posts vs foundation?


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Old 09-02-06, 04:48 PM
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posts vs foundation?

Hi folks

Has anyone had any positive or negative experiences with houses built on posts vs houses built on a conventional foundation? I am looking into building an addition onto my house, and heard that posts can be significantly cheaper than a foundation.

One of the contractors I've talked to said he has not had good experience with houses built on posts. He says he got animals in the crawl space, and had to spend more to insulate the pipes. He also said a house built on posts is more prone to settling than one built on a foundation.

So are posts still worth it? Can those problems be addressed, or does it just end up being a big headache?

Thanks
 
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Old 09-02-06, 09:07 PM
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What kind of soil is present?

Concrete slabs can crack, settle and sink.
Pier-and-beam can settle unevenly.

Currently I'm on a slab foundation with piers that were poured prior to pouring the main foundation. The soil is mostly white rock with blue rock beneath. No clay.

Either base CAN have problems.

What type of foundation is under the existing house?
Are you only extending power and HVAC into the addition or plumbing as well?
 
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Old 09-02-06, 09:39 PM
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posts vs foundation?

IBM5081 is right.

Posts or piers can give you differential settlement. They do offer many advantages also.

Often, there a problems with terminology when describing foundation systems. What you say is a conventional foundation may not be known to most of the world. After 40 years, I have noidea what a "conventional foundation" is.

1. A slab on grade is one type. - Can be difficult to modify, work on and possibly control the environment.

2. A continuous perimeter foundation wall with interior posts (columns & beams) or piers is very common. It can be just a crawl space with limited clearance (crawl space) or a complete basement. - Usually dictated by local climate and water conditions. In cold climates, the basement is the cheapest area in the house. Nationally, it is the most common type for single family homes.

3. A home completely supported by piers with a crawl space. A non-structural skirt wall (brick, siding or lattice) can be used to provide varying degrees of protection from critters and the climate. I had this in Virginia Beach and it was adequte for the climate, but would have been unsatisfactory in a northern climate. This home had almost as many block in the 2' high piers as a home would have in a full basement, so cost is not everything.

Generally, if you can build it, a slab on grade is the cheapest way, but severely limits you when it comes to making changes and accessing mechanical, plumbing and electrical complnents. Because of this the lowest cost is not the best way to determine which way to go.

Rely on the person that is creating the drawings for your home. If you wish, he can show it several ways and you can go and shop it out - this will raise your design cost because of the options for electrical and mechanical, but may give you peace of mind. Unfortunately, you may end up with the low bidder.

Dick
 
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Old 09-03-06, 06:53 AM
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IBM5081,

The existing house has the continuous perimeter foundation wall with interior posts. The addition would be higher off the ground since the ground slopes down in the back and I would want to keep the floor of the addition on the same level as the original house.

The dirt is heavy red clay in northern Virginia.

I *would* be extending electrical, hvac, and plumbing into the addition.

The addition would probably be around 1000 square feet.
 

Last edited by DIYaddict; 09-03-06 at 07:46 AM. Reason: removed quote as it's unnecessary to quote the entire post
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Old 09-03-06, 07:46 AM
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posts vs foundation?

I would use the same type of foundation system for an addition of the that size. The addition will be easier to construct and can look like the orginal.

You will have a slighly higher foundation cost beause of the height, but everthing else will be the same or cheaper. Slab on grade would be very questionable and costly because of the site elevation.

You could change the post spacing if it fits your new layout. If you reduce the relative number of piers, increase the footing size slightly (low cost, long term benefit). If I remember correctly, that clay is reasonably stable, but it is clay. Any settlement or consolidation in clay is slow (months or years).

If the slope is great enough, you could possible use the area below the first floor for storage.

Since you have a reasonably large addition, it would be wise to get some professional assistance to look at the big picture and get your drawings right.

Dick
 
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Old 09-03-06, 08:24 AM
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Concretemasonry,

I would have actually guessed that he slab would have been more expensive due to the costs of grading/excavating and the amount of concrete to be mixed an poured. So in your experience the poured slab actually costs less than piers and posts or the foundation wall?

I doubt I would be able to do slab on grade in any case since the floor of the addition will probably be 4 to 5 feet off the ground in some places.

What is the best way to address settling with piers and posts or a foundation wall?
 

Last edited by DIYaddict; 09-03-06 at 08:27 AM. Reason: removed quote as it's unnecessary to quote the entire post
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Old 09-03-06, 10:39 AM
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posts vs foundation?

Generally, a slab is cheaper if you are on flat ground, but not as easy to work with. You stated that you had a sloping lot. In my previous post, Imentioned that for you, a slab would be questionable and costly because of the elevation.

You should have someone local look at your home and site and have them prepare some drawings. Additions are not as simple as they first appear.

Dick
 
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Old 09-03-06, 08:50 PM
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Since the code requires wood framing to be placed 6" away from earth contact, a curb wall and foundation, "stepped", will be your solution.

Given your geographic location, you will also have to be at a depth which is below the frost line.

Because clay is considered "expansive soil", your foundation will need to be deepened as well as need an increase in rebar.

Building up from a stepped curb wall is simply a series of increasingly taller wood framed walls, upon which your floor system is then placed.
Depending upon your area, additional shear bracing may be required. This can also affect the size and spacing of foundation bolts.
If you intend to install a door as access to this space, maintain a min, of 24" from a corner to the closest point of the door framing.

You made an "interesting" statement;"the amount of concrete to be mixed an poured."
This would indicate you being in a rural community.
If transit-mixed concrete is not readily available and you are intending to mix and pour on site, obtain and adhere to a formulating for concrete that is intended to, at its optimum, achieve an compressive strength of 2500PSI.
 
 

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