Jelly
Veteran Member
Posts:1017
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| 08 Jan 2008 06:43 AM |
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I will be building in Louisiana in an area having a 0.2% annual chance of flooding less than a foot. That means no flood insurance necessary, and most homes are built on a simple slab on grade.
It would however be prudent to elevate the house somewhat - let's say three feet to be good and sure. The latest research suggests that vented crawlspaces cause more moisture problems than they solve, therefore unvented conditioned crawlspaces are starting to show up. You have basically a closed perimeter foundation and an elevated floor (with a vapor barrier under crushed rock on grade or simply a thin "rat slab", more on conditioned crawlspaces at buildingscience.com). But a rat slab would add a substantial amount of concrete cost, although without a rat slab it seems like you'd ultimately be inviting pests into your conditioned living space.
Another option is an elevated slab, which is essentially a closed perimeter foundation which is filled in and then the slab is poured on top of that. Then you build right on the tall slab as if it were on grade. But depending on what is used for three feet of fill I guess it might end up costing more than building the elevated floor.
I suppose a third option is a pier and beam foundation. The piers are far enough apart that the space under the house is adequately and naturally vented. The beam material needs to be considered - wood is out of the question. A steel I-beam would work, maybe reinforced concrete, and we also have to add the cost of under-floor insulation to the equation, or a steel SIPS or ICF floor, all pricey options.
So what's the best method - any opinions? Anyone have any insight or personal experience?
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Alton
Veteran Member
Posts:2157
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| 08 Jan 2008 11:00 AM |
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Jelly,
In your location, I believe using an elevated concrete slab would be the best option. Being in contact with the earth will offer some cooling advantages over the other systems. In my area, with sand and gravel readily available, I have found that it is more economical to build with an elevated concrete slab.
In my opinion, a pier and beam systems makes sense only when the height from the ground is much greater than three feet or the terrain is too rocky.
Since termites are so prevalent in the South, I suggest that you consider eliminating all of the wood that you can from the structure.
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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Jelly
Veteran Member
Posts:1017
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| 08 Jan 2008 02:42 PM |
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Alton, thanks for the input. I guess it depends on the quality of the soil, but if I excavated for a swimming pool would I be able to use earth from the site as fill for an elevated slab? |
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Alton
Veteran Member
Posts:2157
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| 08 Jan 2008 02:51 PM |
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Jelly,
Some code officials will allow you to use dirt to raise the pad. The dirt must be free of organic matter and pack well. I much prefer to use sand or gravel. You might consider using the excavated dirt to slope the yard away from your home. Although you would be on an elevated slab, it is still best to slope your yard to drain away from your house.
Feel free to call or write me. I do a lot of consulting for free. My way of giving back.
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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Jelly
Veteran Member
Posts:1017
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| 09 Jan 2008 07:37 AM |
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Alton, I'm sending you a PM/email. |
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Alton
Veteran Member
Posts:2157
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| 09 Jan 2008 05:41 PM |
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Jelly,
PM on this site says I do not have any messages. You might want to try my e-mail. alton at auburn edu I left out the at symbol and the dots because I get enough spam as it is. |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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Jelly
Veteran Member
Posts:1017
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| 06 Jan 2009 11:11 PM |
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I'm not having an easy time finding anyone around here who knows how to build an elevated backfilled concrete slab. Seems like they all want to use compacted clay fill to get the grade up to the desired level and then pour a monolithic slab. There are various reasons I want to avoid this.
Anyway, I guess I could DIY the foundation, even though that was something I had planned to leave to a professional.
Anyone know if a CMU stemwall typically requires horizontal rebar or is it normally just vertical down into the footers? It would probably only be two courses of block, three at the most. |
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Bruce Frey
Basic Member
Posts:429
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| 07 Jan 2009 05:17 AM |
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I do not know the nature of the clay or soils you have available, but I think your best options are likely to be:
1. Raise your building pad with properly compacted fill (with testing), using sand or gravel for the uppermost layer and a slab on grade (which could also be post tensioned if that is the norm in your area).
2. If you don't trust the fill, use conventional footings and stem walls to support a structural slab, using the fill dirt as a form, not as a support. The interior support structure could be concrete "post and beam" instead of stem walls.
It is easier to raise a large building pad with fill because you can use big equipment to do it than it is to fill an area within stem walls with small equipment.
