage construction fails and determine whether you are on the path of other failed projects. Then decide whether to go forward with this project. Lot's of people not happy with their finished garage because of lack of drawings, contracts, permitted inspections and on and on and on...

I wish you well.

 

[LB-1911]

^

You really need to contact the Bldg Dept -

Building & Development

46 Hughes St., Suite A
Blairsville, GA 30512 Hours:
Monday - Friday 8:00am-4:30pm
Closed for lunch noon-12:30pm daily

http://www.unioncountyga.gov/building-development/

Good Luck

 

[56Mark]

I am in upper east TN, about 3 hours north of you. Frost line here is about 16", most people go 18"+. Either way, I would want to be on undisturbed soil unless I knew for sure it was compacted adequately. Go talk to the local building inspectors. I have dropped in a time or two on ours for advice and they are usually pretty helpful.

 

[ConCretin]

Building codes aside for a moment, there is no functional reason to go any deeper than the desired depth of the thickened slab edge unless you are looking for frost protection. This doesn't apply in your region and wouldn't apply to a mono-slab anyway. You are building on a structural fill. The edges of the slab are supported by the same materials as the rest of the slab. Assuming that organics and topsoil were removed before you started and the fill was compacted properly, you should be fine.

With regard the the compressive strength of the concrete, 3000 psi concrete is pretty standard in residential construction is more than adequate from a structural standpoint so I can't fault your contractor. The loads on your slab will never approach 3000 psi. With that said, I generally like to use a 4000 psi mix for a working slab because the added cement provides for a more durable surface.

 

[wssix99]

On Saturday they dug the footers, which are only 3-4 inches below the rocks. I was under the impression the footers should be 12” deep, (below the undisturbed soil) but the contractor said they did not need to be that deep.

You are fine to be concerned. The 4" of overburden may not be enough to compress the soil below so it will have the density needed to hold fast once the structure is placed upon it.

Your contractor could be right. They could be wrong. The only way to know is to consult your engineer, get a spec for the dirt, and have testing done. A penetrometer test is one way to get that piece of mind. (This test wouldn't be enough for a large building, but it should be OK for you. We used this when building our house and found that we needed to keep digging our footers deeper.)

https://en.wikipedia.org/wiki/Cone_penetration_test

 

[Firebrick43]

I am with llwillysfan on the 4000 or even 4500psi concrete. Also if you don’t place plastic underneath and wet cure it for at least a week you will lose half of the rated strength. Don’t let them put to much water in it to make it easy to move.

I agree with wssix99 o the test. Very few contractors compact enough or correctly. If all your ducks are in a row you won’t be back here in 6 months to a year pissed that you concrete has serious cracks in it. (all concrete cracks even if you don’t see them.) in

 

[imjustdave]

When you say they brought in Dirt.... what kind of dirt? Engineered or just random dirt?
Did they strip the grass first?
how deep is this dirt?
Was it compacted?
Are you building a monolithic and or thickened edge... footer and slab in 1 pour? or traditional foot "upside down T"
Having fill brought is is ok, I brought in 16 feet of it under a hotel once 180X100 feet worth of building and built right after.. but I also had compaction in lifts, engineered fill material and soils engineer testing all day long documenting and verifying everything was correct.


I'm thinking 14 trucks is 200+ yards your talking multiple feet worth of fill correct?
Not trying to scare you but if your backfill is deep, wasn't compacted and or its just random dirt I suspect you may have settling issues. When we built foundations for homes, 2 story typical, we would dig down around 12-24 inches but it had to be native ground If not we dug further down. Seattle WA Area for reference. Then backfill for garage slabs that would be poured latter on.

If you have multiple feet worth of fill, I would seriously consider having it tested for compaction.

Have a photo?

 

[MotorCityBear]

First, thanks for the replies and advice.

The fill dirt was just random dirt. I did not see any compaction except for the skid steer moving the dirt around.

It is a monolithic slab.

The contractor said it is 4” of concrete on top of 4” of gravel and the footer is 4” deep for a total of 12”.

He also said he uses 3,000 PSI concrete because 3,500 or 4,000 is more likely to crack. He also said if I install an automotive life 3,000 PSI concrete is fine.

