I am going to add a 20’x24′ workshop to the back of my garage. I am planning on bidding out the slab. I will be putting in radiant floor heat.
I have been doing some research, but want to be prepared when the different contractors give the different opinions.
I am planning a slab on grade 4″ thick.
I would like some opinions on the details though. Such as is there any benefit to using rebar as opposed to wire mesh. The size (width, depth) of the footer portion along the perimeter of the slab wall, the thickness of the insulation under the slab. is a 4″ slab thick enough. etc. etc. And what can I expect to pay?
Some details. The garage is attached to the house, and the workshop will be going straight back. The ground is level where the shop will be built. I am located in Rochester NY.
Thanks, -Tim
Replies
Tim - I'll give you what we do locally, hopefully it applies for Rochester. Are you doing any of the work?
From the bottom:
Crushed, compacted gravel
2" Foam
2'x2' grid of 1/2" rebar (mesh works too, but rebar is the best practice)
1/2" pex, tied to the rebar with zip ties, pex typically 8" o.c., runs no longer than 300'
4" of concrete is adequate.
Footers should be below frost level for your area - we go 36", and would likely pour footers, build walls with block, fill, compact and then pour the slab.
best wishes.
Footer widths are engineered, but typically 24" locally, since that is the width of a backhoe bucket.
Our heating people here specify the PEX runs to be ideally about 220-250 ft and to be 18"OC and not closer than 12"OCI like the reabr when it is a structural slab like this but often use flat stock mesh 6x6 when it is just a slab because it is easy to layout the PEX to that grid
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Is your garage detached? Is it built on a floating slab or on footings? If it's a floating slab, typically, the slab is thickened to 12" around the perimiter. There should be re-bar drilled and grouted into the existing slab to keep the two moviing together. I would also set your re-bar on chairs to make sure they are in the slab and not laying on the bottom of the slab.
My concern, if the slabs are floating and you are in a cold climate, would be the difference in movement between the old and new slabs. If one is heated and the other is not. Frost may move the old and new slabs differntly with the heat and insulation in the new slab construction. This will create havoc with the joint between the new and old. Your soil composition will be a factor as well. I think it would be worth consulting an engineer.
The garage is attached. I didn't really think about the movement differential. Rochester can get pretty cold in the winter.
I want to attach the workshop to the house for a few reasons. The main one being that the permit will be easy to get. A detached building would be alot harder to get a permit for.
I will need to consulot an engieer for the permit so I will be sure he address the movement issue.
Brian's subfloor profile looks good. I'd add a poly vapor barrier below the foam. Radiant heat is reallly nice.
Some redi-mix suppliers offer fiber-reinforced concrete. The premium isn't that much. I believe the fibers are polypropylene and you wouldn't need rebar or wire mesh.
You should still cut control joints in fiber concrete to allow concrete shrinkage.
In my opinion fibermesh does something different than rebar or wire. Fiber mesh helps prevent surface defects like spauling and tiny surface cracks. Rebar makes the slab more rigid so that it is less likely to crack. Wire helps hold the slab together in the event that it does crack.
You're right, except that WWM and rebar are supposed to do the same thing. I agree with the poster who said rebar is better but wire is adequate.
Matt,
Once Upon A Time.... I had a PTA President who wanted to use the poly mesh on an outdoor basketball court... It was for the school's children and it was paid for by the PTA. The company that poured the slab "tried" to prepare the ground. It was a black clay soil is bad for swelling and shrinking.
After the slab was poured, it took some getting use to all that plastic "Hairy" stuff coming up on the slab. Now with kids on it everyday, they wore away the stuff in a few weeks... But the clay soil had far to much movement and within 6 years, major cracks appeared. In the end, it had to be jackhammered and hauled away when the parking lot was enlarged. So, there's no replacement of plenty of rebar and proper soil prep.
I would suggest that you plan for some floor plugs in your slab. Because I bought my heavy copper wire in shorter remnants and at half the cost or less, I used 3" non-perf. sewer pipe in the sand to provide 4 floor outlets in a 30X40 workshop. The larger diameter PVC allowed the #2 or #8 sizes much easier to pull....
To get the plug to the floor level when the concrete was poured, I used a PVC downspout transition connection that changes the round 3" PVC into a rectangluar shape that just happens to be the exact size of an electrical box. Those surface plugs cost an arm and a leg, so I used outside waterproof electrical boxes and fastened them over the floor opening and to the floor. Yes it does stick up above the floor, but it's waterproof and up and out of the way, in case of a leak or bad spill, when fastened and sealed with a good silicone or the product known as "LexCell". Those electrical boxes can be straddled by your power equipment or an automobile should you need to work around them. I've lived with mine for more than 12 years...
