Replies

1. 
   Guest_ | Oct 11, 1999 07:51am | #1

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   John,

   The last time I gave advice to someone "Up North" on placing a concrete garage floor I got beat up a bit by some of the regulars. I am going to approach your post with a bit of caution. It is very possible to build a garage or shop on a slab in cold country. It is also possible to place a slab inside an existing pole building. In both cases, site preparation is critical. To my way of thinking, he term "slab-on-grade" is a bit misnomer. The slab must be placed over good drainage, usually a bed of crushed stone. This ensures that the water that would otherwise freeze and expand drains away. Reinforcement is also required. Mesh and/or fiber may be adequate in a pole building. If the slab will also be the building's foundation more extensive reinforcement will be required. I am assuming you are not a concrete finisher. I suggest you contact a reputable one and discuss your project. He or she can advise you on customary building practices in your area. If a permit is required your local building officials may be able to help (or at least tell you what they will be looking for).

   Good luck,

   Steve

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   Guest_ | Oct 12, 1999 11:15pm | #2

   *Hi John.I live in the Pittsburgh, Pa region. Our frost depth is approx 36" deep. I work as an industrial carpenter for a local steel mill and we pour slabs all the time. It seems like you are wanting to build a pole building and then pour the concrete floor in after erecting the building? I've seen this done many times. Make sure that your poles are in the ground deep enough. They must be below the frost line depth for your area; otherwise a frost could heave and lift up one or more of your poles and cause your building to rack and all sorts of trouble begins. You don't want that.Steve is right about having good drainage for your floor's site. The topsoil (grass & vegetation) should be removed and replaced with gravel. Ideally, you should remove about 12 to 16 inches of your topsoil. Cover this gravel with plastic and then install your concrete reinforcement. 6X6 mesh is used a lot, but we normally place rebar in our slabs. We place the rebar on "chairs" (your masonry supplier has these) or on old bricks if we don't have the chairs. Using 1/2 inch rebar is fine for your needs. Space the rebar on 12" centers. Set down one course with the rebar running longways, and then place another course on top of the first course, with the second course running crossways. This will give you a grid pattern of 12" squares.You should tie the rebar. You can buy a bag of ties and a wire tyer at a masonry supplier. The ties are a piece of wire with loops on each end, and the tyer tool is simply a wooden handle with a steel hook attached to it. The hook can rotate because the handle has ball bearings set inside it. Put the tye wire around both pieces of rebar where they intersect, and attach both loops over the hook; twist the hook by rotating the handle and the rebar is tied. Tie all the outside perimeter points where the rebar intersects, and then tie every other joint inside the field area. After the rebar is tied, go ahead and pour your concrete. We usually pour a 6 inch thick pad (or thicker in some cases.) Some people will tell you that a 4 inch pad is adequate, but to me, thicker is better. Adding chopped fiberglass to the mix is added insurance against cracking, but this makes it harder to finish the floor. Some people use the fiberglass in place of using the rebar; I would advise against it. Use the rebar, and if you have the extra money, throw in the fiberglass as an added precaution. If you don't have the cash, forget about the fiberglass. The verdict is still out on just how well it performs in this manner anyway.Heres something else to ponder. Instead of using the plastic, you could install 1/2 inch Dowel Chemical "blue board". Its a rigid type insulation panel. This would help insulate your slab and keep it warmer and less prone to cracking. This product is expensive, and most of my friends don't use it, and we don't use it in the mill either; though I hear it works rather well.I built my garage/workshop on a slab. I cleared the ground like I explained earlier, but I also dug a foundation around the perimeter of my intended slab. Instead of pouring a foundation "footer", I backfilled this trench with various size rock and gravel. This is whats known as a rubble trench foundation. Any water near my slab would sink into the trench area and flow away from my slab; preventing frost upheavel. My slab is a "turned down" version; whereby the slab is thicker around the perimeter edge. I did this by pouring some of the concrete in the trench line (I did not fill the rubble trench back to the top, but left about 4 inches), this helps guard against slab movement.Unlike your proposal, I built my wooden walls on top of the slab. Actually, I laid 2 rows of concrete block on top of one another, and then built my walls on top of the block. My block has no mortar joints; because there was no mortar used. Instead, the block were stacked up dry and butted tight to each other. I then used "B- Bond", which is a mortar mix using chopped fiberglass in the mix. You trowel this mix over the face of the block. Both the outside and inside faces get covered. When dry, this creates a strong bond. In order to keep your block running level, use steel galvanized, corrugated brick masonry ties as shims where needed. The first course of the block must either be set in mortar or bonded to the slab using an approved adhesive. I went the adheseive route. I drilled holes approx every 4 to 5 feet into the slab and placed 1/2 inch all threaded rod in these spots. The rod extended up through the open webs in the block. These webs were then filled to the top of the block with concrete. The rods acted as "J" bolts and was the attachment point for my mudsills. Thus my walls are tied directly into my slab foundation; which helps stablize the structure from uplift in windy weather.My garage/shop is 24 X 36 and is now 4 years old. To date, I have no cracks in my floor and no problems of any kind.I gave you my example only to indicate that slabs do work, not to suggest you build your garage in this manner.Whatever you build, good luck in doing so.Davo

2. 
   Rich_B. | Oct 13, 1999 01:13am | #3

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   Consider the previous point on contacting the local building official. In my area, even if it's one of those pre-fab storage buildings from Home Depot, a rat wall is required under the foundation. I'll probably build a garage/shop next summer. Still don't know exactly what I'll do, but it will include concrete going below the frost line.

1. 
   Dave_W. | Oct 14, 1999 12:52am | #4

   *Here in Duluth, MN. where it doesn't get too much colder for too much longer, garages on slabs are built every day. Previous writers are correct in that it requires a built up garvel base approximately 12". The slab will keep the immeditate area under the slab from freezing. To further decrease ground freezing under the slab you can lay rigid 2" foam 4' out from the slab about a foot below grade. Slope the foam down away from the slab. Use a 12" x 12" thickness of concrete around the slab perimeter. Good idea also is to lay a row of concrete block around the perimeter and build walls up from there. Good luck.

3. 
   John_Pucovsky | Oct 14, 1999 12:52am | #5

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   Hi everyone. For a 20 by 24 foot garage/shop is a slab on grade( reinforced I presume) ok in snow country? I was thinking that with a regular 2 by 4 wall or posts 6 feet apart, 4 feet down, with ledgers and B & B siding. I could then put a concrete floor in later. Any advice?