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-   -   Basics of new home slab construction? (http://www.diychatroom.com/f19/basics-new-home-slab-construction-110365/)

J S Machine 07-11-2011 09:48 AM

Basics of new home slab construction?
 
First of all, what are the differences between a crawl space house and a concrete foundation house? I always understood concrete to be the better of the two, but never quite understood the differences. I do know that after walking on several crawl space wood floor houses, I like the slab floor better.

I know that concrete can have crack issues too. My father was a tile man and I used to help him alot when I was younger. Sometimes we had neighborhoods where new houses were being built that were really bad bout crack problems and shifting foundations. We would have to go back and redo pieces of tile that had busted because of cracks in the slab. Eventually we started using crack isolation thinset mortar to counter the problem.

I haven't watched a slab being built, but I think I may have an idea. I know that they have to have footings, but I am unsure if everything is all cement. I see concrete blocks used every now and then and I think I have seen the inside filled with sand before the final slab surface is finished. Is this correct?

I guess my final question is do slabs have to be done in one entire piece or can they be sectioned off into quarters or sperate areas?

AGWhitehouse 07-11-2011 11:37 AM

Both scenarios have foundation wall that extend to below the geographical area's frost depth. Connecticut is 42" deep from grade.

Crawl spaces are done to allow for access to under-floor utilities such as plumbing and electrical. The raised floor also keeps the living environment seperated from the moisture and cold-sink of the ground. elevated floors can be insulated and maintained easier as well.

Concrete floors are usually poured 4-5" thick over a 6" stone base and 6 mil poly vapor barrier. They can be poured monolythic (one big pour) or they can be poured in sections with expansion joints. If poured monolythic then control joints will need to be saw cut into the slab surface to allow for controlled cracking. One aspect of concrete floors that is always missed is the insulation. The earth hovers around 55 degrees F. With a living space at 70F and the earth a 55F you have a 15F differential without any insulation. If I were to suggest a slab composition for modern times it would be 6" stone base, 6 mil poly vapor barrier, 2-4" polyiso, 4-5" concrete slab.

J S Machine 07-11-2011 12:21 PM

Quote:

Originally Posted by AGWhitehouse (Post 683864)
Both scenarios have foundation wall that extend to below the geographical area's frost depth. Connecticut is 42" deep from grade.

Crawl spaces are done to allow for access to under-floor utilities such as plumbing and electrical. The raised floor also keeps the living environment seperated from the moisture and cold-sink of the ground. elevated floors can be insulated and maintained easier as well.

Concrete floors are usually poured 4-5" thick over a 6" stone base and 6 mil poly vapor barrier. They can be poured monolythic (one big pour) or they can be poured in sections with expansion joints. If poured monolythic then control joints will need to be saw cut into the slab surface to allow for controlled cracking. One aspect of concrete floors that is always missed is the insulation. The earth hovers around 55 degrees F. With a living space at 70F and the earth a 55F you have a 15F differential without any insulation. If I were to suggest a slab composition for modern times it would be 6" stone base, 6 mil poly vapor barrier, 2-4" polyiso, 4-5" concrete slab.

So when you say "foundation wall," is this the course of blocks I see running around the peremiter?

I live in AL, and there is not much issue with cold here. I seem to see that mentioned in alot of my threads. I remember talking about the frost line in another of my threads. We don't usually get a whole lot of cold weather a year, maybe two three months of it. Of that, you might have a couple of instances where it gets below freezing. Our enviroment is not like it is in northern states who have to deal with snow. They close schools around here when we get snow :lol:

When you speak of monolithic slabs..I cannot ever remember seeing one that had control joints cut into it. Does it have something to do with where I'm at?

The reason I ask about this is because I would like to take on building my own home one day. I may not be able to build the whole entire thing at one time, so I hate to take on the costs of an entire slab. If it could be done in sections it would be easier on me, but I totally understand the reasons for doing it all at once.

I guess another thing to consider are the repair costs associated with plumbing failures in slab foundations. If I understand correctly, this stuff is burried in the slab and completely incased in concrete, correct?

A crawl space may be easier to deal with if a problem were to ever occur.

AGWhitehouse 07-11-2011 12:58 PM

Yes, foundation can be concrete, masonry, wood, or steel. Depths and materials used differ per geographic region.

Go into a Lowe's or Home Depot. The black joints in the slab and the diamond shapes around columns are the expansion joints. If a slab is small enough you won't need these.

J S Machine 07-11-2011 01:07 PM

I know what they are, I was just saying I have never seen one in any residential slabs. I've seen alot of slabs too. I wonder if it has to do with my region. For instance, would I see more expansion joints up north?

AGWhitehouse 07-11-2011 01:29 PM

Most homes don't put them in and don't cut them in either. The result is hairline cracks that form irregularly. They are usually only hairline cracks. To control the cracking, 1/4" deep saw cuts are sometimes put in the slab surface to create "weak points" where the slab will crack.

The irregular cracks can look something like this: http://www.inspectapedia.com/structu...ack224-DFs.jpg

J S Machine 07-11-2011 01:48 PM

Quote:

Originally Posted by AGWhitehouse (Post 683928)
Most homes don't put them in and don't cut them in either. The result is hairline cracks that form irregularly. They are usually only hairline cracks. To control the cracking, 1/4" deep saw cuts are sometimes put in the slab surface to create "weak points" where the slab will crack.

The irregular cracks can look something like this: http://www.inspectapedia.com/structu...ack224-DFs.jpg

ok. Those look exactly like the cracks I have seen in other slabs and like the ones I currently have now in my own home.

I wonder why they don't do it? Saving cost?

AGWhitehouse 07-11-2011 02:36 PM

The cracks don't really affect anything other than moisture and gas penetration. Since most slabs have a vapor barrier right below, the penetration issue is essentially moot. So yes, savings is the main reason.


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