Post and Pier Foundation Depth

[VannL]

I have spent a lot of time trying to research this one question, and it seems everywhere I turn, I find a different answer.

The general understanding is that you need the base of the concrete pier to be below the frost line. For me in Central Alabama, that appears to be 10". That little fact I found. My question is, based on that, how deep does the hole need to be?

The ground is loamy and stiff through to 3 feet. I will be using the concrete to come just above the ground level and then 6X6 above that to level the beam, as I am building on a slope. I am building a 20X36, using three rows of seven piers to support the structure.

Any thoughts? Thanks!

And yes, I did check the Alabama code. As far as I could tell, it only mentioned the depth of the gravel base (about 8"), but I couldn't make out anything else.

[MountainDon]

Correct on footing depth being at frost line. For 10" that actually would mean 12 inches depth IIRC. I believe 12 inches is the minimum footing depth.

And yes, I did check the Alabama code. As far as I could tell, it only mentioned the depth of the gravel base (about 8"), but I couldn't make out anything else.

The reason you did not find any information on that is that the code*, like all versions of the IRC, does not recognize pier foundations as being suitable for a habitable building.

*(for 1 and 2 family residential structures the code is the 2009 International Residential Code, adopted statewide by the State of Alabama) Available Here

[UK4X4]

Frost depth only considers uplift from frost- and is sited as a minimum...not the ideal !

it does not help with lateral movement
it does not tell you the soil can support the planned weight

If you are building on a slope its even more critical

Lateral stability would be critical for tall piers on a sloped plot

Think of a fence post- how deep would you install that ? and expect a cow to be able to lean on it without movement

Now think of the same cow on stilts 25ft high leaning on the side of the house

Does 12 " still sound like a good idea ?

Loamy soil- does not support weight very well either

How many stories is the build and what angle slope- which way is the house orientated what would be your estimated longest 6x6's from the ground up.

Post and pier has so many variables involved - and all need consideration in order to not have it fail

[Squirl]

Even in shallow frost locations, post and pier foundations are usually placed deep (4 ft is the standard I usually see), to help use the soil to counteract shifting and leverage forces on the pier. 

I'm going to borrow something from Don_P.



Sorry, I am going to go on my occasional small diatribe.

In most instances, both in the short run and the in the long run, a full foundation in areas with little frost depth will be stronger, easier, and cheaper than piers.  The site conditions would have to be pretty unique.  If you are interested in further explanation, just ask.

[VannL]

MountainDon,
Thanks for clearing that up for me. I did some digging in the IRC as well, but like you said, they had no information on it.

UK4X4,
In answer to your questions (Great questions):
I was considering a depth between 2 to 4 feet for my piers.
The land slope is not bad, maybe up to three feet at best. The land on one side of the house, we will call zero. The land rises up about a foot near the middle and then slopes off to three feet below zero. Based on pier position I would say 0 1 -2ish (Maybe 3. I haven't gotten my water level out to find the exact distance of drop yet.
I would estimate my longest 6X6 to be a little longer than my tallest point, so just over 3 feet.

Squirl,
You said something that surprised me. You said that a full foundation would be cheaper than piers. I was under the understanding that piers would be cheaper due to the low concrete need and the ability to not use engineered floor joists to run the span across from one foundation on one side of the house to the other. Did i understand you correctly? And yes, please elaborate. Thanks! (I am assuming that you are not talking about a concrete slab. I'm not wanting to do that due to the ground slope and other issues.)

[JoelE]

I get confused by this topic also. On many of the log home forums they use post and pier and I've read weight often exceeds 100,000 - just logs.
Some of these structures been around forever - shucks, to the north of me around Ely I have been in some that are over 100 years old and solid yet today. Including my SILs 123 year old one on an island up there.
I also have been in homes with block basements that have walls veering inward and I smell big expenses a coming for HOs.

Watch is the magical formula I dunno. Look at some of the monsters in link and you can see what I mean.

http://www.loghomeinfo.com/photosofsomestudenthouses.html

[VannL]

Me to, JoelE. I would rather not do the concrete wall foundation, but I don't want my house to come up with a limp leg either. I have seen a lot of pictures in the Owner Build section and I would guess that over half of them are using Post and Pier foundations. I just want to make sure I am on solid footing here (Pun not intended, but it works!  )

[Don_P]

Sounds like a country song doesn't it 

The "code" is prescriptive. If one follows the methods prescribed within the codebook we know that they work, they have a proven track record of success. The codebook rarely prohibits something outright unless it is very bad. What the law does say is that when you step outside of the codebook prescriptive methods those portions that are outside of code need to be designed by an engineer who is qualified, in this case in geotechnical engineering. I've built a good bit of log and timberframe, you don't find it in the codebook but it is not prohibited. It is engineered.

