Concrete for deck post depth

[Crakajak]

Some SWGa counties require a minimum of 24" wide x 10" deep with 2 runs of 1/2" rebar for single story homes.Some are as wide as 36" because of soil type.
We typically poured a 20"x20" x 10" deep footing for decks in Gwinnett. With rebar.Put in above ground post connectors.
I have dug up and replaced probably close to 500 posts on decks and fences that failed over the years.Never had to replace one on top of a footing with no dirt around it.
We are not getting younger....so in 30 years do you want to be replacing rotted posts under your deck?

 

[Crakajak]

Some years back in W Cobb on a large new construction home for sale multiple groups of buyers happened to meet on the back deck touring a home, they all took about a 20 ft plus ride to the ground together when the poorly built deck failed.

Exactly why the codes were changed and continue to change.
Folks need to remember you can,t do something the same way it was done 30 years ago.Insurance companies are now walking properties ,looking for non code compliant issues that have to be fixed before they will insure the property. (Folks think home inspectors are bad.try dealing with an insurance inspector telling you you have to replace the siding before they will write the policy on a 30 year old home with LP siding because of a documented manufacturer defect

 

[Crakajak]

A friend in the panhandle of Fla had to do a 36" wide x 20" deep footing of 5000 psi concrete with 4 runs of rebar tied into the monolithic slab with wire and rebar every 2 ft each direction.Talk about a budget killer.
Soil was sand and the water table was at 3 ft below grade.
That was 28 years ago and never had an issue.
But back to the OP question.
Depends on a lot of things.
Lots of counties / cities"require " a permit for deck replacement.
2 reasons....proper footings and proper size/attachment of joist.
Insurance companies can also be problematic with no permit....
Again depending on location.

 

[Batjack]

A friend in the panhandle of Fla had to do a 36" wide x 20" deep footing of 5000 psi concrete with 4 runs of rebar tied into the monolithic slab with wire and rebar every 2 ft each direction.Talk about a budget killer.
Soil was sand and the water table was at 3 ft below grade.
That was 28 years ago and never had an issue.
But back to the OP question.
Depends on a lot of things.
Lots of counties / cities"require " a permit for deck replacement.
2 reasons....proper footings and proper size/attachment of joist.
Insurance companies can also be problematic with no permit....
Again depending on location.

Replaced a front porch in one of the "high end" subdivisions in west Cobb that had been built on backfill and was sagging so bad it was about to tear the roof off the house. Once I got the roof secure and tore off the porch had to bring in a tractor with a PTO auger on it and drill 6 holes 2 feet dia. down to solid soil...14 feet deep. Since coal ash has a zero percent compaction factor I bought a dumptruck load from the power plant at Euharlee to fill the holes to within 2 feet of ground level... then build a rebar "cage" and pour 5000 psi concrete to ground level to set the post on. The home owner was quite high up in the county gov. so the inspector was over my shoulder every day. That house might fall of that "hill" it's set'n on, but that porch ain't never gonna sag again.

 

[Nuttin Better]

The minimum footing depth in Georgia according to the 2018 edition of the International Residential code is 12". The actual size of the footing is based on the height of the deck and size of the deck. According to what information you have provided I would recommend footings 16" x 16" 12" deep.

 

[earlthegoat2]

I recommend finding a specialty lumber dealer and find some 0.6 CCA posts if they are legal in your area for your application.

Ground contact Timbers from Lowe’s are pretty garbage. Dock supply places will have them.

.

 

[trial&error]

If delaying rot is the concern. Keep the wood off the ground and no standing water on wood. You can come all the way to deck height with reinforced concrete and those paper form tubes.