Slab on grade with 'curb'

[KDub1]

I am having someone build a 2.5 storey house this year and am currently working out the floor plans. The house will not have a basement nor a crawl space, but rather a slab-on-grade so that the finished floor elevation would be the slab thickness (approx 6") above the surrounding ground. However I am concerned about being at a reasonable high enough elevation above the high-water mark of a river fronting my property. So I was thinking of having a 12" high concrete 'curb' at the base of the perimeter wall, except at the one entrance. (The curb would give me an extra 12" of floor to ceiling height if I am using standard 8' long lumber for the walls!) If the was a high-water event, I would have an extra 12" before the river water would enter the first floor, assuming that the entrance is blocked off good with sand bags.

I was wondering if it is reasonable to expect a contractor to form and pour the SOG and the curb at the same time (like the hand formed curb and gutters along the side of most roads) or whether they would have to do a separate pour for the curb. I prefer it to be monolithic to avoid relaying on a waterstop at the joint. Would 12" be too much to expect for the average contractor to form and pour at the same time as the SOG, especially with all the rebars? I'm also trying to avoid a typical foundation wall. I don't think this would matter, but I would like to have in-floor heating as well.

Thanks in advance for your help!

 

[larry4406]

A competent concrete crew can due this.

The inside of the curb is formed using 2x material and supported by stakes pounded vertically down thru the slab area. The slab and curb is poured in one shot with reserve concrete maintained. As the pour sets up, the stakes and curb form are removed and the stake holes filled with the reserved concrete.

 

[Leaflessshadetree]

The "curb' may not have the effect that you are thinking. Will you have a 12"step down to the floor or will the door openings be flush with the floor? What about after the water comes up the extra foot, will your curb form a pool that holds the water in? You may want to check with a few insurance companies. They have some unique rules for structures that could possibly flood.

 

[Bondo]

The "curb' may not have the effect that you are thinking. Will you have a 12"step down to the floor or will the door openings be flush with the floor? What about after the water comes up the extra foot, will your curb form a pool that holds the water in? You may want to check with a few insurance companies. They have some unique rules for structures that could possibly flood.

Ayuh,... It would make more sense to raise the Whole slab the foot,...

 

[KDub1]

The "curb' may not have the effect that you are thinking. Will you have a 12"step down to the floor or will the door openings be flush with the floor? What about after the water comes up the extra foot, will your curb form a pool that holds the water in? You may want to check with a few insurance companies. They have some unique rules for structures that could possibly flood.

I will only have one door which will be flush with the floor (the curb will surround the entire slab with the framed exterior walls on top except where the one door is. In the event of a flood, I will have 1' protection via the curb before water soaks in thru the exterior wood walls leaving me to sand bag >1' around the outside of the door. This would also create a sump to pump out. Although I will not have any nice finishes, I figure 1' of protection is better than none.

I am above the high water mark, but with the extreme weather these days, you never know.

Thanks for the heads-up about the insurance companies!

 

[KDub1]

Ayuh,... It would make more sense to raise the Whole slab the foot,...

Yes that is a possibility too, but then the house will be a foot higher which is not so desirable due to other reasons not explained. Also, I was hoping that the cost of the curb would offset having to buy 12' long lumber rather than 8'ers for the exterior walls in order to achieve an 8' floor to ceiling height (I would like to have a drop ceiling too). I guess it might be better to leave it up to the contractor to determine what is more cost effective depending if he finds it cheaper to have more concrete vs more wood.

 

[Architorture]

so if the bottom of the slab is essentially at grade, what is stopping the 18" of water outside from simply saturating the base below the slab and coming up through the joints?

 

[themiller]

From someone who has dealt with water issues... Raise the whole house.

 

[volleyball]

Unless you share the reasons, the best answer will be to put the house on stilts, whether 12" or 12', raising the house is the best way. It will also lower your insurance payments which may pay for itself in a year or 2.
I hate tight crawl spaces but hate slab on grade even more.
What is your locations 100 year flood height?

 

[rburke65]

Why could you not raise the entire stem wall, creating a crawl space, and be 3' above the high water mark? Maybe you don't want the "crawl" for some reason. And it might help if we knew from where you are located. The only experience I have with flooding is from TV, but after seeing the devastation, I would plan on total be 'red marking' the 12" and planning on a 4 foot crawl to stay dry. Good luck to you.

 

[MrV]

The way my outbuilding was formed (a 6 inch curb) was to cantilever the inner form by suspending it from the top. I dont see why this couldnt be done with a 12 inch curb but the additional head pressure from the concrete might make it difficult to restrain the bottom of the inner form from blowing inwards without a lot of additional diagonal bracing....which equals more cost. An experienced concrete guy ought to be able to give you a quick answer on this. Agree you dont want this done in two pours if you are trying to make it waterproof.

dean

 

[larry4406]

Curb and gutter is poured everyday in America on any given street. The curb is elevated relative to the pan. Same concept. All one pour.

 

[readhead]

It is going to be a pain to install base and casing.

 

[KDub1]

Unless you share the reasons, the best answer will be to put the house on stilts, whether 12" or 12', raising the house is the best way. It will also lower your insurance payments which may pay for itself in a year or 2.
I hate tight crawl spaces but hate slab on grade even more.
What is your locations 100 year flood height?

One thought is that sog (i.e. a floating slab) is generally cheaper than a strip footing / block wall or post foundation with insulated crawl space and wood floor (I would also like to have an in-floor heating system). So I thought it could be engineered in some practical way. I'm not sure what the 100yr flood level is, so that is something I should find out (thanks for pointing that out). However, the river rose this Spring to just under 3.5' below the finished elevation of the proposed slab which is higher than any other year. Given that the water table is even higher do to the Spring melt, raising the grade would be helpful too.

 

[volleyball]

Remember Katrina, the dam held for years and then it got over run. Those dams went back to holding the water, only in instead of out. so once you get water, it may be hard to get rid of. You put in drains, it just backflows.
I live on a hill. Got plenty of stairs. Not as nice as on level ground but I never worry about flooding. Put a garage or a workshop, storage or even a mancave although the lower level is not to be living space.