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mojo43
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# Posted: 26 Apr 2012 08:25am
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I have been playing around with a bunch of different ideas for the 9X12 cabin that we are going to build in Ontario (where the frost line is deep).
We are on a sloped lot with very moist black earth. I will be building on a higher point, but it will still have black moist earth.
Is this an option? It would be the easiest way for me as getting in a lot of gravel is a much harder option:
http://coyotecottage.com/cabin/cabinconstruction/foundation.htm
If that is not an option then I guess the only other option is to use sono tubes and put in piers. Is that correct??
Thanks in advance. I am very new to this so I apologize for the silly questions!
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rayyy
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# Posted: 26 Apr 2012 09:38am
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My first building which is a 12x12 shed just sits on 2x10 treated boards with the floor joists being 2x6 treated.I laid out a 6mill plastic sheet under it to keep moister down.Not perfect but it will out last me.
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mojo43
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# Posted: 26 Apr 2012 10:13am
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I am pretty new at this. Doesn't the shed move a lot over the years? I will have a bunch of windows and a couple of patio sliding doors in our cabin.
This scenario sounds pretty easy to build, but wouldn't my building move a lot and screw up the windows and doors?
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mojo43
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# Posted: 26 Apr 2012 10:22am
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Sorry, I forgot to mention that there will be a loft in the cabin as well.
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MtnDon
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# Posted: 26 Apr 2012 10:38am - Edited by: MtnDon
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To me a slope, moist earth and deck blocks do not mix well together.
Moist earth will not only allow slippage sideways on a slope but it will freeze and heave.
If you use sonotubes remember to think about how you are going to brace the piers to the beams and joists PT 6x6 timber piers (foundation grade) may be easier all round.
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mojo43
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# Posted: 26 Apr 2012 12:54pm
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Hi MtnDon. I am still pretty new at this. Where can I read more about PT 6x6 timber piers (foundation grade)? Does this mean I dig down 4 feet and put the 6X6s in the ground? Is there any cement to hold it?
Thanks in advance and thanks for the knowledgeable info!
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jkycia
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# Posted: 26 Apr 2012 02:20pm
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I am in somewhat similar but different situation.
I have built by 8x12 cabin in ontario. I just put down 1ft deep gravel and a stack of two 10" cinder blocks in each corner. I built the frame on top
of that. This is on a flat area with clay and a hill going up on once side and a slope going down on the other side. So water passes through.
After one year the bricks shifted a lot. They sank in and tilted. The pairs of bricks are close to tipping over.
Don't do what I did.
I now need to figure out how to fix my foundation.
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mojo43
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# Posted: 26 Apr 2012 02:24pm
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Thanks for posting... I won't take any shortcuts. I'll try to make the foundation solid.
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MtnDon
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# Posted: 26 Apr 2012 02:32pm
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jkycia, thanks very much for the info regarding your foundation failure. Not very many owner-builders will man up to something like that.
For a fix on that you might want to look into helical piles; metal tubes that screw into the ground. Frequently those are a contractor installed item, although several years ago my BIL in Wpg found some that he could install himself.
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mojo43
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# Posted: 26 Apr 2012 02:36pm
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Hi MtnDon, not sure if you saw my questions above??
I am still pretty new at this. Where can I read more about PT 6x6 timber piers (foundation grade)? Does this mean I dig down 4 feet and put the 6X6s in the ground? Is there any cement to hold it?
Thanks in advance and thanks for the knowledgeable info!
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MtnDon
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# Posted: 26 Apr 2012 02:44pm - Edited by: MtnDon
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mojo43 , Piers in wet soil can present a stability problem if the ground is wet most of the time. One issue with piers is that they tend to act as individual units, not as a single unit like a poured concrete foundation. When it comes to piers it's the more the merrier and as much depth in the ground as can be. Soil, especially wet soil, offers little in lateral resistance to movement; not nearly as much as some folks think. If piers must be used I like 4 feet in the ground for every foot above ground.
