Wood pier question

Cole · November 20, 2012, 01:07:33 PM

Hello,
I am new to this great site. I am going to build a cabin, 16x24, or a 14x28 this spring. Always been a dream to build my own cabin. As, of now I am working finishing clearing the land. I still am going back and forth over my foundation options. Either foundation grade lumber piers, or concrete block, or even sonotube. Really trying to avoid bringing in a lot of concrete, I have a bad back, and I wont have to much help. Figure wood piers would be the simplest foundation, however, wondering how long they will last. I have read that the inner 2" of a foundation grade 6x6 will not accept the chemicals, and could rot easily from the inside out. I was going to apply roofing tar to the top foot of the pier, if I add a layer of tar to the bottom of the post, up a foot, would that help slow down organic attack? Since this is my first build, I feel wood post are the simplest foundation option, really want to go that route, just concerned with the longevity of the 6x6 lumber posts. I will place the post on a concrete rebar inforced footer, and backfill with dirt. Any, help would be great. Thanks in advance.

Cole

MountainDon · November 20, 2012, 01:21:40 PM

Just a quick note before I head out the door.

Truw foundation grade piers will not have any heart wood. It is the heart wood that will not absorb the PT chemicals. You can readily see that when you look at the ends of the 4x4 and 6x6 PT that you see on the rack in the lumber stores. Sometimes you need to shave a little off the end to make it really apparent.

There are other deficiencies in the pier foundations that many build. There are lots of variabilty with the suitability of the soil and many use poor or no bracing. You can use the forum search function to find other topics related to piers / posts. drbuilder has a recent topic that does illustrate one of the better looking jobs on bracing.

....till later.....

November 20, 2012, 01:43:00 PM

Quote from: Cole on November 20, 2012, 01:07:33 PM
Hello,
I am new to this great site. I am going to build a cabin, 16x24, or a 14x28 this spring. Always been a dream to build my own cabin. As, of now I am working finishing clearing the land. I still am going back and forth over my foundation options. Either foundation grade lumber piers, or concrete block, or even sonotube. Really trying to avoid bringing in a lot of concrete, I have a bad back, and I wont have to much help. Figure wood piers would be the simplest foundation, however, wondering how long they will last. I have read that the inner 2" of a foundation grade 6x6 will not accept the chemicals, and could rot easily from the inside out. I was going to apply roofing tar to the top foot of the pier, if I add a layer of tar to the bottom of the post, up a foot, would that help slow down organic attack? Since this is my first build, I feel wood post are the simplest foundation option, really want to go that route, just concerned with the longevity of the 6x6 lumber posts. I will place the post on a concrete rebar inforced footer, and backfill with dirt. Any, help would be great. Thanks in advance.

Cole

This came up not too long ago. Might check the conversation in the following post.

http://countryplans.com/smf/index.php?topic=12614.msg164174#msg164174

zion-diy · November 20, 2012, 03:43:31 PM

Maybe I'm missing something here but, wouldn't putting concrete in the bottom of the pier holes and then filling around the post keep any kind of intrusion out thus preventing rot?
Seems like that and PT wood would last longer than me.

Cole · November 20, 2012, 06:18:14 PM

Wow, thanks for the fast replies. I have several questions about construction, and this site is very helpful. I love nature and grew up in extremely rural Missori, and really only have basic carpentry skills. But, been studying and researching and am thankful for this site. I have another question about my post foundation. Could I just build the cabin on post set over gravel footers? At the correct depth and size is this an ok method? I found a link on here that showed me a wood post foundation with gravel footers, alt. to no concrete...just wondering if someone has used that method, and if anyone has cons about that type of foundation. I will add a lot of bracing to cut down on lateral movement, and add Simpson strong ties to tie the entire cabin together. If gravel will work, what size is appropriate? Thanks again for the help, I really appreciate it!

November 20, 2012, 08:02:01 PM

Much has been written / discussed here about the deficiencies of piers used as a foundation for a cabin or home. Nothing deters many DIY builders. So if one is going to use piers for a cabin or home let's do it in a manner that gives the structure a fighting chance.

