Opinions on hybrid slab foundation.

[N80]

My daughter and her husband and going to be building a house in the country soon. They have talked to several builders and they have suggested that they consider what they call a hybrid slab foundation as it costs considerably less than a traditional pier and joist foundation with a crawl space.

The hybrid has stem walls and piers but then they back fill it, pack it and put the slab on top. Plumbing is under the slab and comes up through it.

They are asking around for advice on this. I didn't really have any advice for them other than that if there is a plumbing issue the floor has to come up and holes cut in the slab. They say that they are much more energy efficient. Mice and bugs would be less of a problem too.

Of course traditional foundations have their issues too. I've had homes with termite issues in the sills, humidity and mold issues. So they aren't without their own issues.

Anyway, curious to hear other things to consider.

 

[RNeumann]

If I’m understanding this correctly they are describing a two step pour. Footings and stem walls and then the slab. This is very common and in some cases cheaper. Some places that don’t get frost heave will do what’s called a monolithic pour where the footings and slab are poured at the same time.

Their aren’t really any issues with the proposed two step pour. Of course the details will be critical. Vapor barrier, insulation (if required) etc will all need to be done properly.
The type of pour is typically dictated by the engineer. The contractor should be following plans.
As for downsides of a slab vs raised floor-
You can’t get under the house if you want to make a change or add a wire etc.
However, you don’t have to worry about rot or leaks. You won’t have a bouncy floor and in that region it sounds like it will save you some money.

 

[Tractor Seabee]

Are slabs on grade common there? If not it could be a resale issue. Great opportunity to but the heat in the floor. I hate cold floors, especially places I like to run barefoot. At least put floor heat in the bathrooms, nice resale feature. I grew up in the Phoenix AZ area after WWII. Monolithic foundation/slabs were all there was in new work except custom. Outside walls were concrete block. One builder of subdivisions advertised, "sign today and move in in 30 days". Everything was prefabbed off site. Funny, that is come back as a new idea to cut cost.

Ron

Ron

 

[dmccarty]

Most of the houses my family, through multiple generations, have been on slabs. No plumbing problems I ever heard. Our current house is a slab even though it is on a slight hill. One side of the house, the top of the slab was 12-18 inches above grade. The other side of the house was about 60 inches.

A footer was poured, then concrete blocks were used to build up to the floor height. Plumbing was installed and pressurized for leaks. 67 stone, which compacts to 95ish percent when put in place. Rigid foam was then put down for insulation, I think it is 4 inches.

Our builder was a PE but I was a bit nervous about a load bearing wall being on 4ish inches of slab so he dug out trench in the gravel so there would be roughly a 18inch deep grade beam when the concrete was poured. The concrete was just poured on top of the rigid foam.

We did color the concrete. The color was a dyed powder. One could pour the concrete and then case the dye on top. We said ^&*() that, and had the dye added into the truck at the concrete plant so the color would be through out the slab. Cost a little more but it was worth it. Funny thing was the DRIED color is light tan/brown but wet it is very brown. When the colored concrete came down the chute it looked like brown poo! :shocked:

The concrete guys were standing at the end of the chute, shovels ready and when they saw that brown concrete, they all turned to look at me and the contractor with a WTF is this look on their faces. :shocked::laughing::laughing::laughing:

The slab was cut into a pattern I came up with for decoration and to handle cracking.

The details I used to build the slab are in one of the building guides, from Builder's Guides | Building Science Corporation. I also used the guide for wall and attic details.

We have had not problems with our house after 14 years. The previous home we owned was a slab and 30 years old when we sold it. Never a problem with the slab and/or plumbing.

I REALLY do not like crawl spaces. Horrible. The building science website has details on dealing with the problems with crawl spaces. Our floor does not squeak. It does not bounce. We left the slab as the finished floor and the ability to move stuff around easily is wonderful. If a toilet leaks at the seal, and we did not notice, we won't have floor rot issues. A coworker had to rebuild the floor in a bathroom because of this issue. Twas not cheap to fix.

Later,
Dan

 

[N80]

I REALLY do not like crawl spaces.

I hate them. I don't think I've ever seen one done right (proper moisture control, ease of access, etc etc. And as far as I am concerned EVERY bathroom located on a typical wooden joist system is going to result in rot at some point.

My daughter and her husband are leaning toward this hybrid slab but they keep worrying about plumbing problems, etc. I have to keep reminding them that a standard crawl space type system is not without its own very major drawbacks.

Their house site is fairly flat so there will not be any grade issues. We visited a house being built by one of the builders with a hybrid slab and it was on a bit of a grade. They had to have the stem wall and fill inspected by an engineer before they would approve it. The engineer approved it with no reservations.

My primary concern is that my daughter's house is going to be 2 stories. (I live in a 90 year old two story house.......I think they are a bad idea too). The issue is that if the first floor is a slab there is nowhere to run ducting and it will have to come down from the attic and run through the ceiling space between the floors.

 

[ultrarunner]

My brother's first home was slab neighborhood built in 1948.

65 years in it was common for the copper water pipes to leak... seemed someone in the neighborhood was always chasing a leak... the home 10k homes in 1948 where selling for 650k 65 years later.

When we remodeled and added I put all the water pipe overhead... worked well but needs to be well insulated.

Next door neighbor had the same done on 200k remodel and the first weekend the new copper water pipe froze and destroyed the ceiling and maple floor... it had a drop for an outside hydrant right at an eve vent.

 

[Oldpath05]

If at all possible go with a walk in basement, basements are a good place for washer/drier, well pump/acc, hot water heater, and or furnace, wood fired or other, plus makes it easier for future add on plumbing and electrical work, so without a basement the living house space has to much bigger.

Mine is a 3 wall 24'x32' cement foundation, south wall studded and boarded, I lived in for 5 years than had enough finances to build up, but with basements a dehumidifier is a must in summer months.

 

[RedNeckRacin]

I am building a house currently and did a traditional PA home. 2 story with basement. I'd have no problems doing a stemwall and a slab on grade. I would however make all of the plumbing runs in a pipe chase that I could replace the lines if need be and everything would be home run pex no questions asked. There would also be in slab heat. I have friends that built a pole barn and poured a slab inside to build their house. no issues to date other than they wish they had a basement.

 

[skipperbrown]

Almost every home in FL built in the last 30 years has this type of foundation. My suggestion is to make sure the fill dirt is tamped very tight and the concrete kept wet for a long time after the pour to help control cracks (usually with wet straw or flooding). Have your SIL and daughter go to a few of the concrete manufacturer's websites and read up on how they recommend a slab be poured and cared for. Have them cut and paste the manufacturer's procedures into their contract and then make sure they are followed. Contractors don't care about proper procedures because it takes longer, adds some cost, and they will be long gone before any issues crop up.

 

[Egon]

Slabs have many possibilities. The nicest may be in floor heat. Utilities can have a planned placement to accomadate a slab. A basement construction also has water & sewage lines buried under concrete.

Consider basement cost to garage type storage.

A basement is a poorly lighted damp hole in the ground that consumes many resources to convert it into habital living space.