As to why I was hesitant to reach out to the building inspector, the contractor said he is “friendly” with the build inspector. I am not implying any bias or wrong doing by the building inspectors office but I wanted to ensure I had an objective answer to my question.

 

[GMCGarage]

I am having a garage built in northern GA. The garage is approximately 1600 square feet with an upstairs man cave. Just had the site cleared and “prepped” but I have a concern.

The builder brought in 14 dump truck loads of dirt to raise and level the build site. Then about 4” of rock was added. Concrete will be poured this week.

On Saturday they dug the footers, which are only 3-4 inches below the rocks. I was under the impression the footers should be 12” deep, (below the undisturbed soil) but the contractor said they did not need to be that deep.

I know north GA usually only gets a few days of freezing weather per year but shouldn’t the footers be deeper?

One other question, the contractor said he will use 3,000 PSI concrete. I thought 3,500 PSI would be more appropriate.

Any advice will be appreciated.

What do your drawings show? Post a pic of them

 

[lolaetype]

First, thanks for the replies and advice.


As to why I was hesitant to reach out to the building inspector, the contractor said he is “friendly” with the build inspector. I am not implying any bias or wrong doing by the building inspectors office but I wanted to ensure I had an objective answer to my question.

Hmm. Does that mean he can cut corners and get away with it or does it mean he gets preferential treatment when it comes to getting plans approved?

I'm more familiar with commercial construction, where slabs are signed off by engineers and then scrutinized by city permit departments. Who designed the slab? Does the work require a building permit? Does the city or county come out and inspect and approve the pad before the pour?

It's your slab being paid for with your money. If you are uneasy with what you see you should check with the city or county to ensure it's done to code.

 

[BillK]

Looks like 12 inches below undisturbed soil if I am reading this correctly.

https://up.codes/viewer/georgia/irc-2012/chapter/4/foundations#4

 

[James-W]

I know when I built my garage the city inspector came out to check everything before they poured the concrete. The inspector had a tape measure and he checked all sorts of things before giving the "go ahead" to pour the concrete.

 

[bob15]

He also said he uses 3,000 PSI concrete because 3,500 or 4,000 is more likely to crack. He also said if I install an automotive life 3,000 PSI concrete is fine.

As to why I was hesitant to reach out to the building inspector, the contractor said he is “friendly” with the build inspector. I am not implying any bias or wrong doing by the building inspectors office but I wanted to ensure I had an objective answer to my question.

If not your building inspector, go to the neighboring towns and ask them first. They might say 12" and then when you go ask yours, you have an approximate depth in the back of your mind.

Look at automotive lift install plans before agreeing to 3k. Mohawk is 4k and Rotary is 3k, so do some homework now!!!

 

[Chuckster in NJ]

When the contractor says he is "friendly with the building inspector" RUN AWAY!
I highly suggest hiring an engineer who will certify the soil compaction is adequate for your project. ANY "competent" building department will ask for a compaction report PRIOR to any inspections...... Unless they are "friendly" with your contractor.

Compaction of the soil and soil type is the most important thing with a project to prevent problems that may happen from poor compaction. (EG: Racking, Cracking, Abnormal Settling)

BTW! Take or leave my advice.... Your Project and Your Hard Earned Money.

BOBS ADVICE (post #14) is the best advice about the concrete specs...... Do your homework NOW!

 

[mike93lx]

No chance would I pay for a slab on disturbed, uncompacted soil.

I wouldnt even do that for a shed

 

[imjustdave]

When the contractor says he is "friendly with the building inspector" RUN AWAY!
I highly suggest hiring an engineer who will certify the soil compaction is adequate for your project. ANY "competent" building department will ask for a compaction report PRIOR to any inspections...... Unless they are "friendly" with your contractor.

Compaction of the soil and soil type is the most important thing with a project to prevent problems that may happen from poor compaction. (EG: Racking, Cracking, Abnormal Settling)

BTW! Take or leave my advice.... Your Project and Your Hard Earned Money.

BOBS ADVICE (post #14) is the best advice about the concrete specs...... Do your homework NOW!

Contractor and inspector being Friends or friendly isn't a bad thing, in my book, unless they are taking a cut to approve stuff.

