What's your flavor under slab, sand or rock?

farmfromkansas · May 19, 2021, 09:12:25 PM

If you are going to form a stem wall around the perimeter, just put your foam vertical inside the stem wall and cut it off just below the slab. I like to have a ledge for the slab to sit on, so used to nail 2- 2x4's around the inside of the form for a 8" wall, so you have a 3" ledge. And I like gravel under your slab, you never know what crazy thing will cause you to build up water under your slab, maybe a plumbing leak or some other crazy thing, and the rock will release that water. Sand does not allow the water to flow like gravel. I always used to put drain tile in basements, did that for my daughters house, and used gravel under the slab, ran the pipe outside the footings on the walk out side of the basement, showed my SIL where I left it, and told him to put a pipe in when he had his sewer system put in. He did not. Last year they built up water under the slab, and couldn't get rid of it till they dug in a pipe to drain it. Daughter got new carpet in basement. They didn't have trouble for about 13 years till we had a crazy wet spell, including a big flood. The flood didn't get in their house, but it soaked up everything.

Don P · May 19, 2021, 09:52:47 PM

I'd have to say I just thoroughly enjoyed Joe's article... guess that means I hadn't read it before posting my last comment :

:D.

Doc's pic shows a thickened footing section to be poured monolithic with the slab, don't forget to thicken under load bearing elements. I got called in on a TF job run amuk one time. Went into the basement and a post was doing its best to punch through the slab, the numbnut had put a serious post load on a 4" thick slab. From the pile of concrete debris, that had been his second attempt at the basement

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alan gage · May 20, 2021, 10:56:23 AM

Another Building Science article I just came across you might find useful, Sledgehammer. This time about slabs, stem walls, and how best to insulate them. It's a newer article and has links to past articles on the subject. Not so opinionated but still entertaining. This protection board also needs to deal with critters such as rodents. Don't underestimate the animal kingdom. I particularly hate mice.

BSI-118: Concrete Solutions | Building Science Corporation

I've done slabs and stem walls on 3 structures (a house and two shops). Each one was done a little differently and each one found me laying awake many nights as I tried to think through every step and how best to construct it. Best being a combination of cost, ease of me doing the work, and actually working as planned long term. Articles like the one liked above really helped me visualize the different methods as well as pointing out pitfalls I wouldn't have figured out on my own. I was fortunate that 2 of the 3 buildings were on sites with a deep gravel base and very low water table. Also fortunate that we don't have termites to deal with.

On one of my shops I used exterior insulation on the stem wall and allowed the metal siding to extend below the framing to cover it. When the building was backfilled the metal siding was covered. I figured this would protect the insulation it but I did underestimate the animal kingdom, namely my chickens, who continually scratched away at the soil on the south side of the building as they would sun bathe and uncovered the insulation, which they promptly pecked clean off the wall.

Alan

mike_belben · May 20, 2021, 11:25:07 AM

im jumping in the middle here without reading everything so forgive me if its been mentioned. i have a friend who is an engineer at a boston based architecture company doing interstates and major urban buildings. it is safe to say he is a concrete pro and i get my advice from him via text pretty often. biggest thing he ever warns me about is to "haunch" a monolithic slab. as they dry, slabs want to curl up like a potato chip. if its a 4" center thickness.. make its a 6 or 7" crust around the perimeter. i rarely see this mentioned. and building science guy is the first place i go.. that dude doesnt sugar coat.

Sedgehammer · May 21, 2021, 01:52:00 PM

@doc henderson Thanks Doc for going to all the trouble.

Here's what I am going to do

1. pour a 12" wide stem wall up to floor level. This wood have cut outs for a 1.5"x1.5" lip for siding and a 4.5" wide by 6" deep for the slab to sit on
2. plastic on grade run up sides of stem wall to level of the EPS that is going down
3. put down a 10 mil or so plastic on current grade
4. put down sand to EPS grade
5. put down type ll 2' EPS
6. lay down either 1/2" or 3/4" pex-al-pex. This has memory due to the aluminum core inside, so I can lay it down with few fasteners. if 3/4 I can have fewer runs, as runs can be longer
7. go crazy