concrete slab for shed

[cheapdave]

Well after contemplating several options, have decided to pour a concrete slab for 12 x 16 shed....what I have done so far is to remove sod and put up forms. On one side the top of forms are about 4" above grade, and the other side witch is 5' from property line, top of forms are about 10" above grade. Also I am in Michigan so 42" frost line and I do not have great drainage, but have chosen a location that is higher. I plan to pour 4" slab.

Am thinking to dig out one side to about 6" depth and fill with couple inches of sand..the other side will need to be filled more, my neighbor says just fill with sand, but I think I should fill with 4 or 5 inches of crushed limestone then inch or two of sand on top....is this one of those 6 of one half dozen of the other type deals?

should I put a footing around perimeter? if so how big? or would that be overkill. and would a footing say 1' deep by 1' wide be a problem with frost? what about just pouring concrete 6" thick at perimeter, would that be ok?

With the drainage issues here, think I need a moisture barrier....What should I use?

also thinking 3/8 rebar - spaced about 18" sound right?

any advise appreciated.

[rlz]

I recently poured a 18'x22' slab for a workshop and had a foot difference in grade between the short ends. Our city inspectors required the top of the slab to be 6" above grade and a foot below grade around the parimeter. The perimeter trench had to be a minimum of 8" in width and then sloped inward toward the center. They also wanted #4 rebar to go inside the trench 3" from the bottom and 3" inches from the top. Also remember to put in anchor bolts for your wall studs when pouring the concrete to anchor your walls.

My best advice is to consult with the city inspectors if you're required to pull a permit.

Have fun doing the job.

Robin

[EXT]

You cannot have too much re-rod. Don't use the #3 size, go for the 1/2" or #4 size. I built a shed foundation, 5" slab with 16" deep stiffeners all around. Had a source for some free re-bar and dropped 320 lineal feet into the foundation. That was 25 years ago, not a crack in it. This is in the Toronto area of Canada with a 48" frost line.

[benick]

If you're gonna do a slab for your shed , you more than likely need a permit...Best to start with that and then get back to us with those specs...Without knowing what YOUR building codes are we will just be wasting time trying to tell you what to do....

[trader4]

Along those lines, in many places you can't put a shed 5 ft from your property line. A larger clearance is required. Given the other conditions, ie Mich winters, a concrete slab would not be my choice for this application.

[cheapdave]

There is no permit required if shed is under 200 sq ft. shed can be 5' from property line as long as it has eve trough. I can build it any way I want according to building inspector. my neighbor built a shed few years ago, little smaller than mine. same conditions I have. He poured 4" concrete slab that he filled several inches with sand to level, roughly the same as I am doing, and he used no steel no moisture barrier, and no footing. He did place one row of blocks, and advised me to do the same.(I was planning to construct walls directly on slab but he says 2x4 plates at bottom of wall will rot) He filled and raised area around slab with black dirt and sloped away. so far he is in pretty good shape he is farther from property line though. he has been having some minor moisture problems......would like to know if I put in quality moisture barrier can I then build on slab? Thanks

[Buddy]

Plastic vapor barrier would keep slab drier. Use some rebar, even old fence wire is better than no steel at all. The blocks he used is acting as a footing, but a floating slab is all we use in Florida. Up there, is best to have a footing below frost line to prevent heaving.

[Joe]

Building inspectors, newsgroup warriors, friends, all are good sources of ideas. But don't forget to ask the people who deal with concrete every day, your ready mix supplier and the finishers. Keep in mind that sand is a poor base for concrete compared to pea gravel. Your building code may even specify it for larger slabs, so logically it would be best for yours.

Joe

[Rudy]

I wouldn't recommend the use of "Sand" for your concrete slab base. Sand can wash out and isnt structurally solid unless it's contained within a footing/foundation. The norm seems to be a *compacted* layer of sand/mixed gravel (around here they call it "road base"- other locale calls it Navvy Jack..a sand/gravel/crushed rock type mixture used for making concrete). Thats what I put under my concrete driveway and back porch slabs 5 years ago. No problems so far.

As far as the plastic vapor barrier goes: code here calls for a tamped sand base (within the foundation). Over that they place a 6 mil vapor barrier..seams taped with Red Tuck Tape. Over that they poured the floor. This is for "inside". Its your call if you want to use one outside for your shed. The gravel base should provide any drainage necessary under your slab (as long as you provide a way for any sub-slab water/moisture to flow away) so there shouldn't be any moisture concerns.

[benick]

All the slabs I've seen here in Maine have an above grade compacted gravel pad under the concrete slab so water will drain and run AWAY from it...As has already been pointed out , the moisture barrier is for inside floors like in a basement....Never seen one under a slab in a garage or shed , but whatever..The only thing I've seen under a slab is Styrofoam panels to minamize frost movement.....Bottom plate on the slab should be PT and won't rot but concidering how your neighbor did his I can see his concern...LOL.......But you sound as if you want to build it like your neighbors and have your mind already made up....Let us know how it works out....