To me an builder with this relationship can really help, they will take photos as proof of items being fixed, They usually know the standards the builder builds too and it meets standards.
Worse is ones that hate each other. Inspectors who don't trust a builder are going to nitpick everything "usually with no benefit for you" , maybe cause delays "slow to show up for inspections, spend hrs on what should be 15 min" , possibly case change work that will end up out of your pocket.

 

[tinmanwpk]

Like Bob15 just mentioned, do your homework.

Just this past week I looked on a site for car lifts as I am planning long term for a garage. I distinctly remember 5" thick with 3500 psi concrete.

 

[Firebrick43]

First, thanks for the replies and advice.

The fill dirt was just random dirt. I did not see any compaction except for the skid steer moving the dirt around.
This is bad, really bad!
It is a monolithic slab.

The contractor said it is 4” of concrete on top of 4” of gravel and the footer is 4” deep for a total of 12”. It matters not if the underlying soil is not compacted. And the rule of thumb for gravel, again compacted correctly, is three times the slab thickness

He also said he uses 3,000 PSI concrete because 3,500 or 4,000 is more likely to crack. Wrong again. Maybe if to much water and not post water cured but if plastic is under the slab and the top is kept hydrated for a week no issues. Major Cracking will occur down the road due to settling. He also said if I install an automotive life 3,000 PSI concrete is fine.look up the lift you are thinking about. 3000psi is actually ok but many call out more than 4 thick”, many require rebar in 12” spacing. Do not trust your contractor with this. Something you have not mentioned is the amount and type of reinforcement.

Reinforcement is not necessary(except in the lift area!) in many instances if the slab sub base is good. However yours is not! Reinforcement such as welded wire does little in many instances as the concrete guys try to lift it while standing on it. Every slab I have demoed had the wire on the bottom. Better yet is rebar. However a little rebar is not helpful in your situation but a lot maybe. A slab can be engineered to stay together even if some settling occurs IF there is enough reinforcement. This would have to be on chairs and close spaced. I do not know however what that amount would be.

 

[jhelrey]

You said they brought in a bunch of dirt. Was this dirt spread out at 4 inches deep or 48 inches deep?

 

[ConCretin]

I'm with the other posters who have expressed concern with the 'random dirt' and apparent lack of compaction. The soils under the slab must be capable of supporting the loads placed on it. A slab on grade is not structural in that it can't span very far over soft ground without cracking regardless of whether it is steel reinforced or not. I'd be calling a halt to things until I was comfortable with what is under your slab. It needs to be a well compacted, granular material. The contractor will be paid and long gone when your slab falls apart.

With regard to your contractor's comment about 3000 psi concrete being more prone to cracking, it's not true from a practical standpoint. Theoretically, the increased volume of fine material in the form of cement in higher psi mixes will marginally increase shrinkage but it's not enough to make a measurable difference. The amount of water added is much more likely to increase shrinkage and potential cracking.

Absent detailed specs, it wasn't improper of him to carry the cost for 3000 but it's also fair to ask for a higher psi concrete and pay the difference.

 

[wssix99]

You are fine to be concerned. The 4" of overburden may not be enough to compress the soil below so it will have the density needed to hold fast once the structure is placed upon it.

Your contractor could be right. They could be wrong. The only way to know is to consult your engineer, get a spec for the dirt, and have testing done. A penetrometer test is one way to get that piece of mind. (This test wouldn't be enough for a large building, but it should be OK for you. We used this when building our house and found that we needed to keep digging our footers deeper.)

https://en.wikipedia.org/wiki/Cone_penetration_test

Please disregard most of this for the edges of a monolithic slab.

The fill should be well compacted, though. Compaction after the fact and voids are not good. If properly compacted, you should be able to ask around locally to get better opinions as to if testing is worth it.

