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Eson
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# Posted: 25 Feb 2023 02:45pm - Edited by: Eson
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Hello all,
Lurker here that is finally starting my own project. I am building a 12x12 cabin with a shed roof (back wall 14' high, front wall 10' high) with a sleeping loft. We do not plan on any utilities in this main cabin, as we'll build a lean-to or piered addition next to it with primitive kitchen and bathroom. Main 12x12 cabin will essentially be a few pieces of furniture and a wood burning stove. Walls will be 2x4 construction with 6×6 post and beams for added strength and aesthetic (kind of a hybrid post/beam construction).
To the question...found a local concrete contractor with 40+ years who prefers small jobs like mine at this point as he is winding down workload (perfect!) and he's responsive, so really liking him at the moment. I was expecting him to propose footers below frost depth, and then the 12x12 slab on top of that (we're zone 6, south eastern Illinois). To my surprise he said in his experience, the footers would work (i.e. prevent frost heave) but he had seen frost heave in the middle of the slab that cracks it (since it's a shallow pour). So he is proposing a monolithic 4" slab on grade with 12" thickened edges to support the walls. He says for this project, if the slab moves/settles a bit, better for it all to move together instead of the footers and slab acting independently.
Does this sound right to y'all? Many thanks for any time with this!
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Tim_Ohio
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# Posted: 25 Feb 2023 04:00pm
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Have it set high so you can grade down and away from it. Water will run away and will not be a part of frost heave. Yes, put gutters on it so water isn’t running down off the roof into the slab. Will there be rebar or screen in the slab, to strengthen it? I’d ask for it. Most garage slabs are poured this way. I think it will be alright in the climate. Some people have foam board insulation put under the perimeter of slab, from the edge to about a foot in. Some say it prevents bridging of the cold. I’d put a floor on top of the slab with insulation under it. Concrete floors are cold.
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Peewee86
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# Posted: 25 Feb 2023 04:00pm
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A thickened edge monolithic slab is certainly a viable foundation option. This is how I did my cabin and I am much further north in Minnesota. Five years later, I have yet to find a single hairline crack in the concrete or anything that has suggested frost movement of any kind. The link to my cabin build is below.
It does concern me when you talk about adding a lean-to with peers at a later date. The whole point of a monolithic slab is that if it ever does move, it will move as a single unit. The lean-to footings could move independently and create problems. Will your finances allow you to pour concrete under the future lean-to area at the same time? -PeeWee
https://www.small-cabin.com/forum/6_9417_0.html
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DaveBell
Moderator
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# Posted: 25 Feb 2023 04:07pm
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Yes, sounds good. You should also have a compacted bed of gravel under the slab to help drainage, two rebar in the footers around the circumference four inches from the bottom, and a grid of rebar making two foot open squares or smaller across the slab. All rebar should be steel wire tied together and to the chairs (standoffs). Ask him to install J-bolts every 4-6 feet around the edge while the concrete is still wet, so you can secure the walls to the slab. Probably 2 on each side for a 12x12. Draw a map of your stud walls, 16" O.C. and then mark where the J-bolts should go so the J-bolts end up between studs. You can leave out one J-bolt where the door is. Seven J-bolts should keep your cabin on the slab.
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DaveBell
Moderator
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# Posted: 25 Feb 2023 04:09pm
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Good pics Peewee86.
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Eson
Member
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# Posted: 25 Feb 2023 04:18pm
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Quoting: Tim_Ohio Will there be rebar or screen in the slab, to strengthen it?
Thanks for the response! He was planning on screen and I asked about rebar and he seemed happy to add it.
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Eson
Member
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# Posted: 25 Feb 2023 04:23pm
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Quoting: Peewee86 It does concern me when you talk about adding a lean-to with peers at a later date. The whole point of a monolithic slab is that if it ever does move, it will move as a single unit. The lean-to footings could move independently and create problems. Will your finances allow you to pour concrete under the future lean-to area at the same time?
Thank you for sharing. Thats a good point on the piers, I'll evaluate on my end and see what I can do. Our thought was to save money by simplifying the slab as much as possible, then the addon I could take my time with (i.e. adding a toilet that flows to hobo septic).
