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alliumnate
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# Posted: 6 Dec 2015 04:03pm
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I will be using pressure treated 6x6 posts for my 16x24 cabin. The frost line in my area is pretty shallow (between 5 and 10"). I've read different ideas on using concrete around the posts... so my questions are:
1. Can I fill in concrete around the posts as long as the concrete is below the frost line, or should I be setting the posts on top of concrete?
2. What type of concrete should I use? I have some quikcrete that I've used for setting trellis posts. Is there something better/cheaper that I could use?
Thanks!
-Nate
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MtnDon
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# Posted: 6 Dec 2015 04:34pm
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How deep are you planning on making the holes for the piers? You do understand that piers offer very little in lateral resistance to movement, especially even more so if the piers are very shallow.
The idea of concrete under a foundation is to provide a footing to assist in spreading out the load. Eight inches thick with rebar centered in that layer is pretty the norm. Sixteen inches wide, so for a post that would be 16 x 16 inches. Around the post simply layer and ram the dirt that was removed. Refill and tamp in "lifts" of about 6 inches. You can ram all the dirt that was removed into the space around the pier.
With such a shallow frost depth you can easily construct a superior foundation. Dig a trench 12" deep by 16 inches wide. If the earth is a little tough to dig use a rototiller and remove the loosened dirt with a square nose shovel. Pour a concrete footing around the perimeter with rebar as mentioned above. Three or four courses of concrete blocks on that and you have a very solid crawlspace foundation. Or look into a PWF (permanent wood foundation).
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bldginsp
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# Posted: 6 Dec 2015 04:48pm
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Setting the posts on top of the concrete is better in the long run in terms of rot. Wood posts set in concrete in the ground are doomed to rot at some time. The advantage to putting posts in the ground is lateral stability, but you need to go more than 12-16" deep for that. I'd consider putting the posts on top of the concrete with metal post bases, and providing a lot of cross bracing between the posts and floor structure.
Use regular bagged concrete. I'd avoid the 'fence post mix' which, I think, is cheaper and not intended for structural use. What's most important with concrete is how you mix it and how you design the footing.
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alliumnate
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# Posted: 6 Dec 2015 05:23pm
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Thanks for both of your inputs! I'll probably avoid the concrete block foundation due to the slope I have. I have holes dug with a 12" auger at least 3 ft deep. It sounds like I should dig out and widen the holes a bit. I'll get at least 8" of concrete in the bottom of the holes, set the posts on top, and back fill and tamp the soil.
Should I get some gravel underneath the concrete? My soil is a gravelly silt loam that drains real well.
Again.... I really appreciate your feedback. This is all new to me, and it's nice to have others to bounce ideas off of. 
-Nate
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bldginsp
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# Posted: 6 Dec 2015 08:03pm
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Don't see a reason for gravel under the concrete. The diameter of the concrete pads is a function of the weight carried by each post, and the load capacity of your soil. Requires some math. Good luck
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OutdoorFanatic
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# Posted: 6 Dec 2015 08:12pm - Edited by: OutdoorFanatic
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If you're like me, all I care about is it will last 20 years or so. Plant yer post in the crete. Most 6x6PT are good for a lifetime. After that, who cares?
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SteveF
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# Posted: 11 Dec 2015 08:40am
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Concrete will 'suck' the moisture out of wood so I would caution you not to set your posts in the concrete as they will become very brittle and snap under load over time. Use the concrete pier and then go to HD and get yourself some steel post peaks... do a search for the word "PEAK". You always need something between the cement and the wood even if it is a shingle, tar paper, etc.
I set my cabin on 8x8 posts and you can check out the pictures here:
http://www.small-cabin.com/forum/6_4565_1.html
Hope that helps.
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Don_P
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# Posted: 11 Dec 2015 06:14pm
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Quoting: SteveF Concrete will 'suck' the moisture out of wood so I would caution you not to set your posts in the concrete as they will become very brittle and snap under load over time.
This is incorrect.
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MtnDon
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# Posted: 11 Dec 2015 06:35pm - Edited by: MtnDon
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Isn't it more likely that the concrete around a post will be a damp "magnet" attracting moisture from the soil and promoting decay in the oxygen rich area where the post emerges from the ground?
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