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We’re planning a workshop and assumed we’d just do the frost footings as is typical around here (Northwestern WI) but a friend said floating slabs are cheaper and just fine for pole-barns, storage sheds, etc. I’m skeptical… I want the building to stay in one place for more than 10 years. Any thoughts?
BTW: we’re designing it to look like a 1940’s service station – if anyone knows of great living examples in the MN/WI region, please holler!
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Kathleen,
I adore the 40's. I have the clothes, the furniture, love those diners and the gas stations...send us a picture when you're done, OK?
If slabs are allowed where you are, that's a much easier and cheaper foundation for a structure where you don't need a crawlspace to run pipes or ducts. Some folks think concrete is harder to stand on than wood floors, but you can take the savings and buy a pickup load of really good shoes instead.
I'm assuming your ground can get pretty soggy, so be sure you have a layer of plastic over the fill to keep out ground moisture and protect with 2" of XPS (eXtruded PolyStyrene aka "foamboard") insulation under your slab. There are two p.s.i. ratings for foam board that i know of and you need to be sure to buy XPS rated to go under concrete, not as wall insulation.
There are some relatively new designs for insulating footings that use board insulation fanned out 2' from the thickened edge of the slab and 4' at the corners, keeping the heat that escapes from the footing from escaping to the air. I've not done this, but maybe someone else will chime in with suggestions here.
You can also hit the search button up top and type in some pertinent words to get a cyber-ton more info from this site.
*The Journal of Light Construction had several articles on frost-protected slabs for homes. Unfortunately, they have recently changed their web site and articles that used to be viewed for free they now charge you $5.00 to view with only the first 100 words for free. The foundation articles are at:http://www.jlconline.com/cgi-bin/jlconline.storefront/3c9110b600090503271a401e1d290544/Export/catalogs/1013Slabs_On_GradeI remember reading at some point that at least certain of the frost-protected slab on grade foundations were only for heated buildings, but I don't recall if that was from a recognized authority.
*Hey - thanks for the info. I did look elsewhere in the site and the "frost -protected shallow footings" sounded like what the friend was talking about. Our house has a heated (off-peak electric) concrete floor - stained and with grooves cut to look like 3' tiles. (My husband likes to tell people we imported them from Italy - and they usually fall for for it. So we're used to standing on concrete.) We've thought about putting the electric floor heat in the shop and keeping the temp. very low. But if we do the shallow footing route, I think the in-floor heat will not be a good idea since the thermal mass becomes the entire ground area, rather than the footprint of the shop (nothing to insulate the sideways heat transfer more than a foot down). So that's the tradeoff I'm thinking about. For a guy that wants to build a Cobra kit-car, I would think a heated floor would be a much more comfortable way to putz in the shop, rather than a wall mounted gas-fired heater.I love the 40's too - and take pictures of old service stations any chance I get. Will post a picture when we get that far....
*i I think the in-floor heat will not be a good idea since the thermal mass becomes the entire ground area, rather than the footprint of the shop (nothing to insulate the sideways heat transfer more than a foot down). Kathleen, that's the reason for the insulation underneath the slab, to stop the heat going down. I've had shops with no insulation and they are a pain to heat, but an insulated floor is another thing entirely. I'm in MT, so our winters are similar. Use the slab and at least 2" of foam.
*There are a number of references on the internet if you do a search. These are just a few of them. For the type of foundation that I was thinking of - basically a slab with a tapered thinkening toward the edges overlaying foam board - several of them do say that they are only for heated buildings. Even if you planned on heating it at times, you should pencil out how much it will cost to keep it heated when you decide to go to Hawaii or Key West for the winter - a more conventional frost protected foundation might be the more pennywise choice in the long run.http://www.concretenetwork.com/concrete/foundations.htmhttp://www.nahb.com/builders/frost.htmhttp://www.pbf.org/Archive/BuildingGreenFoundations/BuildingGreenFoundations.htmhttp://www.oikos.com/esb/43/foundations.htmlhttp://www.epsmolders.org/3-below.htmlhttp://www.plastifab.com/foundation/04shallow.htmlThere are some tables in the following for the amount of insulation needed, but it is a ways down:http://www.cs.arizona.edu/people/jcropper/desguide.htmlThese are some sources for gas stations although most of them aren't in your area, I don't know how specific gas station architecture was to specific locals:http://www.chevron.com/about/learning_center/hst_road/http://seattlepi.nwsource.com/neighbors/fife/album3.htmlhttp://www.national66.com/photo_ndx.htmlhttp://www.dca.state.ga.us/economic/redevopp/FloBranch.htmlI particularly like this one:http://www.oldgas.com/visit.htmYou might find something here, I didn't dig that far: Memories (gas stations)
*Casey, you always amaze. Love the gas station sites.
