Slab Foundation?

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Old 01-03-05, 08:02 AM
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Slab Foundation?

I am considering buying a home in Charleston, SC the builder is building the homes on a slab foundation. Does anyone know the advantage or disadvantage of this?
 
  #2  
Old 01-03-05, 10:46 AM
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gusbus,

Unsure what your requirements are for frost footings (if applicable) but slab foundations are less expensive in compared to a crawl space or full basement foundation.

The pros however you call it, whether it is from the builder’s standpoint or yours would be no worry about critters getting under a crawl space. Cost wise, it is cheaper to do a slab than conventional framing – crawl space or basement. They are usually closer to grade and thus entry and exit from home may only be one or two steps. Convenient for those with disabilities or wheel chair bound.

Cons would be that a slab does not permit easy access to plumbing nor heating pipes should a problem arise. Electrical can be placed in the slab (all protected by a pvc pipe) but nonetheless this also could be a problem. In today’s construction, rarely do problems of that nature occur but it does happen. Sometimes moisture is an issue and this is something to watch out for. Depending on the location and if other homes are completed within the area, you should take a look at them and review what problems, if any, they have. A crawl space would be better in this description.

The key is to knowing how to do a slab properly to avoid various problems. Problems could be vinyl flooring starting to come up wherever it is laid. Pour laying of the concrete creating uneven spots with the floor not being level.

A properly installed slab foundation will begin with a four to six inch layer of what is called a "capillary break', which is gravel of a size small enough to provide a continuous layer but large enough to prevent water from the ground from "wicking" up through it to the layers above. The next layer is a 10 mil plastic sheeting vapor barrier, laid so that all seams overlap about a foot. On top of the plastic is a two to four inch layer of sand which serves to keep the plastic in place and as a base for the concrete of the slab itself. The concrete may or may not have rebar steel rods or wire mesh in it, but it should be of a proper mixture of cement, sand, aggregate rock and water and be about 6 inches thick in the center or floor part of the slab and 12 to 14 inches thick at the outer edge or perimeter, the foundation part of the slab. Rigid insulation should be installed, like R-10 – 2” thick. This assists in keeping slab warmer.

If any of these parts of the slab are missing or improperly installed or if the concrete slab dries incorrectly and contains more than expected "shrinkage cracks", water from the ground below the foundation will begin to seep upward and cause problems during the years after the home is occupied. If soil conditions beneath the slab is of the "expansive clay' variety, it may, in its normal cycle of expansion and retraction, cause cracks and signs of upheaval in the slab.

When considering a home with a slab foundation, exercise extreme care in evaluating the integrity of the construction and /or the Builder. If necessary, hire an expert in concrete engineering to evaluate the structure. This would be a wise move regardless of the minimal expense to do so

Hope this helps!
 
  #3  
Old 01-04-05, 05:09 AM
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Doug is the pro here, but my experience as a real estate broker with slab floors vs crawlspaces (we have very few basements in coastal NC) is that slabs eventually present far more problems than the other two foundations.
Slabs make a builder more money in the short term (faster and cheaper to install), but will cost homeowners more money in the long run, IMHO.
It can get very expensive to correct plumbing, electrical, moisture or foundation structural problems with a home on an improperly laid or old slab.
I've sold homes on slabs where tree roots or clay have cracked the slabs, or where moisture has welled up and ruined exterior wall bases and/or hardwood floors throughout the ground floor, or the slabs had to be cut through the finished flooring for plumbing and electrical corrections, etc.
Personally, I wouldn't buy or build a home on a slab, but I sell them for others all of the time (and fully disclose all problems occuring, if any, which affects the home value).
It could be many, many years before a new home on a slab has any problems, or as Doug alluded to, if they aren't done properly, a homeowner could start having problems in the first few years, and you can't tell by looking.
This is just my personal opinion about homes on slabs. I'm sure that the builders who do them right will disagree with me, but that has been my experience.
Your call.
Mike
 
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Old 01-04-05, 09:17 PM
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Gusbus,

There are several advantages to the slab foundation:

1) cheaper to build for the contractor helping to keep the cost of the foundation portion of your new home and therefore the overall cost down.

