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Discussion Forum

A Floating Garage ?

| Posted in General Discussion on January 3, 2001 08:36am

*
b WBA At Your Service

Occasionally I tape some PBS shows and recently one caught my attention. It was a Hometime show with Dean and Robin building a two story, three car, custom garage in Minnesota. (I think) Instead of taking the footers to 42″, they chose a shallow footer with the intention of letting the entire structure “float” during the Minnesota frost. My background tells me to build tool sheds like this but not high-end garages.(Their backers always seem to have deep pockets.) Anybody up in the frozen tundra want to enlighten me on this practice ?

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  1. Gabe_Martel | Dec 16, 2000 02:39am | #1

    *
    On both sides of the 49th you can build homes/small buildings on a floating slab and as long as you found it on properly drained base without any organic content, it's not a big deal.

    Building on the tundra is a different matter. Perma frost requires buildings be founded on piles driven to specified refusal. Even then they move continuously.

    Gabe

    1. tedd | Dec 16, 2000 03:14am | #2

      *Did they use an Alaskan slab (thickened edges)?

      1. Mike_Smith | Dec 16, 2000 03:38am | #3

        *if i could get the code approval, i 'd lay a gravel bed and float the slab... thickened edge...gave up trying to get the approvals..my favorite garage now is pole-barn....... i hate pouring all that concrete to 42 inch below grade..but we do it...

        1. Rein_Taul | Dec 17, 2000 07:07am | #4

          *Great question!I've been thinking about rebuilding (see replacing) a cobbled together heap of junk garage this way myself. I haven't done much research yet, but a quick scan has not given me any insight into how large a project should be before it needs a full foundation. I suppose the weight of the structure would have something to do with it. I don't expect a masonry wall to hold up on a floating slab. Anybody know?I'm thinking of a 12 x 36 ft garage/shop combo. I don't have a big budget, but that shop space would sure be nice. I'm not going to be able to afford masonry, so I'm aiming for a light structure.

          1. Davo_ | Dec 21, 2000 06:53am | #5

            *Rein,My garage is 24 X 36. I laid 2 rows of concrete block on top of an 8 inch thick floating slab, and then built a wood framed structure that sits on top of this block. My framed walls were 9 foot high, and the roof was trussed. I inserted 1/2 inch threaded rod approx every 4 to 6 feet apart, that went down through the sill plate, the concrete block cavities, and were embedded into the slab itself. This tied the whole structure together to guard against uplift from winds.Slabs that are properly reinforced with rebar can easily hold masonry walls. When you refer to masonry, you didn't specify whether you meant concrete (poured in place) walls or concrete block. Block is much lighter than poured concrete and is used all the time on floating slabs. I have installed poured-in-place concrete also on top of floating slabs without no problems. Mind you, the concrete needs to be more than 4 inches thick (which is all you need for a standard floor) where the walls will sit.Davo.

          2. Rein_Taul | Dec 21, 2000 11:03pm | #6

            *Thanks Davo,You confirmed my untrained ideas. I have also seen garage slabs with thicker sections under the parking area for added stregnth. Your 8" thick slab sounds like this would not be needed. Where did you build your garage? What is the frost like?By the way the masonry I was referring to is concrete block.

          3. Train_Driver_Chris | Dec 21, 2000 11:38pm | #7

            *I come from Saskatchewan where frost heave is a major concern. (ie. Mother X@#$#$%!! Cold and lots of clay). Like Gabe says, a properly placed compacted structural granular fill under a reinforced concrete slab with thickened edges will be effective. It is done all the time in this part of the world. Three things to consider, never use wire mesh to reinforce, ensure proper control joints in your slab and if you plan on attaching this to your house in anyway, scrap the idea of a slab alone...foundation should be deep as the structure your attaching to, usually accomplished using piles and grade beam. My garage is 24x36 with a 5" single reinforced slab with 10M bar 12" o.c. with a thickened edge that is 12" thick on the outer edge with a 12" footing width tapered back at 45 to the slab thickness reinforced with 2,15m bar through the footing at 3" from bottom surface of footing. Hope this helps. The other comments are right on as well...TDC