Piers should have a concrete footing; normal size is 16x16x8. With a lightly loaded building smaller footings can be used, especially when the number of piers is increased, as then the load is spread out more.
Perhaps another option should be explored; pole building. Ideally there would be piers set as deep as frost level and they would extend up to the top plates of the wall. That can make for some long poles or posts. 6x6 PT posts could be used as they have enough meat in them to permit notching 2xsomethings into them to provide for floor joist supports. Members that carry loads should be sitting on other wood or notched into them rather than relying on bolts or nails to provide the support. The posts would be 4 feet on center and then can be covered with exterior OSB structural panels to make a nice rigid structure.
Posts/piers in the ground do not need concrete around them, just a concrete footing. Once the posts/piers are in place simply repack the earth that was removed. Ram it into place firmly with something like a steel mushroom topped digging bar. Some folks will argue that pouring bag of dry concrete mix in the hole around the post is good, even better. IMO, that just costs more and you end up with concrete that is sub par.
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mojo43
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# Posted: 26 Apr 2012 02:53pm
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Hi MtnDon, thank you so much for the response. I really appreciate you helping me out.
I understand what you are saying about the pole building, but I don't understand what keeps the poles in the ground. I assume that I would cement them 4 feet down?
Also, I don't understand what the difference would be between using a pier foundation vs using a pole foundation. Are they not essentially going to give me the same thing in terms of heaving and movement?
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MtnDon
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# Posted: 26 Apr 2012 03:35pm
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The big difference between a pole building and a set of piers with the building perched on top is that the piers can still move laterally independent of each other. By that I mean a pier or two may begin to tilt because of earth movement. Not just move up or down but actually pivot or slide in the wet earth. It is very difficult to brace against that sort of thing. Most cabins on piers have poor bracing. I have seen two different buildings have serious pier movement, rotation, in the ground. One to the point of requiring dismantling and starting over in a different place.
The pole that continue from bottom in the ground to top of wall are braced solidly by the exterior sheathing panels that are nailed to the poles and the framing that goes between the poles. With upper cabin structure (floor to roof) reinforced by the sheathing the poles basically are restrained from any tilting or rotating in the ground. The typical platform built cabin, built on piers&beams has a rigid upper structure that is quite separate from the piers. The upper building may be quite rigid but it is sitting on piers whose bottom ends are quite free to move around.
Continuous poles, top to bottom, tied into the upper structure, make a stronger more rigid package than a structure attached to a separate set of legs, so to speak. The connection points where the piers meet the beams and the beams meet the floor joists and the rest of the upper structure are all potential points of movement, hinging. A continuous pole removes those hinge points. It does need each pole to be precisely placed in order to have the 4 foot wide OSB panel joints fall over the poles, but there are advantages.
Hope that makes sense.
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Rifraf
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# Posted: 26 Apr 2012 04:58pm
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Sorry to butt in here, but how much help would it be to add lots of cross bracing in a normal pier scenario. My cabin for example was on blocks, and I had put 5 piers on my high side 3.5 deep (frost lvl 24 inches) My plan was to cross brace between the piers and also from the inside skids to the pier .. sound ok ?
Apologies to the topic author, I dont want to hijack the thread but perhaps knowing how well cross bracing can help will be of use to you as well?
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MtnDon
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# Posted: 26 Apr 2012 07:03pm - Edited by: MtnDon
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Cross bracing is better than no bracing for sure. The problem with much bracing is the strength of the connections.
A 16D common nail (3 1/2" x 0.162"), not a box nail like used in most nail guns, has a shear strength of 154 lbs when used to fasten a SYP 2x to a SYP 4x. That's per nail.
A bolt, bolted through both members with washers and a nut has greater strength. A 1/2" bolt has a shear of 659 lbs using the same wood members as above.
Then there's the matter of too long a 2x bending in the center as it is placed under compression loads. That's where an X comes into play with the crossover point through bolted with the appropriate thickness spacer. Use 3 inch washers under bolt heads and nuts to thwart bolt pull through.