I would look for a civil/structural engineering company that has experience with commercial projects. I am sure that someone locally has done what you want to do.
Personally, I would avoid a crawl space if at all possible.
Good luck.
Bruce |
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Manfred
Basic Member
Posts:200
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| 07 Jan 2009 07:10 AM |
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I have done stemwalls in NC, filled them with sand and compacted. Stemwalls out of ICF filled with concrete. You have to make sure that when you compact the dirt (sand) it finds it right density. You can not allow for settling of the dirt underneath the concrete since that would create voids and stress on the slab. Another thought is, since you are going that way to use the inside parameter of your house as a storage tank (sand tank) for energy supplied through solar gain from the roof. If you are interested I can send you information on this. Just e-mail me.
I concure with Bruce - crawl spaces are lost real estate, harbour disease, are not ventilated properly, nobody understands them, they have been building wrong with them since the beginning of time. |
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| Manfred Knobel<br>Moss Pointe Builders, Inc. |
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Jelly
Veteran Member
Posts:1017
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| 07 Jan 2009 02:43 PM |
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Yes, crawl space is off the table.
Bruce, regarding item 2 in your post you had me until you said "interior support structure could be concrete post and beam instead of stem walls." Do you mean interior load bearing walls when you say the interior support structure? Or do you mean as a different option the whole foundation could be concrete post and beam instead of CMU stemwall?
Manfred, what you describe doing with ICF is essentially what I want to do with CMU. I would be happy to do it with ICF, but the material cost would be much lower with CMU that is externally insulated. I will try sending you an email or PM for more information about the energy storage, but my experience with this site is that it usually doesn't work. Let me know here if you don't receive anything from me. |
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Bruce Frey
Basic Member
Posts:429
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| 07 Jan 2009 05:32 PM |
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Posted By Jelly on 01/07/2009 2:43 PM
Yes, crawl space is off the table.
Bruce, regarding item 2 in your post you had me until you said "interior support structure could be concrete post and beam instead of stem walls." Do you mean interior load bearing walls when you say the interior support structure? Or do you mean as a different option the whole foundation could be concrete post and beam instead of CMU stemwall?
What I meant was that if you use a structural slab (using the soil as a form instead of support), you would probably do stem walls on footings for the perimeter support. For a structural slab, you want to keep the spans short so there likely be some slabsupport needed for the interior portion of the house. For the interior, you have more options than footings and stem walls. As an example, you could do small piers connected by beams. The beams would just be trenches in the soil with rebar and concrete. The slab and foundation design would need to incorporate interior bearing wall locations, etc. All this is to hold up the slab. The walls would be built on top of the slab in the "normal" way for Louisiana and Texas. Yes, doing this or a regular slab on grade creates a cold bridge at the edge of the slab, so the folks up north won't be doing it this way. If you decide to build stem walls first and then fill with a slab to be supported on soil, see if you can find sand to use as fill. It is much easier to compact than clay with small (hand) equipment and sand compaction is a bit less sensitive to moisture content. Sand is compacted with weight and vibratory compactors. Clay sometimes requires "sheepfoot" rollers to do it. Your clay, however, may be easy to work with, so check with your civil/structural consultant. In any case, make sure that the fill has no organic material in it. Bruce |
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Jelly
Veteran Member
Posts:1017
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Bruce Frey
Basic Member
Posts:429
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| 08 Jan 2009 04:51 PM |
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Between the two choices.....
I hesitate to recommend having a slab supported by foundations at the perimeter and compacted fill on the interior. For me it should be one or the other. If you get really good compaction, it will probably be OK, but there is an increased danger of differential settlement and slab cracking.
My preference would be to let the slab float inside the stem walls (also eliminating the cold bridge) as a "slab on grade". You still need to do a really good job of compaction, but in my opinion there is less chance of differential settlement causing cracks.
Will you go up with ICFs above the slab?
Any chance of water getting tapped inside your stem walls? Why are you reluctant to raise the entire pad and do a slab on grade?
Bruce
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Jelly
Veteran Member
Posts:1017
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| 08 Jan 2009 05:58 PM |
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It will be steel SIPS above the slab (99% sure of that at least).