Bill
Edited 4/7/2007 7:32 pm ET by BilljustBill
wire and fiber does the same job. wire/fiber and rebar does not do the same job. rebar is fir load factors. wire/fiber is for stress shrinkage.
I think you ment to adress that to the OP. It's not my slab that is being discussed.
My wife was rushing me out the door when I wrote the thing above about fiber mesh.
I'd like to add that here, 8-12 years ago everybody was using fiber mesh. Thought it was the greatest thing since sliced bread... Now, the trend is back to WWF (wire) and rod. For me WWF goes in every interior slab, and rod (rebar) if there is any question at all about the substrate. If it is a turn down (monolithic) slab, it gets rebar in the footings. Not sure exactly why the migration away from fiber mesh but I have a pretty good idea... :-)
Are you saying that you use both the wire mesh and rebar in the same slab?
Generally just wire, but if the ground below the crushed stone is not the best we will put rebar in it too.
Edited 4/8/2007 8:51 am ET by Matt
Matt,
I thought I'd add to your experience with fiber mesh and to the slab question, too.
thanks for the heads-up,
Bill
For anyone who hasn't seen/used fiber mesh, attached is a pic. The fibers are about 1" long and look like fiberglass although I think it is actually some kind of poly-something-or-another plastic. They come in a bag about 12" square. I believe there are other types available too. Like I said before, I believe that fiber mesh is effective for preventing surface defects, like the small plastic shrinkage cracks that occur, for example, when placing concrete on a hot windy day when moisture evaporates too quickly from the freshly placed concrete surface.
Speaking of concrete cracking, I also attached a pic of some control joints in a driveway slab that is about a year old. Yes control joints actually work! The control joists pictured are tooled in. Sawcut joists are at least as effective, however require a concrete worker to return a day or 2 after the pour to do the cutting. Saw cut joints are preferred on, say a house slab, where the surface needs to remain a flat as possible.
Four inches of concrete over two inches of foam is the standard. The footings will need to go below frost; 36" doesn't sound deep enough in Rochester. The local building inspector will know.
If your ground is properly prepared you don't need rebar. Mesh is good because it gives something to fasten the tubing for the radiant heat to.
You'll want a fairly smooth finish to reduce fatigue on your feet and make cleanup of sawdust easier.
It would be really cool to put the dust collection and power for your tablesaw in the floor.
FWIW, here's what I did on my 24x36 detached garage in central NH:
4' minimum frost wall (back corner is 12' deep, as the ground sloped down there)
bank run gravel backfill, plate compacted in lifts as gravel was placed.
6 mil poly vapor barrier
2" EPS foam
wire mesh
PEX tubing tied to mesh with 12" spacing
3 runs of 1/2" rebar continuous loop around the perimeter, and several pieces bridging over the footing at the entrance. Wanted to run a 2' grid over the whole thing, but steel prices were really high at the time, and the only reasonably priced source didn't have as much as I needed.
5" of concrete in back, 6" in the front where vehicles travel in and out.
I didn't put any control cuts in. No cracks so far after 1-1/2 years. My father has a slab the same size nearby (no insulation, etc.) that has a couple fine cracks after about 20 years.
I haven't hooked the tubing up to any heat source yet. - still trying to get the rest of the building completed.
Don
The stem wall, we just dont do down here, nobody does. I wish I did, I needed the extra height.
>The stem wall, we just dont do down here, nobody does. I wish I did, I needed the extra height
I might have just gone with an alaskan slab if I wasn't trying to build on the edge of a dropoff, but didn't really have a choice here.
Don
Those be grade stakes I see, right?
>Those be grade stakes I see, right?
Yep, the finisher put those in the day before the pour. He set those with a laser - that's his method. I hadn't given much thought to how he would level the floor before he did that - guess I figured he'd set guides for a screed board or something, as we've done on the couple shed floors I've done with my dad.
Didn't like the idea of poking holes in the vapor barrier, but those holes represent a very small part of the overall surface area in a well drained soil - plus I had had to poke a few more to drain the swimming pool that it became when we had an unexpected downpour right after I put down the foam (which, of course, floats).
Don
People used to use grade stakes around here, but now a day the various concrete subs I use just use the laser during the pour. They will have some reference points set up ahead of time that were done during the forming/dig-out process. The receiver will typically be just clamped to a 2x4. Sometimes crews will have more than one receiver so sub-crews can be working on different areas of the slab simultaneously. In the case of a garage slab, since it is sloped toward the "big door", the stem walls will either be masonry, or concrete poured earlier during the day, and chalk lines are snapped on these. The laser receiver/2x4 is set up off the snapped lines. The accuracy ends up being around 1/8".