 

[72Camaro]

We went 2-3 feet, depending on what was needed to get into undisturbed soil. We also brought in 280 tons of select fill, roughly 250 yards. Footers are 12 inches wide around the perimeter and cross crossing the middle. 5 inch thick 4,000 psi. Shop is 30x40

 

[WNYflyer]

From Georgia Residential Building Code:

Whether it applies fully to '"Accessory Structures" I don't know but no doubt proper practice.


https://up.codes/viewer/georgia/irc-2018/chapter/4/foundations#4


R401.2 Requirements

Foundation construction shall be capable of accommodating all loads in accordance with Section R301 and of transmitting the resulting loads to the supporting soil. Fill soils that support footings and foundations shall be designed, installed and tested in accordance with accepted engineering practice




R403.1 General

All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, crushed stone footings, wood foundations, or other approved structural systems that shall be of sufficient design to accommodate all loads according to Section R301 and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil. Footings shall be supported on undisturbed natural soils or engineered fill. Concrete footing shall be designed and constructed in accordance with the provisions of Section R403 or in accordance with ACI 332.

 

[MotorCityBear]

Thanks again for all of the replies and advice.

So the building inspector will meet me tomorrow to inspect the foundation.

I asked the contractor to hold off pouring concrete until I am comfortable with the foundation work. The contractor put in 6 Sono tubes to support the fill dirt used. He said the sono tubes go down to hard pan.

I will try to upload the plans.

Stay tuned my friends......

 

[Samh]

I am having a garage built in northern GA. The garage is approximately 1600 square feet with an upstairs man cave. Just had the site cleared and “prepped” but I have a concern.

The builder brought in 14 dump truck loads of dirt to raise and level the build site. Then about 4” of rock was added. Concrete will be poured this week.

On Saturday they dug the footers, which are only 3-4 inches below the rocks. I was under the impression the footers should be 12” deep, (below the undisturbed soil) but the contractor said they did not need to be that deep.

I know north GA usually only gets a few days of freezing weather per year but shouldn’t the footers be deeper?

One other question, the contractor said he will use 3,000 PSI concrete. I thought 3,500 PSI would be more appropriate.

Any advice will be appreciated.

No advice other than to say I am related to about half that town.

 

[ard]

Thanks again for all of the replies and advice.

So the building inspector will meet me tomorrow to inspect the foundation.

I asked the contractor to hold off pouring concrete until I am comfortable with the foundation work. The contractor put in 6 Sono tubes to support the fill dirt used. He said the sono tubes go down to hard pan.

I will try to upload the plans.

Stay tuned my friends......

Be careful, the building inspector is (1) not an engineer, (2) not on the hook if the slab turns to **** in 4 years and (3) only making sure the plans are being followed.

Tossing in a few sonotubes, without any engineering around **how** they will work seems like a half assed move.


Kudos for being proactive and stopping this. There are quite a few threads here along the lines of “I had questions but let them pour...figuring it would be fixed later”. One turned into a years long delay and total tear out as I recall.....

GL!

 

[Chuckster in NJ]

Contractor and inspector being Friends or friendly isn't a bad thing, in my book, unless they are taking a cut to approve stuff.

Being "friends" and being "friendly" are two different things. A good contractor should have a good working relationship with the inspectors BUT they shouldn’t be FRIENDS. Do your job according to the approved plans and don’t cut corners and being courteous to inspectors is a good working relationship...... Playing golf and going out to dinner with inspectors (who are your friends) is bad and unethical. Trust me on this one, because I am from NJ and this is why we call it the "Soprano state"..... Tony knew how to get things done!

BTW! A friend will help you move...... A "VERY GOOD" FRIEND will help you move a body!

 

[ConCretin]

I asked the contractor to hold off pouring concrete until I am comfortable with the foundation work. The contractor put in 6 Sono tubes to support the fill dirt used. He said the sono tubes go down to hard pan.

There are applications for this type of foundation but they are carefully engineered. The 'sono tubes' need to be able to support the weight of the structure and everything in it -and- the other foundation elements must be capable of transferring those loads back to them. Unless all of the above is true - and I can almost guarantee it isn't - they will do more harm than good. It sounds incredibly half-assed to me.

 

[James-W]

Being "friends" and being "friendly" are two different things. A good contractor should have a good working relationship with the inspectors BUT they shouldn’t be FRIENDS. Do your job according to the approved plans and don’t cut corners and being courteous to inspectors is a good working relationship...... Playing golf and going out to dinner with inspectors (who are your friends) is bad and unethical. Trust me on this one, because I am from NJ and this is why we call it the "Soprano state"..... Tony knew how to get things done!