          4. Davo_ | Dec 27, 2000 08:42am | #8

            *Rein,I reside near Pittsburgh, PA. The frost depth in this region is listed at 36 inches; though I've never seen frost this deep in this area. We do get snow and plenty of cold temperatures. Temperature has been in the teens all week with a wind-chill putting the temp in negative digits.Like TDC said, I too used rebar (1/2 inch diameter) in mat form with 1 foot centers. I also used chopped fiberglass in the cement mix. (Some people claim the fiberglass will do as good as the rebar, and therefore rebar is not needed. Don't believe it. Go with the rebar. The fiberglass was just added "insurance." Note that a slab with chopped fiberglass won't finish out to as smooth a finish on the surface.Concrete block can easily be supported with a slab.Like TDC said; floating slabs are for single, un-attached buildings. If you were wanting to attach your garage to your existing house structure, you definately need to tie the two together; and use the same foundation structure as the house; so that both dwellings will move together in harmony (if they move at all) as a single unit and therefore stay stabilized. Slabs can move (though you would be hard pressed to see it with the naked eye). If your garage foundation was not tied in solid to your house, then any slight movement would cause the framing (at the tie-in point) to pull apart from the house, and cause other problems as well. If you are going to build a seperate, detached garage, (which was what I built) then a slab is plenty suitable, and of course there is no tie-ins to be concerned with.BTW, after compacting the ground, place 2 inch Dow Blue-board rigid foam on the ground instead of the plastic. The foam will better insulate your slab and help control heat loss. You may have been thinking about pouring vermiculite into the open cavities of your concrete block as an insulator. This does help, but a rigid foam on the outside and /or inside wall with furred out drywall would be better in a cold climate. If you didn't want the extra expense of the furred out walls, then by all means, at least pour in the vermiculite. Thats the norm around here...block walls with just the vermiculite in them...better than nothing.Also, if you go with a thickened edge slab,(i.e. .. a 4 inch slab in the middle with an 8 inch thick edge that is 12 to 16 inches in width) make sure the dirt is chamfered at the 4inch to 8 inch transition point. Don't allow the 4 inch slab to simply drop into the 8 inch trough area with the trough being square edged in shape. At this point where the concrete flows into the 8 inch trough, if the dirt is not chamfered or rounded, the concrete will form a 90 degree angle at this point and is a likely point where a stress crack could form. By rounding the dirt at this point, you are eliminating a stress point. Hope this description is not too confusing.Good luck Rein. Hope all goes well. Davo.

          5. Matt_G. | Dec 27, 2000 02:45pm | #9

            *Chris:How thick is a 10 or 15M bar? Down here get our rebar in what we call #5 - 5/8", #6 - 6/8", etc.TIA

          6. Pete_Draganic | Dec 27, 2000 02:52pm | #10

            *Mike,Next time fill that 42" trench up with lumber cut-offs and lunch wrappers first. You will be amazed at how much concrete you save. Think of it as improvised rebar. PeteDISCLAIMER: This was a joke. No intention to use such a poor building practice was truly expressed or implied by the author of this post. (you've got to be careful with these things around here these days)

          7. Gabe_Martel | Dec 27, 2000 03:20pm | #11

            *Chris, 10m bars are about 1/2 inch and the 15m are 5/8, 20m are 3/4.Hope this helpsGabe

          8. Matt_G. | Dec 27, 2000 03:33pm | #12

            *Thanks buddy.

          9. Gabe_Martel | Dec 27, 2000 03:34pm | #13

            *Davo gave great advise. The only thing that I would advise would be based on the experience of the user.If your uncertain about your experience with this type of layout, then you would be advised to use a frost footing/wall instead. It has other advantages as well. You can delay the installation of the slab until later, saving initial costs that can be applied to insulation and exterior finish.Gabe

          10. Bill_Flather | Dec 27, 2000 03:36pm | #14

            *Seems to me there was an article in FH by whatshisname Benson (timber framing and old house renovation stuff) about building houses in cold areas on a thick, compacted, well drained crushed stone base and concrete slab thickened at the edge. I believe the point was that if there is no water to freeze and heave under the structure, there should be no movement. He may have been doing this in NH.

          11. Bill_Flather | Dec 27, 2000 04:05pm | #15

            *OK, It wasn't Ted Benson, it was George Nash, find the article in #60, p52, "Slab-on-grade foundations for cold climates". Another article was #107, p76 "Frost-protected shallow foundations".

          12. Rein_Taul | Dec 27, 2000 07:58pm | #16

            *Thanks all,I can't wait 'till spring (and the budget) to get started. I've checked this out with several locals. Slab on grade works fine for this type of detached structure. I was mostly concerned as to whether there was a size limit. If the structure gets to a certain size, I presume a foundation wall is the best way to go. After all not everything gets built on a slab!