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Eson
Member
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# Posted: 25 Feb 2023 04:25pm
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Thanks @DaveBell, I'll confirm those specifics, he had mentioned some gravel and I asked to add rebar in addition to the metal mesh he was planning, but I don't have exact details of rebar/jbolts.
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Tim_Ohio
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# Posted: 25 Feb 2023 04:46pm
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If you have the lean-to slab poured with the main, how do you keep the weather from bringing water under the walls of the first structure, before completing the addition? That brings up a wall detail, btw. Use pressure treated lumber in the wall plates. Your siding will come down over the outside of the wall to cover the seam between the wall and the slab, but moisture is still a problem on that plate.
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Eson
Member
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# Posted: 25 Feb 2023 04:55pm
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Yeah, I guess I'm not sure what's different with doing a piered addon later, than a typical house with a foundation and a porch (our current home has a wraparound porch with posts on concrete footers).
@Tim, yes that's exactly what I'm planning to do regarding sill plates and siding covering the seam. I'm a decent carpenter, but concrete/foundations are new to me!
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gcrank1
Member
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# Posted: 25 Feb 2023 05:12pm
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Do I remember correctly that there should be a barrier material between any wood, treated or not, and concrete; ie, under the sill plates.
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DaveBell
Moderator
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# Posted: 25 Feb 2023 07:05pm
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Sill Gasket
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DaveBell
Moderator
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# Posted: 25 Feb 2023 07:39pm
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12x12 slab 4 inches thick is 1.83 CY of concrete. A concrete truck typically hauls 8 CY. If you bought 1/2 truck load (4 CY) you could pour a 12x24 slab and avoid problems building a lean-to or piered addition next to it.
Are you going to use metal roofing?
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Eson
Member
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# Posted: 25 Feb 2023 07:45pm
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Quoting: DaveBell Are you going to use metal roofing?
Yes, metal roofing is the plan
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DaveBell
Moderator
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# Posted: 26 Feb 2023 07:26am
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Quoting: Eson Yes, metal roofing is the plan
Don't crush the screw rubber washer. So many people do that with power drivers and then the roof leaks. Screw in part way and then use a rachet and socket to finish. There is caulk or dual sided tape made to connect the sheets together water tight.
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Tim_Ohio
Member
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# Posted: 26 Feb 2023 07:34am
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If you have the lean-to portion of the pour set below the main floor, a step down, the pour could include that part as well. Then water would not be a problem, before you get to around to it. Your contractor would be adding a little more to frame that part, though. It might make the floor plan a little more interesting to have two levels. I dunno. Maybe others have better ideas.
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Steve_S
Member
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# Posted: 26 Feb 2023 07:48am
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When I built, I used a Frost Protected Slab Foundation which is pretty much as you described. I have the Radiant Flooring PEX embedded within the concrete and BOY as a Geezer I totally love that heat !
If Curious, I used a preconfigured "kit" from here:
http://www.radiantcompany.com/
BTW: It was cheaper than I expected and well worth it.
Slab Poured and smoothed, 7 days later (curing is required) Sealer Applied to concrete.
Slab Prep: Because the soil here is a Sandy Loam and I am inside an unmolested forest, we excavated 10' deep to remove the loose soil and erratic boulders. Then it was backfilled with Pit Run and rock, packed down with a 35 Ton excavator with a fine gravel topping, the Slab was installed on top of that very solid base. Whatever you do, you have to be certain that you are pouring that slab on a really solid base.
My Walls are 2x6 using Advanced Framing method.
Double Top Plate & Double Bottom Plate (1 against the Concrete is PT then the normal SPF), all attached with Galvanised Nails & screws of course.
The PT is attached to the slab with Concrete Screws.
Between the concrete & Lumber I used Owens Corning ProPink Sill Gaskets with a Bead of Accoutiseal between the concrete & Gasket and another bead between the gasket & wood.
REF Links:
Owens Corning ProPink Gasket
1/2 inch x 4 inch Concrete Anchors
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