Post and Pier or floating slab?

[pericles]

Unfortunately, our project in New York got put on hold for a while (there's an update coming in the Owner/Builder forum for those of you who worried I dropped off the face of the earth,) but we're hoping to be able to get it started again this summer.  Here's a question for the group though:

We're hoping to build the 16'x24' Little House.  Where we're building, frost depth would require us to sink cement piers to maybe six feet.  The ground is too rocky to dig the footings by hand, so in addition to the cost of cement, forms, and a mixer, we'd have to pay to rent an auger or an excavator. 

With that in mind, would it be cheaper for us to put down a floating slab? 

-Jack

[Windpower]

6 FEET ?!?!

must be far north

[pericles]

It's not, it's in Southern New York State, just across the PA border.  The frost depth is only 4 feet, but I've been told it can go deeper during real cold snaps, especially in the clay we're working with.  Long story short, code is six feet.

[Squirl]

6 ft does sound extreme.  Last time I checked Broome, Deleware, and Otsego county were 4 ft.  Did the building inspector tell you this?

[pericles]

He did, although going back over my notes I'm seeing that he might have said code was less, but recommended six.  (This conversation was a while ago!)  Even if the depth is only 4 feet though, that's still quite a lot of concrete plus equipment.  Will piers really be more cost effective? 

[Don_P]

The slab would be a shallow frost protected slab, there are details for its construction in the codebook chapter 4  here;
https://www2.iccsafe.org/states/Virginia/Residential/Res-Frameset.html
(If you poke around I think NY code is on there somewhere as well, it'll be similar)
there's a diagram and insulation table etc around R403.3 and going on for several pages. This clears one possible code issue, its prescribed exactly how to do it in the codebook, find the column in the chart that satisfies the inspector and everybody's happy.

If you are going down to that depth I came across this company last week. Neat looking system, no idea what the cost is;
http://www.permacolumn.com/downloads.htm
The nerd page is here, this connection has moment capacity, that's a big plus;
http://www.permacolumn.com/manuals/Design%20and%20Use%20Guide%2011-2-05.pdf

[Squirl]

I will let other speak to putting in slab foundations in colder climates.  I do not have experiance with this.    But my concrete calculations are as follows:
For a 6" slab 16' x 24' with no footings would be 192 Cu ft. or 7.1 yrds of concrete.
For 15 x 4' x12" diameter piers you would need (3.14r^2*h) = 47.1 cu. ft or 1.74 cu yrd of concrete.
So in short you are looking at least 4 times the concrete.  This does not include the differences in rebar, steel mesh, insulation, labor, or footings on the slab.  

There are also practical considerations with a slab.  You have to have a team of people ready to work the concrete.  You also have to have good site access for the concrete truck to come and pour. The nice part is everything gets done in a day.  Piers can be done alone and over time even in an area with poor site access.

[pericles]

Squirl - doesn't your math just cover ONE pier?  I'll need (the plans aren't in front of me right now,) but call it eight piers. 

I guess I can't just put the whole thing on deck footers huh?

[Squirl]

No, that is 15 piers.  One pier 12" diameter 4' height would be 3.14 cu. ft. times 15 would be 47.  I went with a slightly different design for more piers because you said it is clay and I was worried about the load capacity of clay soil.  It is outlined in the link to the VA residential code that don provided a link for. If you look on the homepage there is a 16 x 24 cabin that used 15 piers (3 rows by 5 piers)

[Squirl]

I will break down my calculations

Volume of a box Height X Length X Width

Height 6"
Length 16'
Width 24'
So .5 x 16 x 24 = 192

Volume of a cylinder Pie * R^2 * Height

Pie = 3.14
Radius = 12"/2 = 6"
Height = 4'

3.14 * .5^2 * 4 = 3.14 * .25* 4 = 3.14 cu ft. X 15 piers = 47.1

[John Raabe]

The Scandinavians have been doing frost protected shallow foundations in some pretty nasty climates for many years. Not all inspectors know about this.

I have a free download with details at the CountryPlans Home Page: http://www.countryplans.com/Downloads/shallowfound.pdf

You can even use this technique to protect pier foundations



Several builders at this site have used variations of this technique to build and insulate the shallow Little House foundation in cold climates like Wyoming.

[Squirl]

The original question asked about price.  So I got out some of my notes and used the web.  I like using the Lowes site, because they list there prices and goods online.  I used the prices for Binghamton.  This is a good place to start.

In the 16x28 foundation plans on the homepage it calls for 10 piers of 8" diameter with footings.  So by my calculations this would require around 40 cu ft concrete.  If you used premix 80 lb bags, this would take about 70 bags.  At $3.45 a bag that would come to $241 for the concrete alone.
The cost of 8" tubes is $5.68 multiplied by 10 piers or $57. Half Inch by 10' rebar costs $4.78.  By the looks of the plans you would need two 8" bars in an X and one 4' piece.  So my estimates would be 8 pieces.  For a cost of $38.  This brings me to an estimate of $336 for the post materials.

Now I do not have a lot of experience with slab foundations, but I do have some experience at pouring for slab for a driveway and sidewalks.  People with more can correct me where I am wrong.  It would take around 7 cu yd. of concrete at a conservative estimate of $80 a yard would be around $560.  You would also need 384 sq. ft of steel mesh at  a 5 x 50 roll for $45 x 2 = $90  These calculations are without the insulation or the footings or extra help to work the concrete.  This brings and estimate for the concrete to $650. 

I hope this helps with your cost/benefit analysis.

[Source_to_Sea]

The math gets real fuzzy once you add insulation, beams, plywood, strapping, etc. In the grand scheme of things, it's probably a wash cost-wise, depending on how fancy you go with your flooring, etc. Piers are easier to deal with in remote sites though.

[pericles]

I'm sold!  Piers it is.  Thanks to both of you!

[jr1318]

I wouldn't rule out frost protected shallow footing with a slab, I did a 22 x 22 slab having someone doe the pouring for about 2300.00, the nice thing about that is the floor is done no need for joist, decking, post etc. I am building in NW Wisconsin wher -30 degrees is not uncommon.

[pericles]

On an unrelated topic; I'm starting the construction process by building a 10x14 to keep tools in while I'm not on site, and sleep in when I am.  I was going to use the Little House plans for that too, and I'd ultimately like the option of relocating the 10x14 to the larger cabin and converting it into an addition.  In order to do that, obviously, I'd need to put it (at least originally) on precast blocks.  Is a 10x14 with a little loft (like the one here: http://countryplans.com/smf/index.php?topic=4394.0) going to be too heavy for that?

-Jack