          13. Tim_Kline | Jan 02, 2001 06:50am | #17

            *b WBA At Your Service I threw the VCR in reverse the other day and took another look at the show. The perimeter of the slab was formed to be 18" thick for a 16" to 18" width. The slab then appeared to taper up to an undetermined thickness. (I would imagine 4" to 6") The slab was done as a monolithic pour. About four courses of concrete block were laid on top of the concrete, (all above grade) which provided the base for the 2 X 6 walls. Over the years, I have developed a healthy respect for Mother Nature, and I would think she might be rough on a foundation like this in a cold climate. After seeing concrete slabs heaved by frost, I think it's pretty obvious that frozen ground rises and falls in an uneven fashion. Imagine a structure with exterior walls described above and an interior load bearing point such as a column or even a wall. I envision that the exterior walls, due to their exposure, will lift first, followed by the column or wall (This is a big maybe, depending partly on interior temperatures). With a 4" to 6" slab and a distance of 15' to 20' to the interior load bearing point(s), this uneven lifting would logically lead to a hefty crack in the floor. I'm wondering what parking an 8000# van on this floor would do. No matter how you reinforce a 4" to 6" slab, I don't see it taking this abuse. On another topic,what if you stone under the footers as mentioned above, and later on junior rips the main downspout off of the garage with his hockey stick. Depending on the soil's ability to drain, I see the possibility of water collecting in the stone and again leading to uneven lifting frost. I'm not trying to condemn this practice of building, just trying to understand it, and get the opinion of someone who has a structure like this. If it's cheaper, it will be done around here shortly. Later, as the builder disappears into the sunset, we are called in to provide the impossible repair.....

          14. Jeff_J._Buck | Jan 02, 2001 07:08am | #18

            *Here in Pittsburgh, a slab with a shallow footer is used all the time for garages. Even with an attached, I agree matching the house foundation is best, but these types of garages go up all the time. Remember to build up(dig down) at the location of any supporting posts. Jeff

          15. Rein_Taul | Jan 03, 2001 07:15am | #19

            *Tim,I think the unevenness is the real culprit. Floating is fine, if it doesn't find resistance. As my post stated, I'm concerned about the size of structure. At some point the floor area or the weight will cause uneven loads during floating. This must break up the slab or walls.The only personal experience of this I have had was once working in an industrial park building on a slab that I think was a floater (I'm not sure). Over the course of a few years of our office being in this new building, I watched parts of the floor drop several inches and finally a brick wall started to fail. It all appeared to be motion related, in a non-seismic area!

          16. Davo_ | Jan 03, 2001 08:36am | #20

            *Rein,I've been involved with floating slabs that were 45 feet X 100 feet (and larger) with no ill effects. The primary concern is the soil condition before pouring the concrete. The ground needs to be firm, free from decaying debris and expansive type clay soils. The ground needs to be compacted with either layers of sand or gravel. If the ground is adequately compacted, it will bear the weight of the concrete on top of it just fine. The thickened edge of a slab that will bear the weight of any walls must be adequate in thickness and in width to bear the wall's weight; just like a footer. If you skimp on concrete here, your wall may be too heavy and cause the slab to crack. Also, any posts that you may have in the interior of your garage floor that would be a load bearing post, requires a heavier footing underneath it. This area may require a 15 inch square (or larger) block of concrete that would be 10 to 12 inches thick; instead of just supporting this post on a 4 or 6 inch floor. If you do throw a load bearing post on top of a regular 4 or 6 inch floor; again, the floor will crack because it can't handle the concentrated load.AS for water getting underneath the foundation: Water that passes through gravel will not freeze and expand the gravel (the way it does normal soil) and cause uplift. Also, if you put some drainage lines underneath the gravel bed, the water will be diverted away from the foundation, thus adding extra protection. Railroads, and roadways themselves, are designed with a gravel bed base. They hold up rather well to all the weight and abuse.Get yourself a copy of the book titled; "Complete Book Of Cordwood Masonry Housebuilding" , by Rob Roy. He uses the floating slab technique on the buildings he built in upper New York state. His own house is a 2 story affair, placed on a 44 foot Diameter slab. He goes into good detail about soil preparation, drainage, and concrete formwork. Also, some very good articles relating to slabs, and rubble trench foundations can be found in Fine Homebuilding's re-released book, titled "Foundations and Concrete Work." It was earlier titled; "Foundations and Masonry." You can purchase this book at Home Depot or stores similar to this.Davo.

  2. Tim_Kline | Jan 03, 2001 08:36am | #21

    *
    b WBA At Your Service

    Occasionally I tape some PBS shows and recently one caught my attention. It was a Hometime show with Dean and Robin building a two story, three car, custom garage in Minnesota. (I think) Instead of taking the footers to 42", they chose a shallow footer with the intention of letting the entire structure "float" during the Minnesota frost. My background tells me to build tool sheds like this but not high-end garages.(Their backers always seem to have deep pockets.) Anybody up in the frozen tundra want to enlighten me on this practice ?

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