2) When heating if radiant heat is used it keeps the heat radiated at 4' off the floor down where you live rather than at the ceiling as with forced air.

3) If tiled or Vinyl the floor becomes another source of heat through radiant heat gain through the glazing (windows) in the room.

4) In summer it keeps cooler at floor level through tranferrance of cooling from the ground.


The disadvantages have been spoken from both Mike and Doug. My biggest concern is not having plumbing accessable in the event of a leak or break.

good luck.


Brian Garrison
General Contractor/Professional Building Designer
 

Last edited by Brian Garrison; 01-04-05 at 09:33 PM.
  #5  
Old 04-16-05, 06:43 AM
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Charleston Slabs

New slab construction is better than the older slabs found in Charleston. Most older houses were built with slab on grade. The newer ones are called elevated slabs. These are usually 18 inches higher. If done properly, slab construction will last as long as any other foundation, maybe longer. My home of 27 years is slab. I had it replumbed a few years ago due to pin hole copper leaks. Not the slab's fault but since the pipes were in the slab a replumb was necessary. The newer homes don't have water pipes in the slab. I am moving to a new elevated slab home shortly. I have no qualms about this type of foundation. If you live in a ranch home with kids, you will find the floors noise free compared to wooden floors. A leaking toilet or shower pan will not rot a slab floor. There are pros and cons for each type of construction, but I wouldn't fear slab construction. As long as it is well inspected and found to be sound, you'll be OK.
 
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Old 04-16-05, 06:47 AM
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jmm706,

You're kinda answering this thread late, like 4 months but let me ask you something. If the slab is 18" high, are they building a block wall perimeter and then soil filling the center?
 
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Old 04-16-05, 12:56 PM
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Doug,

I can see no advantage to have a slab 18" in the air except in flood prone areas. In unstable or expansive soil we went to post tensioned slabs. We worked with the engineers to reduce them from the 12" thickness they were originally done with down to 4" thick. The biggest advantage of a slab on grade is wheelchair accessability but once you move the slab up 18" you lose all those advantages. Can you imagine the "swimming pool" you would have to fill to get 12" of gravel or fill under the slab? You also lose the advantage of a monolithic pour and get stuck pouring stemwalls on one pour and then filling and pouring the slab on another.

I have done this on hillside lots to get the garage slab level on a downhill slope and it was days of trucks with gravel to fill.

To each his own.

Brian Garrison
 
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Old 04-16-05, 03:41 PM
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Brian,

I totally agree with you, 18" above grade seems strange to me if it is concrete. At that point, I would do the traditional crawl. True, its more money but overall seems the better choice.

Have a great day!
 
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Old 04-17-05, 11:27 AM
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I went and measured it. It's 12 inches from grade to floor level. I know all that depth isn't concrete. Of course the footers run a lot deeper. I'm not sure what's under it, but I know a lot of rebar was used. (Yea, I noticed the date of the post after I replied - doh.)
 
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Old 03-10-07, 12:17 PM
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Red face Elevated Slabs

Looking at the dates on the posts I might be a little late but hope to receive an answer from a knowlegdable person anyhow.
I need to know what is the reason for the elevated slabs as compared to the usual monolithic slab. I have a hillside problem. I'm looking for reasons why my ideas are either correct or incorrect. To level the slab I have to doze out three feet in the front of the proposed building. That leaves me looking into a bank at the front door. I could grade away some of this but there're too many utilities in the ground ahead for me to remove the entire dirt bank. I have no problems with inspection, I'm in the middle of 80 of my own acres in the state of Missouri. I have a plenty of rocks and red clay in the ground. I prefer a concrete slab because I want to use ICFs {Insulated Concrete Forms} for the walls. The slab would also eliminate the millions of bugs we entertain through warm weather. I expect to run all water & sewer pipes through even larger pipes so they can be replaced if there is a plumbing problem. All wiring would be run through large PVC. If I use an elevated slab I would have to fill the {"pool"} with "minus one crushed limestone" compacted. That's alright with me, costly, but it would solve my problem. I would estimate 18 to 24 inches of minus one.
Analog
 
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Old 03-10-07, 12:39 PM
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Analog,

Crawl spaces can have more than one purpose

First being the grade issues. Sometimes, it is necessary if grade is too low to meet or match floors of an existing structure. It also depends on floor layouts - raised levels and such.