Ok I'll admit it, one major reason I don't want to raise the pad and do slab on grade is aesthetics. The raised floor home just looks better. Ok now that I've got vanity out of the way, it's also for comfort - I want to get my living space away from ground level. You can drive through several postwar parts of town and see slab-on-grade-ranch-home after slab-on-grade-ranch-home with the bottom two feet of the house covered in damp and moss. I'm sure plastic barriers help, but getting up and away from the spongy ground would be even better. The ground stays wet here for days at a time. Getting away from pests is another plus.
I don't trust using clay fill because of numerous problems (already discussed in that concrete floor thread). It's probably adequate if it's well compacted but I've known lots of people round here with problems when built on fill. Another issue is cost, where it seems like clay and gravel/sand are close in price, I think one would be using more clay if you're building up a berm with a gradual rise. It's more material being used (and more potential floodwater displaced). The gravel/sand would be limited to the footprint of the actual house.
But having said all that, I like to keep an open mind and I know I have to stay flexible if I'm going to get this house built. And I very much appreciate the input in this forum.
Anyway, Bruce, it does make more sense to let the slab float and use the stemwall as a screed. Wouldn't there still be a cold bridge though? I mean the CMU is concrete, too. I had planned to insulate the stemwall externally to combat bridging. |
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Manfred
Basic Member
Posts:200
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| 08 Jan 2009 06:09 PM |
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jelly, did you get my e-mail? Thanks |
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| Manfred Knobel<br>Moss Pointe Builders, Inc. |
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Jelly
Veteran Member
Posts:1017
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| 08 Jan 2009 06:36 PM |
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Manfred, I did get the email about the energy storage. Seems like a good idea - somewhat confusing though, hard for me to wrap my mind around it. I imagine it would be expensive. |
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Bruce Frey
Basic Member
Posts:429
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| 09 Jan 2009 03:17 AM |
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Posted By Jelly on 01/08/2009 5:58 PM
It will be steel SIPS above the slab (99% sure of that at least).
I don't trust using clay fill because of numerous problems (already discussed in that concrete floor thread). It's probably adequate if it's well compacted but I've known lots of people round here with problems when built on fill. Another issue is cost, where it seems like clay and gravel/sand are close in price, I think one would be using more clay if you're building up a berm with a gradual rise. It's more material being used (and more potential floodwater displaced). The gravel/sand would be limited to the footprint of the actual house.
...........
Anyway, Bruce, it does make more sense to let the slab float and use the stemwall as a screed. Wouldn't there still be a cold bridge though? I mean the CMU is concrete, too. I had planned to insulate the stemwall externally to combat bridging. Now I understand. I thought you were also planning to increase the grade around the house as well. Accordingly, you will be putting an exterior finish on most of the stem walls, correct? What will you do at the entrance(s) and driveway? Bruce |
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Jelly
Veteran Member
Posts:1017
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| 09 Jan 2009 07:37 AM |
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Bruce, yes I was planning on eps foam board placed on the stem wall for insulation, then thin stone veneer placed over that. Or brick would be nice, too - I guess it could bear on the edge of the footer for the stem wall? Not sure about the brick - seems stone veneer would be easier because it's just lick and stick. The entrances will have steps with either stone veneer or brick.
The garage is detached, so the driveway runs along the side of the house. I wasn't going to pour concrete where I didn't have to - will use flagstones or pavers in sand for driveway, sidewalks and patio. |
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Bruce Frey
Basic Member
Posts:429
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| 09 Jan 2009 12:09 PM |
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I know you read Lstiburek and you will watch the grades areound the house. Think through all the what-if's of water issues, like what happens to the water that would likely end up in the fill under your house if it does flood. How does the water get out? Depending on its nature, clay can be impervious, or nearly so. What is the path for it to dry? Do you waterproof the inside of the CMU to keep it from wicking upward? What you really have is a crawl space filled with sand/gravel.....and I am not sure whether that is good or bad.
The moisture issue could make me rethink floating the slab inside the wall...a continuous slab would certainly be better protection against moisture.
I still like the idea of making an island and building on top of it.
Bruce |
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Jelly
Veteran Member
Posts:1017
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| 09 Jan 2009 12:39 PM |
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Well, I didn't think I was re-inventing the wheel. Maybe it's a regional thing? Perhaps I should be calling it a raised slab. Anyway, here's a pic of one at the Building Science website: slab pic
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