One thing to remember is that we really need a solid footing under the pier to spread the weight out in an effort to prevent having the pier sink or settle. Soil can vary considerably in its ability to bear weight. Since the footing is at the bottom of everything it would be wise to err on the side of conservatism and pour a concrete pad about 16 x 16 x 8 inches thick, with steel reinforcing bars embedded at about half thickness. That's two 80# bags of Quikrete. That's my opinion. So no I don't think gravel makes it better. It is not better, or as good as dirt even, unless it is packed tightly. But then we still have only the area of the pier cross section supporting the structure load. Not a good way to do it, IMO.

IMO, there is no advantage to pouring concrete around the pier. Concrete shrinks slightly as it cures. Wood also shrinks as it dries and who hasn't used PT wood that was fresh from the treatment plant. So there is a possibility of shrinkage providing room for wiggle or wobble. That's theory; I have no proof.

Is that space going to provide a space for water to invade and sit there? That's not good for wood. Not good for concrete either if it freezes. When the concrete cracks there goes any advantage it provided as mass "attached" to the pier. At least that is the way I see it.

As I'm not sure I plan on asking my engineer friend some questions on concrete after Thanksgiving. I'll pass on his professional thoughts.

Personally I believe that shoveling the dirt removed to make the hole, a few inches at a time and ram packing it is the better thing to do.

This does not even touch upon bracing which is often ignored or poorly executed. Nor does it address the building codes issue regarding piers.

JRR · November 21, 2012, 09:27:01 AM

My theory ... and that's all it is: If a wood pier foundation is desired, I would still try to include reinforced concrete footers for each pier. This comes from my experience with termites and rot.

It seems to me that if the hole for each pier was dug to suit the frost depth, and the hole made large enough (say a 3 foot circle), it would support upward and downward forces much better than a post alone. Each concrete footing detail could include a concrete "post" formed in, that would be the same cross-section as the wood post ... would extend several inches above the ground ... would include pipe inserts placed horizontally for achoring. Of course, all of this would have to include bent-to-suit steel reinforcement.

Just my theory.

alex trent · November 24, 2012, 06:31:27 PM

From my experience building a couple on piers and lot of research.

For non pressure treated wood

1. The consensus is that concrete does not seem to be a good choice...water stays in between it and wood and is a problem.

2. Roofing or road tar gets mixed reviews...same as for concrete. While I have no tdone it, it seems the tar is not as likely to leave a space n between it and the wood. People here, where there is a lot of insect and mold pressure use it and seems to work. some of that may be the wood as well as the treatment. I have seen 15 year old posts that are sound below soil level,

3 As has been stated but bears repeating, need ot have the area under thet pier or post to support the weight.

4. I put my posts on top of concrete piers that come about 6" above the soil line and go 4 feet down. Secured pier to post with Simpson connectors. I worry about that connection a bit. Wind and uplift is not the issue. but if the ground shakes the post will likely wind up on the ground and I can only hope it all falls level.

5. RedoverFarm sent me a link to a plastic jacket that goes over the posts underground (cannot seem to send it so you'll have to look). So you treat the wood posts and put this over and keep water out from inbetween and looks like a winner. About 25 bucks each and i am going to use on borate treated posts..no pressure treated here.

Redoverfarm · November 24, 2012, 06:57:31 PM

The site is contained in this posting. http://countryplans.com/smf/index.php?topic=12660.msg164768#msg164768 If concrete is used from the footing level to above ground then without much more effort or concrete it could be elevated in a sonatube to the desired beam connection location and the wood post could be eliminated. Yes there is some additional concrete but in the long run IMO it would be worth the extra protection. No rot problem or no insect problem. Bracing attachments could be made in the tube to accept either wood or metal bracing in the future without much efort. Something as simple as "All thread rods" leaving them protruding out the sides of the sonatube 1" or so on each side. Paired in both directions would accept a piece of angle iron to anchor the bracing in either/ both directions. I am not a big fan of wood post and prefer cement block or poured concrete. Just a thought

November 25, 2012, 10:34:40 AM

Yeah, concrete all the way sure get around a lot of the issues with rot, bugs and connections to the posts.

I never really did an analysis of running the concrete all the way up rather than putting posts on the piers. Likely should have. On the other hand if I can protect the wood in the ground, posts all the way up from a footer are sure the easiest and cheapest for the builds here where there is 4-5 feet of slope.

