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Attreadwell
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# Posted: 11 Jan 2013 16:55
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Building a 16x24 cabin with loft in central Virginia. Not sure on foundation type. Was thinking 6x6 pressure treated on gravel base and back filling whole with gravel. Below frost line of course. Placed every 8ft. So 4 rows of 3 post. Everything I've read says it will drain properly and water level is no issue. Question is will this last long term? I'm deep is woods cement truck can't get to this site. I'm three months out from build. Help me.
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Attreadwell
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# Posted: 11 Jan 2013 17:15
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Ok more specific. 6x6 in a 18"x18"x30" hole. 12" of package gravel, then post in place. Add fill gravel, packing it every 3 to 4". Would 8x8 post be better? 16x24 with a loft will be pretty heavy.
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MtnDon
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# Posted: 11 Jan 2013 18:07 - Edited by: MtnDon
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IMO, there is no point to digging a hole and placing gravel in the bottom. Nor is there any point to using gravel as fill around the pier.
Why not use gravel? Placing gravel at the bottom and around the piers simply creates a dry well. Or at least we hope it stays dry, but it actually becomes a place for water to drain into. If the soil is moist enough water will collect in the gravel. It will not drain away into the soil just because that is what we want it to do. Water will go to a place that is easy to get to.
IF you had sloped land, and IF you dug channels from the post bottoms laterally out with a slope so the water would eventually come out at the surface down the slope, then gravel might be a good idea. But you still need a concrete footer if you want the piers to have the best ability to bear weight.
1. The bottom of the hole. The best bottom of the hole is to just dig it deep enough for the pier plus 8 inches. Either try to "bell out" the bottom or make it larger than is needed. The point is to try and make a poured concrete footing that the pier will sit on. The pad distributes the weight over a larger footprint. The pad concrete is best placed on the undisturbed soil at the bottom of the hole. Quikrete is good for this. Add a X or # of rebar placed at about the 4 inch depth mark. Let concrete a couple days before continuing with installing the piers.
2. Use the dirt you dug out to make the hole for packing in around the piers. A mushroom head digging bar is ideal for packing. They're about 6 feet long, steel with a chisel like tip on the other end. Great for digging hard soil; great for packing the layers.
I wouldn't recommend this in sandy loose soil, but then piers would be a bad choice for any soil that does not pack hard.
Look for foundation grade PT 6x6 or larger. With piers larger is always better as they resist lateral loading better due to the increased side surface area.
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Attreadwell
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# Posted: 11 Jan 2013 19:41
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So. Put the rebar at 4 inch depth mark. Bell out bottom of hole. With the 6x6 pier, what diameter hole should I dig? Should I stand the rebar up in the belled out part of the hole? Maybe drive into the hard bottom of the hole? Sorry I know it's a lot of questions. Thanks
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Attreadwell
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# Posted: 11 Jan 2013 19:43
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I plan on the pier being at least two feet at lowest point for my crawl space
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MtnDon
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# Posted: 11 Jan 2013 20:10
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The rebar X or # is placed in a horizontal plane at approximately one half the depth/thickness of the concrete.
The footer should be about 16 x 16 inches (x 8 inches thick) The 16x16 usually means the bottom needs to be belled out
The rebar should be encased in at least 2 inches of concrete. It is not desirable to drive the rebar into the dirt. That provides a pathway for rust on the rebar. Rust occupies more space than non rusted steel. That causes concrete to crack.
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Attreadwell
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# Posted: 11 Jan 2013 20:20
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Ok. Thanks for the help. I was going to use .60 RET pressure treated 6x6. Kind of nervous about the soil touching it but everything I read says it will be ok. This is a link I was looking at that gave me the gravel idea I talked about earlier. It incorporates the concrete footer you recommended and the gravel I spoke of.
http://www.decks.com/deckbuilding/The_Buried_Post_Footing
Let know what u think. Also I heard about a cover or sleeve, plastic I think that can go around lumber the is in contact with soil. Maybe use that with these plans.
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MtnDon
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# Posted: 11 Jan 2013 20:41
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PT wood that is tagged .60 is foundation material; fine to bury in the dirt. That is what is is designed for.
I do not agree with the gravel packing approach. Neither does an engineer friend. In fact he does not like pier foundations at all, but he does say that if you are going to use piers pound the dirt back into the hole and "hill" the surface to encourage surface water to run away.
Plastic sleeves are not needed with the right PT wood.
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bldginsp
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# Posted: 12 Jan 2013 16:35
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I don't know what species of wood they use for the PT east of the Rockies, but here in Calif. you have to watch out not to buy the PT white fir, or hemlock. Total garbage- the inside will rot out in a few years no matter what you do. The good stuff here is Doug Fir PT, but even that is only good for 20 years max, kept away from constant moisture exposure.
I'd check with local woodcutters regarding local species with high rot resistance values. Here in redwood country, they used to build houses on old growth redwood mud sills and some of those are still there 100 years later. In the applachians there are some hardwood species that are rock hard and naturally resistant to rot, probably better than new growth PT. My general experience is that anything you build on PT is temporary, but maybe that's all you want.
One critical aspect of PT use is to dry and treat any cut ends. Make your cuts, let it dry a few weeks at least, then dip it in preservative at least over night. If you dont, the rot will begin at the cut end almost immediately. IMO it's necessary to dry it because at the PT processing facilities they kiln dry the lumber somewhat before treating, but the core is still wet. You cut into that core and expose fresh, untreated wet wood that cannot absord preservative because the fibers are saturated, or close. But your end grain cuts will dry quickly in warm weather, then when you dip it you'll get a lot of absorbtion.
Wood preservative is ugly crud but if you don't use it properly where wood is exposed to moisture, you will be replacing the pieces sooner than you think.
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MtnDon
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# Posted: 12 Jan 2013 18:29 - Edited by: MtnDon
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Best wood for PT is southern pine. For foundation grade it should not contain any heartwood. The heart does not accept the chemicals as well.
Woods like DougFir usually have to have the surface incised to permit any decent penetration. I've never seen a DF that was foundation grade.
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Attreadwell
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# Posted: 13 Jan 2013 08:09
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Found precast piers for my cabin. They are 20x20, circular and 9" thick with rebar running laterally through the middle of them. Dig the hole, level them out and sit them in. They are heavy as hell but this way I'm not loosing two days of work waiting for poured concrete to set up. I may go 8x8 instead of 6x6 on the post. Only concern is if the concrete is in place and not completely level.
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bldginsp
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# Posted: 13 Jan 2013 17:16
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You don't need 8x8 or 6x6 posts on piers for a small cabin. 4x4 will be fine for any vertical posts.
Best way to use any precast pier is to wet set it in concrete. This gives it a larger footprint, and allows you to pour the concrete onto firm native undisturbed soil. A pier alone has a smaller footprint and in the process of levelling it you might disturb the soil beneath, which will result in the pier settling later. I built a small A frame with piers on dirt, no wet-set concrete beneath, and I used a lot of piers to distribute the weight.
I think you are better off with any kind of concrete pier rather than putting any kind of PT wood, or other wood, on dirt. It has to rot eventually.
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