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Standalone 20x24 workshop foundation questions

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  • guitarchitect
    replied
    I hope this isn't overkill but here's my thoughts - based on what you describe you want to maximize two things... drainage and insulation.

    Drainage

    if you have a lot of water, you may consider excavating a lot more than you need in order to put in a soakaway / french drain... basically you dig a big hole, fill it with gravel, and cover it with earth - you slope your excavation or weeping tile towards it and it gives the water a place to go if it wants to hang around, and then it "soaks away" true to the name. that's what dave_k is describing. your geotech guy can tell you if there's too much clay in your soil for this to be effective, but it is a good/cheap approach if your circumstances permit because you need the equipment and materials on site to excavate for your pad anyway.

    when I did my shop, I didn't build on undisturbed soil so I was able to slope the excavation 2% to one side so that any water that doesn't soak into the ground will find its way out from under the garage. I extended my soakaway up to grade at the back with all of my leftover gravel, and now I have a nice dry pad for building materials / lumber / toys / etc.

    Insulation

    If the ground can't freeze, it won't heave. you can accomplish this by implementing the principles of a Frost-Protected Shallow Foundation to your eventual slab approach. Think of it like this - if your frost line is around 5' down, and you put an insulated wall 5' down, the ground under the wall won't heave and neither will the foundation. This is why foundations are made the way they are. But concrete pony walls that deep are expensive. So you can accomplish the same thing by going horizontally. 5' out is the same as 5' down if you use the right thickness of insulation for your climate. I didn't have the space to do it horizontally (tight urban site) so I extended my perimeter insulation 2' downwards (see my detail below, although i didn't draw the extended insulation). If I had the room I would have extended it 4' horizontally. Here's some more information about it.

    Beyond
    Assuming you take care of the drainage and you properly insulate it against freezing, you've put on your belt and your suspenders. The last thing left to do is over-engineer it! The first 4" of slab are the most expensive... every inch you add is a marginal increase. I did 5" with a wider-than-typical footing at the perimeter, with added rebar and wire mesh in the slab. I used 36mpa concrete just like iamtooler suggests (typical for the heavier loads in a garage, a regular basement slab would be 20mpa). Altogether it makes the whole thing very strong and rigid for the heavy loads of a garage... so putting shop tools in it is no big deal, and it should be pretty resistant to movement. If it does move, it's moving as a single unit.

    Attached is a detail of my own foundation. In order to accomplish good drainage you might consider something similar - establish a 6" compacted bed and build your foundation in insulation foam on top of it. I was originally going to use a Legalett system (which should be considered by anyone hoping to build a garage, it's a great system) but the way I built it ended up being a lot cheaper with many of the benefits. The form of the monolithic slab was achieved with sand infill rather than gravel or foam. The only reason the perimeter foam isn't thicker is because it would have been proud of the wood siding, forming a little "shelf" that would require thick metal (and a tricky forming job) to protect it. Too finnicky for a shop! But as described above the whole strategy basically allows for full drainage under and away from the foundation. It's not as frost-protected as I would like, but such is life - that's why I thickened the slab and the perimeter. I'm lucky to be in a very sandy area so heaving isn't as much of a concern or problem - there's no sign of it anywhere, even on the parking pad out front or the 80' of driveway back to the shop. I was definitely over-doing it with my approach but I didn't want any surprises in 5, 10 years!

    That's a lot of info so let me know if you have any questions!

    Attached Files

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  • iamtooler
    replied
    I second Dave's recommendation but I would bring the floor a good 8'' above grade. As time goes by the surrounding terrain always builds up and you do not want the floor to be the low point! I also highly recommend using a 5000psi concrete, a minimal extra expense on such a small slab and far superior floor. Look at installing heating lines in floor, not too expensive if you lay them yourself.
    Rob

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  • pmtottawa
    replied
    my recommendation is a thickened edge slab on compacted non frost susceptible gravel. 6x6x6 WWF mesh reinforcement. (2) 15m cont bars at bottom of thickened edge footing.
    weeping tile around perimeter and a sump pump should your water be high.

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  • dave_k
    replied
    Frost heave? No I did focus on quality though. I made sure sub base was well compacted, consistently thick and properly formed .... as I recall I spent a lot of time with a rake and a plate packer. And Styrofoam was very popular back then but I didn't insulate the slab. I did insulate the walls and the ceiling but I just wanted to keep the foundation simple and the building cheap .... I had just spent 3 years in school.

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  • geshka
    replied
    Thanks a lot Dave. This is very helpful.
    Did you take any measures to prevent frost/heave ? I believe in 1990 styrofoam was not popular as insulation, anyway ...did you use anything ?

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  • dave_k
    replied
    I built a shop with a Slab on Grade foundation in London around 1990. Clay soil, high water table and the whole neighbourhood has seasonal ponds. I drew up the foundation and my structural design teacher (I was just finishing college at the time) stamped the drawing. It was a 6" slab reinforced with 6 x 6 mesh and 12" ? (coulda been 16" I forget) drop footings around the perimeter. I used 3 - 10m reinforcing bars around the perimeter.

    I stripped the topsoil to undisturbed soil and replaced it with compacted A gravel to grade. I built the center up with compacted A gravel leaving the perimeter full depth to form the drop. I used 3/4" form ply ripped to width to form the sides. I forget exactly how I formed it except that I used snap ties and walers simply because I had them and I then used the snap ties to tie the steel and wire mesh to.

    It turned out beautiful. I checked the grade regularly with a builders level and then a laser when I finally owned one and it may have moved but it always stayed 100% level. The key to success is in designing the subgrade. Water has to be able to flow freely under the slab. To achieve this I extended the excavation beyond the back of the shop then filled that with 6" stone and topsoil over that.

    That's my experience ... yours may vary.

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  • geshka
    started a topic Standalone 20x24 workshop foundation questions

    Standalone 20x24 workshop foundation questions

    Hello there.

    I am planing to build standalone workshop 20x24 size. Soil has a lot of clay and water is high, plenty of movement at Winter/Spring season, so lots of issues around there. I was talking to several specialists, including geo-tech engineers and got quite confusing and oppositely directing recommendations. So far I am inclined on concrete pad on grade foundation.
    Asking this community - from experience what someone may had in this situation - what are my option(s)?
    Also, I am in Ottawa area - is there any recommendations for foundation contractor: not too expensive, not too big , willing to have small job.

    Thanks
    Gene.

    P.S. I have workshop already on concrete posts with wooden floor on joists and its not holding my heavy machines (lathe, mill, surface grinder), but I've got what I payed for - flow is uneven and sinking . So I will take it down !!! and rebuild on better foundation/floor
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