Redoverfarm · November 25, 2012, 10:43:52 AM

Cheaper yes. Long run maybe not. I modified my original post to include a few more comments. I am not an engineer but I would think that 12" sonatubes would be sufficent in most applications. I know that Simpson makes connectors for concrete applications to beam connections. But even with concrete it should be re-enforced w/rebar. The foundation is an area that should not be skimped on. Other areas of the build maybe, but not the foundation.

alex trent · November 25, 2012, 11:34:48 AM

The more I think about it, the more wood all the way in the ground sounds "iffy"....at least over the real long term. My way with concrete in the ground and posts above has the post to pier connections as a possible weak link...but I am not too worried about that if done right and the right bracing done. As said before, I think a big shake and it likely fails. But so do most other things.

With concrete all the way up, IMO you still have some connection issues at the pier/beam in a shake..not sure how much less than with the ones I have on post to pier. But those connectors are not a whole lot different than post pier connectors and while you do not have the leverage of the long post, good bracing takes care of that...so seems like a wash to me.

For me, the issue is not high wind or a lot of fat people dancing in the house, but the earth shaking and that is something we have a lot of down here. There are always compromises. The guy near me build a big continuos foundation place with lots of rebar and all the columns tied together under the ground as you are supposed to do here or seismic protection...but it's likely the whole slope where he built will slide down and the boulders above him will roll into his house. Might be he should not have built there. I'd rather have mine just fall off the piers.

Cole · November 28, 2012, 01:50:12 PM

Thanks for the help. Turns out I can not track down foundation grade lumber

so going with concrete piers. Thinking of 3 rows of 4 with 12" piers. Going to have to work on my plans. Probably will have more questions soon. Any thoughts or help on a 16x24 will ALWAYS be helpful. Not able to go to my property for another 2 weeks, going to be a long 2 weeks!

Cole

alex trent · November 28, 2012, 06:24:50 PM

You are in same position I was with mine. I was not aware that in the USA there were places you cannot get PT lumber.

Are you going to go all the way up with concrete piers or put them on wood posts above the ground? That is a big decision needs to be made up front. How high are the piers above ground? That will affect what you do with wood vs all concrete.

Note that 6 feet apart is about the max you want in a pier line so even at 24 feet likely need 5.

I have 12 inch piers, but at the bottom i beveled out to 18 inches..ez to do with bar/fence post digger. Gives you 2,25 sq feet vs 1 sq foot at the bearing on the post bottom. Lay some rebar in flat and use rebar upright in the columns...3 pieces of half inch or greater.

Before you give up on the wood...Have you checked out the plastic sleeves that go over the posts in the ground ....they look like the ticket to me? If you treated the post first with Borate (BoraCare liquid type that penetrates) then zinc-naphta and put it in the sleeve would seem to be about as insect proof as you can get. Keep water out from between them and the wood which is ez to do. HD can get them on order. About the same cost as a good post to pier connector.

here is one site...

http://www.plasti-sleeve.com/plasti-sleeve-info/

November 28, 2012, 07:20:55 PM

I'm surprised you could not find foundation grade. Try the Wolman locator?

http://arch.know-where.com/wolmanizedwood/

Redoverfarm · November 28, 2012, 07:31:48 PM

Another alternative to wood would be concrete/cement chimney flue blocks. They can be laid up and the interior poured concrete and rebar. But you have to make sure they are plumb and level (At least the first course block if dry stacking). This can be done carefully laying and leveling on top of the footing concrete when poured. BTW if you choose this method you will need to drive grade pins (short portions of rebar) to make your top course at the desired elevation. Use's less concrete than sonatubes. A standard 16"-18" square gives you a about a 8-10" void in the middle for rebar and concrete. When used in this fashion I would notch the edge of the two horizontal meeting flats to accept rebar horizontally(still allowing the blocks to sit flush) and then tied to the verticle rebar from the footing. Finish off with Simpson fastner to accomidate your beams.

Blocks climb at a rate of 7-5/8" dry stacked or 8" with 3/8" mortar joint.

The individual flue blocks are heavy ( 90#) .

Again you can notch out the same as for rebar but instead you can use ready rod or all thread 1/2" or so which will enable you to have an anchor point for your bracing.

