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Pole Barns and Post Holes
I think that around here codes for basement walls as well as sono tubes used for pier foundations require frost barriers. I believe codes are met by wrapping vapour barrier around the sono tube cases. I think the idea is that frost can't adhere to the vapour barrier, and the tubes are less affected by frost heave. Something similar might work for posts set in the ground.
Our post building is a 10' x 10' wash shed, so the building won't carry much weight except for snow load. The building problem at out camp is that the frost goes down to 4' and the spring water table comes up to 4'. If the posts go below frost, then they're in water for part of the year. The easiest construction is to float foundations on the frost.
However, we needed a floor raised 3' above grade, and building that on a floated foundation seemed like a problem. We just stuck 6x6's down 4', left the post ends 2' above grade and used conventional framing above the posts.
We didn't bother with pads at the bottoms of the holes or even gravel around the posts. The soil is very sandy and drains almost instantly-at least when the water table is down. Gravel would have helped the posts set faster but the sand will do the same thing in about a year. We x-braced the posts while the sand sets. We figure that pads in the holes really wouldn't do much good if they're going to be in the surface water for part of the year. A guy that did most of the building and has built around here for years thinks the design is OK, but it is sort of a wait and see design.
If the building settles too much, I'll have to support it with concrete-block piers to grade, but the posts would still provide the lateral support a raised building needs. One good thing about the design is that adding concrete block piers would be easy. The conventional framing sets on 6x6 rails that are on top to the posts. It really wouldn't be too difficult to pick up the whole shed and move it off the posts. In fact, an additional benefit is that it qualifies as a temporary building of less than 100 sq ft. It didn't require a building permit, and it's unlikely to affect the property assessment.
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Pole Barns and Post Holes
Many old log buildings around here are built on top of logs laid on the ground. The old house at our camp floated on a few courses of unmortared concrete blocks. Floating construction works well here and lasts (too bad it doesn't meet codes for dwellings). What killed the old house wasn't that lack of foundations, it was the lack of vapour barrier on the cellar floors and walls. The house had simply rotted from the bottom.
I faced the deck and floating building problem when we put a deck and screen room on our construction trailer. The building materials store design had us hanging a ledger board on the trailer and supporting the front on sonno tubes below frost.
I thought it wasn't such a good idea if the trailer and ledger board could float and the front of the deck couldn't. We ended up building the deck freestanding on deck-blocks and 2' x 2' paving stones. Now the trailer and deck can float and heave all they want without arguing with each other. The building inspector said the design was OK, but not to come back in two years wanting a building permit to enclose the deck and make it into an addition to the trailer.
I like the idea of tiling building sites. It probably reduces rot on posts, and around here frost heave can do a lot of damage to unheated foundations and slab on grade. Keeping water away from them minimizes damage. However, a building guy did point out to me that there has to be some place to drain the water. The years that are problems are very wet falls followed by flash freezes. In such conditions, the soil is saturated, and a French drain would have the same trouble draining that foundations do. Of course, lucky people who are on hills can just point the drainage downhill. At our camp, we’d have to pump it out to the middle of the field.
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Pole Barns and Post Holes
I'm wondering if anybody has suggestions or anticipated problems for a pole shed project.
I want to build a machinery shed beside an existing gabled roof garage. The shed as an extension to the garage would be ideal. The garage is 2x4 stick construction on concrete blocks that float on the frost.
The shed walls would have to be taller than the garage walls, and nee-walling a flatter shed roof to the garage roof might seem an easy approach. However, I'm not sure how much more loading the garage walls would take, and I don't want to spend my winters raking snow off the roof. There's too much of that as it is.
To use the existing wall/roof for loading, I'd probably have to reinforce the walls and build queens-post trussing to reinforce the roof under the joint between the two roofs. And then, I'd still wonder about the floating block base.
What I thought might work is pole construction. The poles for one wall could be installed close to the garage wall, and eaves could extend over the garage roof. I'm not sure how far eaves could extend or if a singe span of reasonable width rafters is feasible. The span would have to cover both shed width and eaves and I'm guessing one span isn't reasonable. I suppose a handbook would answer these questions.
What a handbook probably wouldn't tell me is if there's an issue for joining a roof on a structure that floats to a pole construction roof that wouldn't float.