A bolt will possibly be stronger than the wood when the hole is close to the end of the wood. So many times a series of nails can be stronger than a single bolt. Fasteners should be kept 1 1/2" from the ends and edges for retaining best strength of the wood. That sometimes doesn't leave much room for fasteners when working small members; 2x6 doesn't leave much good area to nail to something like a 4x4.
A good strong wind against the long side of a building with a steeply pitched roof can generate thousands of pounds of force. That's one reason I've grown to dislike pier and beam foundations.
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jkycia
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# Posted: 26 Apr 2012 07:11pm
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Mtndon
Pier is ok if you go deep enough right?
I found a nice site with descriptions.
http://countryplans.com/foundation/index.html
In ontario the frost line is 4ft.
So maybe go down 5.5 ft. right?
That would work for mojo43 as well right?
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MtnDon
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# Posted: 26 Apr 2012 07:31pm
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That would be a happy depth as far as I am concerned. Might not meet local codes but is better than a post stuck a foot or two in the ground.
I am well acquainted with that site; I'm the MountainDon that helps administer their forum. I believe somewhere in that is a disclaimer that pier and beam is not suitable for all soils or areas with a history of tornadoes and hurricanes or earthquakes. That is one way people go wrong. Using 4x4 for piers is another. 4x4 aren't even approved for holding decks up anymore in many places. And even though concrete piers make some sense over wood they are more difficult to brace. Not impossible, they just require more fore thought to make the job easier. One of the worst ideas, IMO, is to use concrete piers with a wood "leg" between the concrete and the beam. Concrete piers should have the beams sitting directly on top using a metal connector like one of the Simpson products.
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mojo43
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# Posted: 28 Apr 2012 09:42am
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It does make sense. Thank you so much for taking the time to explain.
Just so I understand properly, I should have each pole set with a footing of 16"X16"X8"? And these poles should have the footing 4 feet deep?
Thanks again for your help.
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MtnDon
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# Posted: 28 Apr 2012 10:20am
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yes
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mojo43
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# Posted: 29 Apr 2012 09:05am
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Thanks MtnDon, I am a little confused about the footings.
Is this something that could be precast or do I make them myself? I have used sonotubes in the past, but I am not if this is what I should be using in this situation? If I am to use the sonotubes, I assume that I would set the 6X6 in the tube and pour concrete in?
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mojo43
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# Posted: 29 Apr 2012 09:07am
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Or do you have a reference site where I could read more about it? I googled it, but unfortunately I didn't find the answer.
Thanks in advance...
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MtnDon
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# Posted: 29 Apr 2012 11:10am
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I don't have any pole building links; sorry. Google should find lots of pole building info that should be useful.
A footing is usually poured in place. That way if the dug out area is a little uneven you don't have to waste a lot of time trying to level a precase flat square of heavy concrete. Plus ready made flat pads don't have any steel rebar in them. That is needed.
Sonotubes.... You don't need them with pole building. The poles are PT and go in the holes, on the footings and then it's backfilled with the earth that was removed. Ram the earth tight.
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mojo43
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# Posted: 29 Apr 2012 01:38pm
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So the poured footings are dried without the PT poles and then the poles are placed in the holes and backfilled with dirt?
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MtnDon
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# Posted: 29 Apr 2012 02:57pm
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That's pretty much it. Concrete actually cures, not dries to gain strength, but that's just me wanting use the technically correct term. If you do the concrete one footing at a time you can let is set for a short time, maybe 45 to 60 minutes and then gently place the pier/pole and tamp it slightly to help level out the concrete where the pier/pole will sit. Brace it well so it can't shift. A piece of heavy plastic, self stick flashing or pre-applied and dried tar on the pier/pole end will help keep the pier/pole end from soaking water into the end grain.
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mojo43
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# Posted: 1 May 2012 03:42pm
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Thanks so much again for the explanation MtnDon. When you say "Brace it well so it can't shift". Do you mean brace it while the concrete is curing? After it cures I can remove the bracing, correct?