BTW! A friend will help you move...... A "VERY GOOD" FRIEND will help you move a body!

I get what you are saying, and I can see how this could pose a problem. But you are making the assumption that because they are friends, or at least on friendly terms, the inspector will let the builder get away with things that are not done right. I don't think that is fair. You MAY be right, but you could just as easily be dead wrong.

 

[MotorCityBear]

In case anyone is still following this, the building inspector stopped work due to the footers being too shallow.

They had to remove the rebar and vapor barrier and then dig the footers to a depth of 16” to the hard pan and 24” where the fill dirt was added.

It will be Reinspected tomorrow.

 

[James-W]

In case anyone is still following this, the building inspector stopped work due to the footers being too shallow.

They had to remove the rebar and vapor barrier and then dig the footers to a depth of 16” to the hard pan and 24” where the fill dirt was added.

It will be Reinspected tomorrow.

Now you know the builder was not doing the work properly according to the local code. You also know the inspector doesn't let friendship affect the outcome of his inspections.

 

[ard]

In case anyone is still following this, the building inspector stopped work due to the footers being too shallow.

They had to remove the rebar and vapor barrier and then dig the footers to a depth of 16” to the hard pan and 24” where the fill dirt was added.

It will be Reinspected tomorrow.

Any issues with the not-compacted fill under the MIDDLE of the slab?!?

 

[bob15]

Have you figured out what PSI you're making the mix? 3k or 4k

 

[Samh]

In case anyone is still following this, the building inspector stopped work due to the footers being too shallow.

They had to remove the rebar and vapor barrier and then dig the footers to a depth of 16” to the hard pan and 24” where the fill dirt was added.

It will be Reinspected tomorrow.

Good deal. Where in Blairsville are you located?

 

[theoldwizard1]

Assuming that organics and topsoil were removed before you started and the fill was compacted properly, you should be fine.

These two are KEY !

I would still want about 4" below the ORIGINAL grade.

 

[ford33]

OP, it doesn't appear you have drawings for this structure otherwise there would be dimensions for the footings. You got lucky that a building inspector enforced a local code and is making your contractor fix the footing issue.

Is the building inspector your architect? Are you going to rely on the building inspector to design this building correctly? What's the next code you wish for the inspector to check for you? Wall stud spacing? Pressure treated wood on the sills? The correct nailing pattern for sheathing. It's not their job to design your building.

Realize that not everything is in a code. Read the thread about the contractor who built garage walls out of plumb due to the foundation wall not level. Contractor response was there is no code requirement that walls be plumb. And he was correct! See quote below.

It is clear, your contractor doesn't know the local area and lacks fundamental knowledge of building codes and construction methods.

If you decide to proceed without taking corrective action, your next post will likely have the title "I didn't get the garage I wanted and it still cost me $$$$".

"there are no specific requirements for construction tolerances of light-frame wood construction in the International Building Code (IBC) or referenced standards such as the American Wood Council’s National Design Specification® (NDS®) for Wood Construction"

 

[ptgarcia]

Every time I read threads like this I cringe a little. Lots of opinions and recommendations being tossed around supported by little facts and almost zero data. My suggestion is hire an engineer, test the soil, and based on the results determine the appropriate building foundation.

 

[MotorCityBear]

ard- They said the weight of the Bobcat and mini-ex did compact the soil. They also added the 6 sonotubes and rebar. I had a soil engineer on-site today who took core samples down to 6 feet.

Bob15- after discussion with the contractor and the concrete company I am getting 3,000psi concrete

Samh- I am between B’ville and Blue Ridge off of Blue Ridge Hwy. and Hicks Gap

Ford 33- I did not use an architect. The builder is well known in this area with a positive reputation. I have toured a few houses and garages he previously built and was comfortable with the quality of his work. I believe in this case he has too much work and brought in a few subs that did not do a good job. I have addressed this with him and now have his employees doing my build.

Ptgarcia- you said what I was thinking. I appreciate the advice and responses however a few may have been less than helpful.

Thanks again for your help and advice.

 

[wssix99]

They said the weight of the Bobcat and mini-ex did compact the soil.

:cringe:

I had a soil engineer on-site today who took core samples down to 6 feet.

Given above, this is really good insurance.