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Pole Barns and Post Holes
Thanks for the note Bob. You mention the problem I also anticipated. Only I wasn’t thinking about leakage as much as wondering if frost heave would break something if the two roofs were solidly joined. I've seen buildings with basements here that were virtually destroyed by frost heave when left unheated for a winter or two.
I guess it would be great to eliminate the overlapping roofs all together. I sure thought about going straight up next to the garage. The rafters probably can be turned around. an increased span would result, but that's probably manageable.
There always seems to be a 'But Then' problem. But then, going straight up, the shed wall would be higher than the garage wall and snow would drift into space and couldn't be easily removed. That's probably a loading question, and an experienced builder could design something that would work and reinforce the garage if necessary. That might be the easiest solution.
At the moment, I have some vague ideas that keep the overlapping roofs idea. In effect, they amount to building part of the garage into the new shed and joining them in a soft way to be relatively weather proof but so they wouldn't tear each other apart. It's safe to say that what I come up so far ranges from 'probably not viable' to 'simply dumb.' I suppose floating both structures and joining the adjacent walls might be easiest, but I'd sure hate to give up the pole construction plan.
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Pole Barns and Post Holes
Thanks for the notes. I'm still getting my ideas together about pole construction. It's a brand new idea for me that a building on poles below frost (4'+ here) still move. Most design books I've read say the poles have to be below frost, so I guess I assumed that they wouldn't move. That's why relying entirely on book learning is risky. Maybe tying the garage and new shed walls together and then joining the roofs together is a viable design. I would still check with people who've done a lot of building here to see what works best though.
The pyramid idea to keep posts from heaving is interesting even if I end up wanting my posts to move. I guess a pyramid reduces the amount of concrete needed to get enough post in concrete so they won't pull out. Perhaps the shape also reduces the chance of the concrete cracking and releasing the posts. It sounds like you guys have got augers bigger than my 12-incher to do some of this stuff.
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Pole Barns and Post Holes
Jim: We had a cold winter too but with fair snow cover so the frost didn't go too deep. I've got to get back to my own shed ideas. Our neighbour's well line did freeze and I know it's 5'+ deep because we dug up the old head to install a pitless adapter last summer. I wonder if the exposed well casing caused the freezing? Ours didn't freeze but the casing gets covered with snow.
When I read your comment I looked at Corm's previous post. That might be something to think about but I guess the bottom of his forms would have to go below frost. I wonder how Corm gets 24" squares at the bottom of post holes?
In my case, my shed would attach to an existing garage that I think floats on the frost so it probably would be better if the shed did as well. The house we demolished at our camp floated on frost for 40 or more years, as do many of the older buildings around here. Floating them does work if it's consistent with codes. Frost heave wasn't the reason for demolishing the house. I saw somebody in town last week doing about what I want. However, it looked like he had just cut rafters to the angle of the garage roof and was using post and beam for the other ends. He seemed to be applying the rafters directly to the shingles on the garage roof. Maybe things are simpler than I think.
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Pole Barns and Post Holes
Randy: I believe that our old camp house as well as our new elevated shed bears out what you say. The old house floated on about 2' of concrete blocks and survived just fine even with a 5' cellar under the house. Nearby buildings with conventional foundations and basements that were left unheated over several winters almost destroyed themselves.
Our new 10 x 10 wash shed uses conventional framing built on top of posts and beams. The postholes are 4' deep and at 5' centres. We didn't use pads in the holes because it would have made leveling the beams tricky. Besides, a pad on the bottom of a 12" hole probably doesn't spread the load much more than the ends of the 6x6 posts we used.
I never was certain if frost can grab and lift timber posts or poles but frost barriers are now required by code for new residential construction. I think barriers must be installed on foundation walls and sonno tubes but I don't know about poles. The fact that our shed hasn't moved through three winters probably means that frost doesn't grab timber. Eaves on the shed are very wide so roof runoff is away from the posts.
Peters: In the AF in North Dakota we used to call that stuff 'snrt.' You know, snow and dirt. I guess snrt is easier to say than 'snust.' It was strange to be driving when snow was blowing across the highway, drive through it and then smell dust. We finally got rid of the last bag kiln-dried sand we bought to spread on ice. The building supply place must have left the bags stacked in a mud puddle because the sand was anything but kiln-dried. I can attest that wet sand freezes solid as concrete. We had to bring a bag inside several days before we needed to use it.
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