I am replacing the roof and deck on a house. While I was there the homeowner asked me about a problem he has. He has a dormer where the whole room comes about 8′ out from the house. Below it is a porch.The room is supported by two 4×4 PT posts sunk 4′ in the ground.The area around the posts had been backfilled about 3′ deep. He says that the post on the left pushes up in and then settles back down. It tares up the sheetrock joints in the room being supported. There is a 4×6 pt beam on top of the posts. the post on the left had about a 3/4″ gap on top. I can not remember now what that corner was resting on, but there is other members in there. Anyway, our frost level here is 30″. Being that the post is 4′ down and in concrete, it seems odd that it would frost heave.Although that is what it appears to be doing. I grew-up in Alaska and worked for years in Prudhoe Bay so I’m use to frost issues.But, the frost line is so shallow here. Would the looser back fill allow the frost line to go deeper? If so, how many years until it is packed enough to bring the frost line back up to normal? Or could this be caused by other forces that I’m not considering? let me now your thoughts. Thanks
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I bet a layer of frozen soil above the 4' mark is gripping the concrete and lifting it.
That's something that crossed my mind, but I've never encountered it before. I was thinking that the moister in the ground expands as it freezes. This would push into the concrete like a vise. Then it could move the post with the frozen ground. Also, the post would not have the resistance that a footing would have with it's combination of weight and reinforcement.
Could also be a function of how much moisture is in, or getting in the soil.
If moisture is high, it is possible or likely for the frost to extend deeper than it would in well drained or dry soil.
Or maybe it is clay and the presence or lack of moisture is causing movement?
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A few things:
First off, two 4x4 PT posts doesn't sound like sufficient support for a room of this size.
Second, unless it's foundation grade, the PT will rot out in 10-20 years.
Third, there's no guarantee that the concrete the post is set in hasn't cracked open (or that the post has not already rotted off at the top of the concrete), freeing the post to rise/fall with the frost. Even if neither of the above, the post may have shrunk enough to be loose in the concrete.
Probably the best solution would be to remove the posts and pour concrete piers to grade, using Sonotube or some such to assure a smooth outer surface.
A more "quick fix" would be to excavate around the problem post to a depth of about 4 feet and wrap it with something to prevent frost grab, though I'm not sure what.
The post does seam too small to me to. There are many things on this house that are not right. That is why I am not only replacing the roofing, but also creating venting and doing several repairs. A lot of these houses I refer to as being built by "hippy Joe" or "farmer John". I have nothing against either hippies or farmers I just don't want either one building me a house. But, I have had a lot of work repairing or replacing their work.
The documentation on shallow frost protected foundations goes into depth about frost lines and how they vary all over the place, even though as builders we're used to thinking of them as being at a certain depth and no more.
Soil with good ground cover loses heat more slowly to the low surface temps so the frost line is more shallow than average, while rocky areas with no cover allow the frost line to plunge deeper than average. It could be an area where the frost line drops below the 4' level, although it's likely to be something else.
I'm with the others who suggest pouring new piers and if there is still a concern about the frost line being lower than 4' I'd say to ring the piers with 2" rigid foam, 2-3' out, buried 1' under the surface. That keeps the cold surface from being able to suck the heat from that area and effectively makes the frost line more shallow.
Good building!
PS Spent some time on Kodiak and in Anchorage. Sure miss the summers and am thankful for the lower-48 winters.
Beer was created so carpenters wouldn't rule the world.
I hadn't thought in terms of ground cover. There isn't any around these posts. I was thinking of how much air was in the ground. I've used ridged insulation around foundations before to help the crawl space maintain heat. Also, the sona-tube and bracket sounds like a good idea for this application. I've been thinking of doing that for the decks I build. Right now I sink pressure treated in concrete.But, a sona-tube with a bracket sounds good. By the way, how did you like Ak. I really miss it sometimes. I don't like the summers here much, way to hot,(I live in northeastern WA). One thing I like down here is, in the winter, no matter how cold it is,when you go out into the sun you can feel heat from the sun. Back in AK all you feel is cold, no warmth at all.
I miss the fishing and having brown bears in the back yard. Cant say I miss Kodiak's rain, wind, lack of blue sky or $27 pizza. All summer I'd catch pinks and silvers on ultralight gear 5 minutes from home or work until my shoulder would hurt. Living big.
Beer was created so carpenters wouldn't rule the world.
I miss the salmon a lot. I use to get tired of it growing up. we would stock up the freezer. My mom cooked salmon about every way imaginable. But, fishing for them was fun. I live in the mountains near the Canadian border straight north of Spokane,WA. I met some new people at a friend's house a couple of weeks ago. During the conversation one guy asked me if I fished much around here. I do very little. When your use to Alaska fishing style, everything else pales. This here is wonderful country too. It's not a dramatic as AK, but it is awesome in it's own way. A few days ago I was sitting at my desk, which sits in front of a window looking across our pasture, and a young black bear came out of the tree and spent about an hour eating berries. I've been in 47 states and every one has it's own beauty. Although, I wouldn't want to live in most of them. What part of Idaho are you in? Looks like your state is taking a real beating with the fires this year. That's my one fear living here. We have 160 acres bordering national forest on three sides. Most of our land is timber. If a fire comes through here, it could be pretty bad.