*Last fall I poured a floating slab for a new workshop here in Toronto. I have not finished it so it was not heated this winter. As it turned out we had a record warm winter. However, this also turned out to be a great frost test. We have had several fast freeze / thaw periods that have caused havoc around here. Some days have had over 15 degree Celsius temperature shifts. I have seen well established slabs and retaining walls move considerably in the last few months. My slab has held up perfectly, and that is unheated!Granted I overbuilt a bit, but it was not that expensive. There are two key things to remember: drainage and drainage. I have 10" of compacted gravel under my slab. On top of that there is 2" of foam insulation. The bearing edges are on undisturbed soil with a gravel drainage perimeter. The slab is 6" thick high strength concrete. The bearing edges are 14" thick and 10" wide at the bottom. All more than code but only a few dollars more in cost.I formed the edges with 2" foam as part of the form. It is left in place for future heat retention when I heat the building. The whole shop is about 450 sqft with one monolithic slab. Rebar is in the slab in a 4' x 4' grid and there is also 6" wire in the slab. As I said, it is new, but this winter did no harm to it.I did not go for infloor heating as I don't plan to constantly heat it. Instead I will be installing radiant heaters. They will fit my usage better.Good luck in your project.
*Thanks, Splinter, you do some pretty good posts yourself. I just again stumbled across a "great buildings" web site that might be of interest to some of you. They have lots of pictures and some 3D "tours" of some famous places, including a Fallingwater. The Fallingwater 3D requires downloading some files and I didn't do it. I might warn that if you are on a dial up modem, you might want to go out for a cappuchino during the download of some of these pictures...http://www.greatbuildings.com/buildings.htmlRegarding the old gas stations, we still have a few of them around this part of the country, but most are not very imaginative. Most that remain are pretty run down and have been converted to other uses. There are a few famous building in Portland, but the ones that I found most interesting in my youth are long gone. Driving from my home town to Portland was only 85 miles, but was a half days trip on the old narrow, curvey Columbia River Highway, Highway 30. But on entering Portland there was a building shaped like an old shoe, another like a derby hat, and a number of others that were unusual. There were also all kinds of animated neon lights such as a big 7Up bottle with bubbles coming out of it. It just ain't the same no more with all of them gone... They just don't show the same whimsey in buildings any more (of course, those old buildings probably violated every code that was ever written, but they were fun to look at...)
*thanks everyone - I'm off and running.Casey - you saved me tons of time, as if I'm not on the computer all day as it is. Outstanding! (as my dad would say...)
*There used to be a Frank Lloyd Wright designed gas station in Cloquet, MN. But I haven't been through there in about 15 years.
*I've done quite a few monolithic slab jobs and therre are some concerns that apply here, in my mind.Be sure to have a good base of stone compacted to allow for drainage and compression. If the ground can move under the slab, it will and you'll have cracks.Some contractors think a 4"slab is all that is needed. Not true. The edges need extra depth and steel. Basicly you are pouring a footer and slab in one shot so make it good.That also leads to this next point. a post and beam structure places the load on individual stress points under each post instead of spreading it evenly along the entire perimeter, increasing the need for depth of slab edge and steel reinforcement to spread the load on the soil base. Otherwise the foundation will crack. Build a large enough (heavy weight) building and you need so much more depth to your edge that there comes a point where you can be better off building on individual piers or a stem wall/grade beam footer.Only your enginnewer can tell for certain after assesinmg the soils and the size of the building.