Cole · November 29, 2012, 11:08:34 AM

Well Mt. Don, that company slipped past me. I guess not the right key words in my search. I don't know how I missed this lumber company! Thanks again for the posts guys. This is why I enjoy this forum, very helpful! So, going back to the wood post route. Also, thinking of going down to a 16x20. This is my first cabin, so decided it would probably be easier for me to just get 2x10x20s to build up my beams, instead of splicing.

alex trent · November 29, 2012, 12:50:54 PM

Splicing is not all that hard.

Glad you found the wood.

Sheldondesigns · November 30, 2012, 06:03:31 PM

Cole... What you are doing with the 6x6 posts is not too different than standard pole barn construction. Your 16' x wide cabin is definitely small enough to have small loads along it's length. I would suggest you go to:

http://ufdc.ufl.edu/UF00008517/00001/1j,

which is a free on-line article on the construction of pole-type clear span buildings (think barns). In there they have a very clear description of the "pole-foundation" (page 8, I think).

They are talking about much larger buildings, but much of this will be applicable to a small cabin. For the chart on soil bearing capacity, I'd choose 1500psi... that will cover most situations. Go ahead and use the 20' width and you will be slightly over designed (not a bad thing).

As an architect, I would strongly suggest that you not reduce your cabin length just so you don't have to splice the beams. You'll be fine if you stagger the splices (only one 2x10 spliced at a post) and every splice over a post and not mid-span.

You don't want to be living unhappily in your cabin, cursing yourself that you lopped off several feet, when the splicing is so easy.

Best of luck,
Andy

Cole · December 01, 2012, 10:47:03 AM

I have now been going back and forth with my pier options...you guys all give great advice. The area I am building should have good drainage. My cabin will sit on a pennisula of land, with hills all around on 3 sides. I will try to post a pic tomorrow, so I can give a visual. So, all water should run down the hills, and keep my foundation dry. I wanted to see how the rainy season went this year, just so I could get an idea of how water was flowing...but its been so dry here in the midwest, the 3 days it happened to rain this summer, I was not at my site. It did appear that water flowed away from my cabin site, so think I should stay dry. Another question, does it matter the lenght of the boards, if I decided to go back to a 24'. Example, slicing either 16' & 8', or 18' & 6', just as long as I stagger the joints, and don't have any fall over the center post.

December 01, 2012, 12:12:57 PM

Note: As was said above "every splice over a post and not mid-span"
I mention this because it appeared that you misunderstood this, when
you said "and don't have any fall over the center post", but
I could be wrong

You do realize that midspan means points between two posts
not a point in the middle on a post, right?
Bruce

Redoverfarm · December 01, 2012, 12:19:34 PM

Quote from: Cole on December 01, 2012, 10:47:03 AM
I have now been going back and forth with my pier options...you guys all give great advice. The area I am building should have good drainage. My cabin will sit on a pennisula of land, with hills all around on 3 sides. I will try to post a pic tomorrow, so I can give a visual. So, all water should run down the hills, and keep my foundation dry. I wanted to see how the rainy season went this year, just so I could get an idea of how water was flowing...but its been so dry here in the midwest, the 3 days it happened to rain this summer, I was not at my site. It did appear that water flowed away from my cabin site, so think I should stay dry. Another question, does it matter the lenght of the boards, if I decided to go back to a 24'. Example, slicing either 16' & 8', or 18' & 6', just as long as I stagger the joints, and ~~don't have any fall over the center post~~.

I think what was mentioned was this: As an architect, I would strongly suggest that you not reduce your cabin length just so you don't have to splice the beams. You'll be fine if you stagger the splices (only one 2x10 spliced at a post) and every splice over a post and not mid-span.

alex trent · December 01, 2012, 01:14:21 PM

I also made my splices so they went across at least three posts, not just two. Not sure how much difference that makes, but seemed better to have three post per beam pice rather than two.

Also, I nailed through the two 2x10's all along the length with 31/2 inch nails at 20 inch spacing and a row top and bottom staggered.

I put the beams on notched posts and put up the 2x10's one at a time and then the other so my first set nailed directly to the post and the second one to the first one 2x10. Used 3 1.2 throughout. all nails, no special straps or hardware.

I really agree with not cutting the size because of this. You may have to make other compromises but this is not one.