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MtnDon
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# Posted: 1 May 2012 06:26pm
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Prior to placing the concrete the tubes / forms need to be braced in the X and Y directions so they can not move while the concrete is placed and vibrated or tamped to eliminate any voids. The bracing has to be able to resist any movement. Concrete is heavy. Mix as dry as you can; most concrete mixing errors are having too wet a mix. If you mix and let it sit a few minutes and water rises to the surface it is too wet.
Then that bracing can be removed after the concrete cures. It can be reused for the next pier.
How high above ground level will the pier tops be? If more than 12 - 13 inches you will also need bracing that is permanent. Pier to beam and in the 90 degree direction, pier to joist.
Across the width is more difficult to brace effectively. Effective bracing can be made between the piers at each end by constructing a brace wall (shear wall) between the piers. It's all PT 2x and PT plywood. I can find some info if you want or use Google; foundation brace/shear wall, or something like that should work.
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Liamey
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# Posted: 29 Aug 2014 11:59am
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I came across this useful post and have gained valuable insight thanks to all the contributers. My question would be what would be recommended 2x material wise for the post bracing for something like a 16x24 pier and post cabin - 5 posts per row, 3 rows total (i.e. 15 posts) would 2x4 be suitable or 2x6?
Thanks
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creeky
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# Posted: 29 Aug 2014 01:26pm
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hey guys. have a look at apb pole barns he has some pretty good / really good professional grade instructions. I built my washroom with his plans and it worked out for me. I know he changed his site so not sure where the details are, but they're in there somewhere.
Mojo, my building is 8x16. I used 6 posts. I've attached a drawing: This is what I did.
I drilled my post holes with a 12" auger to 4' (I actually went around 4'6" and fiddling with the augur up and down I got my hole a bit bigger). Then I lined the hole (moist soil/clay) with ground cloth to keep the gravel in place and not sinking away.
Then I put in 4-6" of gravel for a bit of drainage.
Then I put in my post. Before I sank the post I drilled and nailed rebar to help the post hold onto the ...
I then poured a concrete collar. I mixed the concrete in a wheel barrow. Each post got a 50 lb bag with the recommended amount of gravel. you think it's a lot but when it goes in the hole you're all like, wha??? I mixed my concrete a bit dry 'cause it will set up okay anyway.
I then filled the remainder of the hole with a mixture of dry concrete powder and lots of gravel. i forget. maybe 1/4 bag powder per hole for the remaining mix.
Make sure your pt post is rated for ground contact.
my .02.
I'm not on a slope, but my locations is exceptionally wet in the spring. The first winter was fine. last winter was exceptionally cold and I got some frost heave in the spring. I'm about 80% sure it was caused by levering action off of the stairs added during the second summer. They use those pound in posts... After the spring thaw the building settled back to level.
good luck.
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MtnDon
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# Posted: 29 Aug 2014 03:49pm - Edited by: MtnDon
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Quoting: creeky Then I put in 4-6" of gravel for a bit of drainage.
I see this recommended a lot. The theory sounds great, who wants their wood sitting in water. However, why will the water drain away from the post? If there is water down there won't it also flow into the gravel voids just as easily? Water does seek it's own level. Give it a place to fill and it will. That's the conclusion I came to. Then one day I was having a conversation with an engineer and asked him. He said there was no reason to place gravel in there. Just dig or drill the hole to the needed depth, drop in the post and backfill in lifts (layers) of about 6" and tamp the earth back in.
Another controversy comes with the use of concrete. A concrete mass near the top, above the frost depth, with earth below it can provide a lift zone. If the earth below the upper collar freezes it can lift the post and the concrete if the two adhere well to each other.
Just something to think about.
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Mainiac
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# Posted: 21 Sep 2014 07:48am
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If there is water in the hole,it cant drain.MY frostless barn sill cock wont work.It has a hole to drain out the water,but it cant because the water table is high,About 4 feet down we have ledge.
One wonders how long PT will really last.
Cabin foundations are a critical start so do it rite.
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