Mat Foundation price: 2 Story dwelling.

[HSNIC]

I have a boring report showing a site on loose to medium sand with no groundwater present. There were borings done in the street at an earlier date with groundwater found at 20 ft below the surface. I know there is not much engineering involved for a 2 family house. The build area is proposed @ 1845 Sq ft.

I have called a few contractors to get prices here in the NYC area. I have been getting ballpark figures in the $60,000 range. Does that seem right?

 

[oldestguy]

Sorry, but if you want experts to advise you on foundations, you have to explain more of what was advised by the report, etc. Perhaps you didn't explain too well to the contractors who are giving an explanation. Is a basement involved?

 

[HSNIC]

Sorry about that. Yes. It is a basement footing.

Are house foundations usually always a shallow mat type?

I have yet to come across a dwelling on a deep or special foundation.

What percentage of the total price should the foundation be?

 

[jgailla]

If you have an average mat depth of 2 feet, you have about 140 yards of concrete.
At $120 per yard, the concrete cost is $16,800.
Add 50% of this cost for rebar and you get $25,200.
Double that for labor and add 20% overhead and profit and you get $60,480.
Any of these numbers could be well off depending on local conditions.
If you have several prices and they are all about the same, you're probably not getting ripped off.

 

[TDAA]

Not in your area, but I usually see more of a spread footing, with stem wall, or piers with the basement wall as a grade beam. Both have used a floating slab, or structurally supported floor over a crawlspace. The floating slab can result in some differential movement from the structure, but that risk is offset by the reduction in materials and cost. It depends on the costs and the tolerance for risk.

Thought of looking at PT slab? may give you a similar product with less materials (more labor), and may work out to be more cost efficient.

 

[Ron]

I don't see a mat foundation for a two story house on loose to medium sand. Water table is not an issue. I would look at a conventional shallow footing. Even some ground remediation would likely be less expensive than the mat foundation. With such a small footprint, you could use a "Terra Probe" approach (30 to 36 inch diameter open pipe attached to vibratory pile driver, inserted and extracted on a 5-foot grid....good results...have used for major structures with excellent results.

 

[HSNIC]

I saw a Post-Tensioning video on the HGTV website. They do a good job of selling it.

Myself, I was thinking of the grade beam method. Having shallow footings at the walls and columns with it being thinner everywhere else.

Shopping around is the thing to do. Right now, I just don't believe in contractors making those 30+% profit margins anymore.

 

[concretemasonry]

If you are asking for a raft price, that is what you are getting for area.

What is so unusual about the site and soils? Is it an unusual layout or are the soil conditions really bad?

Some soil borings are obviously necessary, but they needed be excessive and should give the depth of the existing water table.

Every day, quality homes have been built on strip footings with concrete block or concrete walls. If the soil does not have high bearing strength, the footing width could be extended slightly beyond the normal 20" - 24" and the thickness should be increased slightly beyond the 8" minimums.

In many areas, the longitudinal reinforcement in the footings is not required because the wall is so much more rigid due to the depth, but most codes do require a minimal amount for continuity (not shrinkage/temperature). The wall thickness and reinforcement must be adequate for the lateral loads. The vertical loads are not that high unless you have an unusual layout.

There are many advantages to this type of construction. Beside the decreases amount of concrete, the strip footings and walls can be up in a few days. The post footings, if any, can be poured at the same time as the footings are laid out and poured. This allows the mechanicals to be installed below the basement slab. The floating slab (usually 4") is poured on the top of the interior portion of the strip footing, which usually must abut the bottom 4" of the wall base.

It would be wise to look at the latest IRC and portions of the companion IBC to see what is allowed, since this is the the most common residential standard used. Familiarity will also allow you to defend your reasoning if your design is compared to an "off-the shelf" type that some builders like to use.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.