Second, depending on the location, this can serve as great place to run plumbing, heat ducts below yet still be accessible if need be.

Monolithic pours can be troublesome for any type of plumbing repair. Heat ducts are really no issue and as far as electrical, these are run in a protective plastic pipe.

Unsure if this is the answer you are looking for.
 
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Old 03-10-07, 03:16 PM
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Question Elevated Slabs

If I put a slab on top of 18 to 24 inches of minus one compacted limestone with a three foot footing around the perimeter, what in your opinion might go wrong? In order to have a concrete floor I must build up the height of the slab to compensate for the digging out of the ground to level the house on a hillside. Otherwise the front door will be looking at somewhere close to twenty inches of dirt right at the doorstep.

The important thing is: Would all that compacted gravel give me some kind of problem?

Analog
 
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Old 03-10-07, 04:51 PM
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Analog,

I would have to say no as a presumption that all would be OK to do so. Good fill, compacted would be a must along with good wire and steel reinforcement to avoid problems. The other issue is the bearing walls, internally, where they would be and the need for adequate footings to support them. All of this would need to be sitting on virgin soil to construct. If you are going to build, I would assume that you would be consulting initially with a designer/architect/contractor on what you are wanting to do. These are the ones that could provide direct responses, at least they could see the site and determine the most cost effective way to construct.

Again, unsure of your question as it relates to what you are planning on doing. Hard to answer when I cannot see, i.e. what your property and proposed structure looks like.

In most cases, there are alternatives to what you describe, if I am understanding it all correctly.

My first question is why would the front door be looking into a hill? Sounds like you will be doing excavation of some type regardless.
 
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Old 03-10-07, 05:46 PM
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Thumbs down Elevated Slab

I just took the backhoe out and dug a three foot hole to check for footing depth. Very disappointed, the red clay is down about two feet further than expected. It's not practical to build on a slab floor in that location. The soil around here is all virgin, I just have to find another more suitable location. Thanks for the help.
Analog
 
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Old 03-10-07, 06:35 PM
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Analog,

You're welcome.

Sorry to hear what you found but this is good to "investigate first" then procede as needed.

Good Luck!
 
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Old 06-11-07, 11:50 AM
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Question

So this doesn't really have to do with this thread, and I realize that this is kinda old. I was wondering if it is possible to lay down a new foundation (preferably a slab foundation) without tearing down the house I am in. As of right now the house has an abouve ground (crawl space) foundation, and since the house and foundation are very old it has shifted and is not very stable. If anyone can help, I appreciate it.
 
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Old 06-11-07, 12:37 PM
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Slab Foundation?

You want to put a slab foundation over an area that has soil settlement problems?

How to you propose to hold up the house completely, compact soil (if possible) and then pour a slab?

You are better off by underpinning (supporting) the house, excavating down for better soil and then building the foundation walls. Your contractor can probably work around the house in segments. After that he can level the house and put in new posts and footings for the interior. - You may even get a basement as a bonus if you go deep enough.

Dick
 
  #18  
Old 06-16-07, 09:31 AM
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I appreciate the advice. I have some more questions. First I don't really want to level the house I was wondering if it was possible to put a new foundation without tearing down the house (although I doubt it is). I don't really know anything about this, thats why I am asking. Also I live in Texas and basements aren't really around here. Almost everything is on a slab foundation that is why I suggested it. I know the best way to find out would to be to get a contractor or something to come out and give me advice, but I don't plan on remodeling for a couple years. I just want to get some info for when I do decide to do things. Thanks.
 
 

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