What you want to do is engineered... hire an engineer.

What are your chances of designing a good pier foundation on your own? I've already heard your thoughts on design matters and it doesn't sound like there is a good understanding of the forces involved.

In the south, at shallow frost depths, to me it's just too easy to do it right. Forget the piers, forget the perimeter girders, use that time and money to pour a code minimum perimeter footing and block up a minimum crawlspace, pier and curtain wall if you want to beat every penny out of it,  then build a conventional floor system. Bingo, load traces begin to work, the house has lateral stability, it is solid,  it is anchored against the wind. The house is safe, it is insureable, it has resale value, and best of all, you stand a chance of surviving within it's walls.

Chapter 4 of the codebook has all you need to build a typical foundation. If you wish to stray outside of its prescriptions it is incorrect to ask non engineers how to properly design an alternative foundation. They are certainly capable of telling you what you want to hear, but they are incapable of knowing whether what they have told you is correct.

[VannL]

DonP,
Thanks for the info. Do you have a picture, or several, that can show me what that would look like? I want to make sure I fully understand what you are saying.

[Squirl]

No, piers require other expensive items such as simpson brackets, sonotubes, large PT wood posts, bracing, and large beams.  Most people don't factor in the all the costs and compare.

First, with your frost depth you could probably borrow or rent a roto tiller and a shovel to get the 12" needed for the full footing.
I checked these prices last year.
So a full perimeter 20x36 foundation.  An 8" tall x 12" wide footing should take about 3 yards of concrete.  Around me that would be about $300 delivered. 20ft rebar at $7 a piece about $80.  I figure any wood for the forms can be reused in the build.
So for about $400 you would have a footing that would usually beat the pants of a pier foundation stability wise and you could build it in a day.

For the walls, I don't know what kind of slope you have, but let me guess a little over a 3 ft wall for a decent crawl space.  5 blocks high for 112 linear ft, an estimate of 420 blocks.  When I built mine a little over a year ago, it cost me $1.25 a block plus a $40 delivery fee, or an estimate of  $600.

As far as the easier part, I will use a surface bonding cement instead of just mortar.
[embed=425,349]http://www.youtube.com/watch?v=d2gJTbNkfYQ[/embed]

You need to mortar the first row of blocks to position the wall on the footing, the last row to level the wall, and a little to cap the blocks and secure the J-bolts.  I would give a rough estimate of $150 in mortar.
The SBC bag is supposed to cover 50 square ft.  So around an estimate of $250.
J-bolts $1.50 each x 20 = $30.
2x4 pt sill plate $3.50 each for $50.

So a total estimate of materials of $1500 for a to code full perimeter foundation that is going to have $50,000 in materials and years worth of labor above it, that is almost twice as strong.  Using mortar instead of sbc would probably save you $150.

Borrowing again from Don_P.



Now let me do a quick look at post and pier.
16" minimum footing, 2 bags of concrete $8.
8" Sonotube $8
Rebar $4
2.5 80lb bags : $10
Simpson Bracket $20
6x6 post: $9
So $60 x 14 piers = $840.
Now you need large girders to distribute the load.  My estimate is 3 2x10's for that span/load.  So 18 2x10x12s at $15 each is $270.
Add in another $100 in bracing wood, and a little more for any rodent protection.  Also if your jurisdiction requires skirting, a little more.  So an estimate of around $1200 or more for a post and pier and it is a much weaker foundation method.

[Squirl]

Me to, JoelE. I would rather not do the concrete wall foundation, but I don't want my house to come up with a limp leg either. I have seen a lot of pictures in the Owner Build section and I would guess that over half of them are using Post and Pier foundations. I just want to make sure I am on solid footing here (Pun not intended, but it works!  )

Over the years, I have seen a few with shifting issues in just the first few years.  And that is only the ones that people have posted.  How much extra risk is tens of thousands of dollars and years worth of labor worth to you.  $100? $200? An extra day or two of work?  This is going to be your 1500 square foot house and not some small little cabin in the woods. For the average American, it will probably one of the largest investments that you have.

[Don_P]

LOL, thanks Squirl, and you remembered where the pics are  I guess to put a caption on that one, look at the difference in lateral stability... rack bracing, between the piers and the walls. Folks it just doesn't matter how well you build above if you're on shifting stuff below.

I've been on the phone with a structural engineer for 2 hours this morning. His help has been invaluable we bounced past my opening shot, through one of his and have a third I'll sketch and send before our next conversation, and we'll talk more, that is how it's done if you need to step outside of prescriptive.

I'm not sure what you're asking for pictures of Vann, clarify?

