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Old 11-06-2009, 07:41 PM   #1
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THoughts on garage foundation


Hello everybody. I thought I'd run this idea past you guys to see what you think of it. I am planning on building a two car garage measuring 26' wide by 32'. Garage doors on the gable end. Anyway this is what I have in mind. instead of digging in concrete foundation walls, I would drill and pour concrete pillars 8-10" in dia. These pillars would go down 4 feet, which for our area is below the frost line. The pillars would be spaced about 5.5' apart. I then plan to lay a triple laminated treated 2x6 across these pillars to form the sill. Then its building as usual on top of that. Its getting colder here and not sure if I am able to pour a floor this year or not. I can always add this later. Before you ask, I do not have to attain a building permit or consult a building inspector as this is on private property in a VERY rural part of my state (ND). ANyway I would love to know what you guys think of this plan and if I should consider anything different?

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Old 11-07-2009, 06:09 PM   #2
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THoughts on garage foundation


I would not do it that way. I don't think it would be stable enough and not worth risking all the work and $$$ above the piers. I would not know how to pour the floor properly using the pier system you describe. And what about where the doors go? With out a proper foundation how do you keep the threshold intact?

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Old 11-07-2009, 08:00 PM   #3
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THoughts on garage foundation


I think that you are better off just doing a pole building. This way you will be using a building method that is recognised by the building industry and you are avoiding possible problems by trying to invent your own system.
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Old 11-08-2009, 04:55 AM   #4
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THoughts on garage foundation


That type of design works for a car port but not a garage. If you do that, then in between each pier you will have frost heave movement, which will likely damage the structure since there will be walls on top of your slab. If you don't want to dig the full four feet below your frost line, there is a new technique that allows you to wrap your concrete with Styrofoam so you don't have to dig below the frost line. You can research it, I don't know much about it because it's not a concern where I live, our frost line is only 18 inches.
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Old 11-08-2009, 08:28 AM   #5
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THoughts on garage foundation


I would not be so quick to dismiss this idea. The posts would be below frost line, so the posts themselves would not be subject to frost heave. The poster does not indicate that the concrete floor would extend underneath the sill beam, so even if the floor itself is subject to heave, if the floor is not connected to the walls, the heave would be localized to the floor. As for the rest of the building, it is essentially a pile supported structure with a grade beam. This is totally conventional construction in the commercial world that I have spent the last 30 years working in, except that normally we use a reinforced concrete grade beam, and the OP is planning to use a wooden grade beam.

So on to some comments. The grade beam (the triple 2x6) should be sized by an engineer or architect, since it will take all of the load of your building. I am assuming the beam will be in close contact with the ground, since you are planning to use treated wood. I would ask the engineer or architect their opinion on whether the specific product you are considering will work long term in the exact configuration you are using. There are definitely wood products designed for contact with earth, but they are a bit specialized, and need a little research. Your engineer should also size your footings, since they will be holding up the entire building, including resisting lateral loads.

As for the floor, frost heave is a complex subject. If you have sandy or gravelly soil, you are not likely to get frost heave even if you are in North Dakota. The problem is that much of North Dakota is covered with silt, which is about the worst possible soil for frost heave. Your engineer can advise you on special preparation for the floor to minimize the potential for frost heave, you may need to remove soil and replace with gravel or crushed stone. At the worst, if it is too expensive to do the required work to eliminate frost heave, you may elect not to pour a concrete floor, just use gravel or block.

Conclusion: I don't think this is necessarily a bad idea, it isn't a new construction technique, and it certainly could work. Just needs a little engineering.
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Old 11-08-2009, 11:30 AM   #6
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THoughts on garage foundation


As someone had posted above, an Alaskan slab or floating slab with styrofoam underneath would be your best method. Piers are going to allow the frost to get underneath. They are mostly used here in the south where we also have expansive clays but don't need to worry about freezing. Scrape off the topsoil and lay down 2" of styrofoam. After the slab is done you should also add 4' of styrofoam around the exterior before final grading. Block up a couple feet off the ground to keep the framing dry. If you think you might ever heat it you could also add some pex before you pour.
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Old 11-08-2009, 02:13 PM   #7
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A pier can still heave when dug to or below the frost line (ice lenses pushing laterally on the sides or soil freezing from above through the wrong back-fill material). It is all about how you treat the piers: with a footing attached, gravel under and around as back-fill, rigid foam (as mentioned) in the first foot of soil, plastic wrapped around the tube, etc.: http://books.google.com/books?id=1gg...num=3#PPA31,M1
Be safe, Gary
http://rockproducts.com/mag/rock_agg...prevent_frost/
http://www.oikos.com/esb/43/foundations.html
http://www.waltersforensic.com/artic.../vol1-no11.htm
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Old 11-09-2009, 11:52 PM   #8
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THoughts on garage foundation


Thanks guys for the input. It's welcomed to a new guy such as myself. I guess to answer a few of the comments and questions, the concrete floor (whenever it is poured) will not be connected or poured beneath the wooden grade beam. So any movement of the floor will have no effect on the building. I guess I don't see this system as that much different than a traditional pole structure in which posts are buried to the same depth. In our area there are plenty of pole barns built that have lasted for decades. The site has great drainage as its a hard clay/rock hill. I like the idea of burying rigid insulation around the outside below grade to further protect the piers and piers would be backfilled and tamped with gravel. Is there any other pro's or cons? Thanks again for the discussion.

Justin
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Old 11-17-2009, 09:48 PM   #9
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THoughts on garage foundation


I wouldn't look at a barn to get ideas for your house. Hard to picture what you're doing, just remember the ground above the frost line will move, taking everything else with it.

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