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ROR slab construction in a cold climate

observatory
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#1 Scott123

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Posted 01 June 2020 - 01:47 PM

I live in a cold climate and am building an unheated ten by fourteen roll off roof observatory soon, I hope. The soil freezes to about six feet/1.8 meters here. The soil is rocky silty loam. It is on a hillside, and sees runoff in the Spring.

 

If you built your observatory on a slab in freeze/thaw territory, what issues did you have?

 

  • Did you insulate the slab? If so, how much insulation?
     
  • Did you place wing insulation in the ground surrounding the slab? (The idea being to trap ground heat, so the area under the slab doesn't freeze and heave.
     
  • Someone told me the slab should be built with grade beams on the perimeter. Thoughts? It doesn't add that much to the final cost.
     
  • They also speculated that I could extend the grade beams past the north end of the observatory, and rest the roll off rail support posts on the beams.
     
  • Another person said, no grade beams, just make the slab 6" or thicker. (I was going with 4" slab and grade beams.)
     
  • I am placing a french trench on the uphill side of the observatory, to control runoff.
     
  • Do you have trouble with the ROR rails twisting dues to frost heaves?
     
  • Rail supports - bore holes bellow frost level and run cement to ground level, then attach the rail posts?
     
  • Or set blocks on the surface for the posts?

 

Thank you for your help,

 

Scott


Edited by Scott123, 01 June 2020 - 02:44 PM.


#2 TOMDEY

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Posted 01 June 2020 - 02:29 PM

Based on my own experience... I'd worry about it. Our freeze code construction depth is 42 inches, your 72 sure is challenging... and, as you know... occasional winters can penetrate even deeper than code. Any foundation footer less than code is termed "floating", and not allowed to be rigidly mated to any other, else violate code and risk losing your e.g. homeowner's insurance coverage!

 

Anyway, my point... I went down 42 inches on all 12 footer pours for my little 12-foot dome. Within ten years it had morphed and moved a bit, and within 30 years I had to demolish the otherwise nice little building. On my 24-foot dome, I went down so deep that I had to climb in and out of the holes with a little ladder! The footers are heavily reinforced with welded up steel rebar. That's been 35 years and a couple have shifted slightly, but really very nicely holding. 

 

Your tile etc on the uphill side great idea. It also depends on how long you want the building to last, without distorting... Alternative is to make the supports adjustable. Many modern buildings actually have that built in. I added that to my big dome, so I can always tweak it flat and level. Only small adjustments have been needed.    Tom

 

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#3 Garyth64

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Posted 01 June 2020 - 02:35 PM

Have you checked with the local codes?

 

Here is SE Michigan, on our decks, we have to have our supports go about 42" into the ground.

 

For my 16 x 24 garage, code required to have a 2' rat wall around the perimeter.  But that meant in the back of the garage where the ground slopes 1', the rat wall was 3'.

Even my little 10 x14 shed had to have a 2' rat wall.

 

I think you should have something around the perimeter for support and strength.

 

What is the slope of the ground?

 

I like what Tom did for his supports, and I assume it supported a deck.  Maybe go with something like that instead of a slab.


Edited by Garyth64, 01 June 2020 - 02:37 PM.


#4 Scott123

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Posted 01 June 2020 - 02:59 PM

It's a 7% slope, south to north, north being the highest part. The whole lot is on the side of a ridge, this was the best site I could find.

 

I have considered setting piers below the frost line, it's what I did for the telescope pier. I had to hire a local utility contractor to bore the hole for that, his rig yanked up lots of stones, several over ten inches on the long axis. My concern is, if I bore holes with a two man auger, I may hit similar stones, and have to move where I bore. If I swiss cheese the building site I may introduce lots of holes for ground water, and will have to change the deck layout.



#5 Garyth64

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Posted 01 June 2020 - 06:14 PM

Well, 7% for your 14' slab is only a drop of about 1'.

 

With a slab, you still would want to have some type of perimeter support for the walls of the building.  But it won't be that much.

 

Maybe you could trench around the perimeter, maybe about 8" to 12" wide, and only go down 2 or 3 feet, or until you hit rock.  That may be good enough.  Trenching down 6' may not be required, but check with a contractor.  The slab, I would make about 6", and have mesh or rebar in it.  And have the steel going into any perimeter trenching too, as Tom suggested.


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#6 speedster

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Posted 03 June 2020 - 07:25 PM

Howdy Scott!

 

Anything above the frost depth is going to heave.  Pour grade beams and they will heave just like the slab.  A 6" slab with #4 at 16" on center both ways can easily be lifted by a corner without breaking (up to a size of some thing like 14' square).  You can't economically prevent the movement so design for it with a slab rigid enough to keep the building rigid and plenty of space between slab and pier.  Insulation, in the application, is a waste.  Avoid the French drain and grade the area on the high side to divert water rather than catch and collect it where you don't want water in the first place.  If you decide to sink piers below the frost depth to support the slab, pour the slab on void forms so it is truly de-coupled from the heaving earth.


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#7 HunterofPhotons

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Posted 04 June 2020 - 10:14 AM

....  Avoid the French drain and grade the area on the high side to divert water rather than catch and collect it where you don't want water in the first place.....

 

Grading helps but so do French drains which can move water away from an area.  

Used around a foundation water is directed away from the foundation leaving no water left to heave.

 

dan k.


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#8 Scott123

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Posted 05 June 2020 - 09:09 PM

Howdy Scott!

 

... A 6" slab with #4 at 16" on center both ways can easily be lifted by a corner without breaking (up to a size of some thing like 14' square).

Thank you for the input, Speedster! Should I make the edges that support the load bearing walls thicker, or is six inches enough?

 

Also, what are your thoughts on mesh vs. rebar? 

 

Thanks again,

 

Scott


Edited by Scott123, 05 June 2020 - 09:15 PM.



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