[SouthernTier]

First, with your frost depth you could probably borrow or rent a roto tiller and a shovel to get the 12" needed for the full footing.
I checked these prices last year.
So a full perimeter 20x36 foundation.  An 8" tall x 12" wide footing should take about 3 yards of concrete.  Around me that would be about $300 delivered. 20ft rebar at $7 a piece about $80.  I figure any wood for the forms can be reused in the build.
So for about $400 you would have a footing that would usually beat the pants of a pier foundation stability wise and you could build it in a day.

Squirl: I agree with you 100%.  One thing, though, he said his land slopes about 3 feet over the length of the building.  So either a slightly deeper trench on the uphill side or a stepped footing.  But neither of those issues would negate your suggestion, just add slightly more time/money.

[Squirl]

Slope- SouthernTier is right.



Just type in "step footing" into a search engine, you will find hundreds.

Rather than repost a lot of other points I had once made.
http://countryplans.com/smf/index.php?topic=12174.msg157082#msg157082

[Squirl]

and the ability to not use engineered floor joists to run the span across from one foundation on one side of the house to the other. Did i understand you correctly? And yes, please elaborate. Thanks!

Another common misconception.  Remember that center row of piers and girders is going to cost you $600.  You can then use smaller wood for the floor joists and space them 2 ft on center because they span 10 ft.  Then you have to get 3/4 inch plywood for the subfloor for more $.

Or you can span 20 ft with 2x12's 12" O/C.  You completely skip the center beam and foundation and can go with 1/2 plywood subfloor.  You can add lots of insulation this way.

Or you can get engineered lumber and go 16 to 24" O/C and skip the center row of piers too, but if you go 24" O/C you have to get the thicker subfloor.  You can also add lots of insulation this way.

When you add it all up,  all the options have about $100-$200 difference in cost.  6 one way, a half a dozen the other.

So when if you did 37 2x12x20's, which around me cost $25. = $925
23 sheets of 7/16" T&G osb subloor at $15 = $345
Total $1250 floor

vs.

$600 center beam
38 2x8x10s =$304
23 sheets of 3/4" T&G osb subloor at $24 = $552
Total $1456 floor

[VannL]

Ok, I get it now. Between the picture of the stepped footing and the video, I see what you guys are talking about. So, a bed of concrete for the base and block wall on top. So, I would NOT need the center row of piers and beams, working 12" on center with 2X12X20. Does this mean I could get a basement in here?

Would the blocks get filled with concrete and rebar as well or am I looking for a solid block? I ask this because you mentioned the J bolt, which I am assuming is used to bolt the sill plate to the top of the wall.

[Squirl]

Would the blocks get filled with concrete and rebar as well or am I looking for a solid block? I ask this because you mentioned the J bolt, which I am assuming is used to bolt the sill plate to the top of the wall.

If you want.  It is a "best practice" but probably not required to code in your area.  You can use 8" hollow blocks. The main purpose grout and rebar in concrete block is to help in earthquakes and with the force from the weight of the soil against the wall.  Since your walls won't have soil pressing against one side and not the other, rebar and grout are not required.  Many people add them for extra strength, but it is optional.

Top block with mortared in J-bolt.  It is called "topping" the blocks.  I stuffed crumbled up plastic in the second row (old tarp, plastic packaging) filled the top block with mortar and put in the J-bolt.



Basements usually have a slab floor.  I thought you were looking to avoid the slab, so I just suggested the cheaper crawl space.

[Squirl]

Here is a little guide I did to the block wall tables found in the IRC.
http://countryplans.com/smf/index.php?topic=12374.0

[SouthernTier]

I have seen a lot of pictures in the Owner Build section and I would guess that over half of them are using Post and Pier foundations.

I would add one thing: where it makes sense to go with the post and pier is where it is not really practical to get a concrete truck in to pour the footers (unless you are up to schleping in and mixing many 80# bags of mix).  Some of the buildings you see here fit that description.  As Don_P said, these types of foundations can be made to work, but they need to be engineered (note, I'm guilty as anyone for my 10x10 shed, but that's only a shed).

Old post of mine and followups by MtnDon and Don_P on a house with long piers done (apparently) right:

http://countryplans.com/smf/index.php?topic=8555.msg111371#msg111371

[MountainDon]

I have seen a lot of pictures in the Owner Build section and I would guess that over half of them are using Post and Pier foundations.

Read my signature line below....

If you read my project topic you will see I too built on piers. Read all the way through to near the end and you will see that I spent extra time, extra money, lots of effort crawling around under it retro-fitting foundation shear walls, a system engineered to strengthen the piers against lateral movement. I don't know that I will ever build another cabin, but if I did I would likely pursue the PWF (permanent wood foundation) because our property is one that we'd have to have a Cat on hand to ensure a concrete mixer truck would make it.

[VannL]

My desire not to have a slab poured is the jack hammer needed to fix a plumbing issue. That and we like the sound of a raised floor.

It might still be something I can explore for the sake of a small root cellar, which has been a desire of ours from the beginning. That doesn't need a slab floor and actually works better if it isn't. I will give it access under the stairs, but have that access closed up, or a small hatch on the block wall. Got to have access to the crawl space anyway. Then a small root cellar under the house.

[Don_P]

I poured a "rat slab" in our crawl, a ~2" thick slab... hey how many trucks are going to drive under there. we store stuff in there although it is really a bit too warm. I put the water heater, a short boy 40 gal, underfloor beside a drain. the waste heat helps keep the plumbing under there from freezing. One of the round tuit projects is a hobbit cave root cellar into the hill behind the house.  Being nice dry, secure storage all kinds of treasure has ended up under there  . Our house is 28' wide, its a raised slab for half the width, the dry side of the house and the crawl is under the wet side. I debated making it all on a slab and sticking a 3' culvert under the wet area. Drop the plumbing through the slab into the culvert where I could crawl and change out stuff if needed. My hair was alot longer then, we thought of a woodstove at one end and the flues under slab and up the wall at the far end. Sort of a cross between a roman hypocaust and a korean k'ang.

[cholland]

I just did a 20x32 perimeter wall and 5 piers for a middle beam.
I have a 12" frost depth.  I went with an 8" thick wall. The site slopes about 2 feet from back corner to front opposite corner.
I rented an excavator for a day and dug the footings. They ended up a little deeper and wider than necessary, but more is better in the foundation. I hired a contractor by the hour to help me with layout and building the forms. Used his steel stakes and wedges for the form ties. It took about 3 days for the forms, rebar, etc. and another half day for the pour.
My wall is about 2' on the uphill side and about 3' on the downhill side, with footing below that. I know from the bottom of the footing to the top of the wall at the tallest point was 4' 3". That leaves a fair crawl space and I will back fill the uphill outside to 10" from top of wall (8" is code). The piers are about 30" tall with a 24" dia footing.
All together it was 17 yards of concrete. I will reuse the form boards in the framing so its hard to say where that cost fits in with the entire build.
$1100 labor
$200 rebar and sono tube
$300 tractor rental
$360 concrete pump
$1700 concrete
$400 lumber (2x6s)
Plus other items like screws, form ties, etc. probably another $50 plus
Total about $3650 without the lumber included.

I placed a few prices of 2" & 3" ABS pipe for water/elec, etc, and a 6" steel pipe I had laying around for sanitary drain, at the bottom of the forms. Don't forget to put a UFer ground in somewhere.
I probably over built the foundation for a 1 1/2 story and had planned costs at about $4500.
I am pleased with the result and know there shouldn't ever be problems with the foundation. If I had it to do again I would dig out more and make the crawl space taller, maybe closer to 5 or 6 feet. I think the extra cost at this point would be out weighed by the benefit of extra storage space down the road.

[VannL]

My design at this point is to dig out the area about two feet and go with a five or six foot wall. This will give me access at near standing headroom. It will let me use 8 inch hollow blocks. Very happy with this set up.

[C.Oden]

You people have far more experience on this than I do so take this with a grain of salt as it is my personal experience.
My family has had a retreat up near the MN-Canadian border for close to 113 years this spring-thats all we can document anyway!
The frost line here is at 48' - honestly seems to little some years.
My G.Grandparents built the main log cabin home (small) then the main lodge a few years later and it went on and on. We have 9 buildings on the property now that are cabin- homes, 3 used year round.
The first building were just dry stacked on rocks - looks like they dug down maybe a foot and started stacking. And then used stone and mortar and filled in around the foundation.
The newer generation buildings were dug down deeper on the corners (no idea if to the so called frost line) and they used cement piers that are about 14" sq and 6' apart.
Other than one log building which has a slight slope now (that water in a glass on table proves it for sure !!) they are level and solid as the day they were built I bet. The one with the slope is a structural issue related to dry rot in the lower 2 logs on one side. The roof did not overhand that far on that corner do to being damaged in the 50's when a tree came down on it. Somewhat funny - they cut out the roughly 3'x18' overhang with a hand saw, actually looks fine if one didn't know any better,  leaving maybe 18" there. All the other building have 4' or more overhangs. My thought is they had a drink and looked and looked and said ahhhh heck, its fine and dandy. 
And never again considered it. It happened in winter and the old calendars say it was a -40 daily winter routine. I guess I don't blame them.

So is it possible that the sheer weight of log homes vs stick built makes a difference in some way? There was a post above that seemed to support our experiences over the generations of use and abuse. Its just worked fine here