Advice for setting posts to use as piers for cabin

Hello,

I would like to use these timbers/posts as piers for an off grid cabin/house and am looking for any tips or tricks to do it right. These timbers are supposed to be used guardrail posts but they seem larger than the ones I see on the road. They are also supposed to be treated.
I've also included the layout for these posts and they will not be more than 8.5' apart.
Our frostline is around 18" and I was planning to place these about 3' in the ground. They are all around 6' long so they will be 3' above ground. I want to have a little crawl space underneath the house which is why I chose these measurements. I have a small tractor with a 12" backhoe that I will use to dig the holes. So, the holes will be bigger than the posts. (They are around 6"-8" square.)
This will be a stick-frame build
Since they are supposedly already treated, should I just pour some concrete in the bottom of the holes, set posts and backfill or in case completely with concrete?
Should I still treat the posts in someway?
Any tips, tricks or changes I should make to my plan?

Thanks
Jim

What kind of soil would the posts be sitting on?  When I built my cabin I used 8" sonotubes and the architect said they were big enough to not need footers.  So your posts "may" have large enough footprints relative to the weight of the structure to not need cement.  I would slightly worry about cement causing the posts to rot faster than if they were just in the soil.  If you're looking for a solid/level spot at the bottom of the hole, you could pour some cement in there just to create a pad.  Once it's dry, install the post and backfill.

The posts seem awfully close to one another on your layout.  Do you need them that close for some reason?  

Mike Jay wrote:What kind of soil would the posts be sitting on?  When I built my cabin I used 8" sonotubes and the architect said they were big enough to not need footers.  So your posts "may" have large enough footprints relative to the weight of the structure to not need cement.  I would slightly worry about cement causing the posts to rot faster than if they were just in the soil.  If you're looking for a solid/level spot at the bottom of the hole, you could pour some cement in there just to create a pad.  Once it's dry, install the post and backfill.

The posts seem awfully close to one another on your layout.  Do you need them that close for some reason?  

Thanks Mike for your reply. We live in Northern Arizona at an elevation of 6700'. We have 4 seasons but it's mostly dry except for monsoon season. Our soil is clay/cinder/rock which is why I need to use our tractor instead of an auger for the holes.
Although I am a little cautious on making sure the timbers are protected from weather, I am more concerned about shifting/tilting if just set in dirt.
If your concern about the closeness of the piers as a whole, I understand. This will be my first house build and I just want to make sure I have a solid foundation.
If it's about the center row, these will just be set with a beam on top to provide center support for the floor joists. I plan on using 1 piece 12" engineered wood I beams and even though it might be overkill, I want to prevent center sagging.

Hi Jim, that soil seems to me like it would support the posts nicely.  I can't really see them shifting once they're backfilled (except for earthquakes?).  The closeness of the posts I was questioning is in the left to right direction.  The posts down the centerline is a good idea.  I'm just thinking that you could get by with spacing them out to 10' or more.  Depending a bit on what sort of rim joists or beams you're using around the perimeter.  Then again, if you have the posts and digging the holes is easy, more posts certainly can't hurt.

You may find it better to not have concrete at the bottom of the hole. Let the timber sit on the soil.
It lets the 'hole' around the timber drain.
We find posts encapsulated with concrete rot.
If you can bore holes it would be a better end result, because the hole itself will support the concrete
firmly.

Here's a few options to consider. I helped build a deck 20 years ago and we painted the below grade portion of the wood posts with foundation sealer, and set those on top of the concrete footer, and its worked out nicely. This will prevent water from contacting the wooden posts. I know, I know, it's toxic gick. Another option is to treat the posts with a borax solution. It helps prevent the wood decaying microbes from getting established to rot the wood. Borax is a mineral mined from the earth (they do make synthetic borax too) so this can be much less environmentally bad compared to petroleum foundation waterproofer. A third option is to pour a concrete footer and using a form, such a sono tubes, bring the footer just above grade, and slightly mound the concrete so water sheds, and set the wood posts on those, then everything is above grade, and can dry naturally.

I think a good question to ask is what sort of longevity do you desire from these timbers.

So we have both use concrete and do not use concrete opinions. I guess I'm stuck on that for now and not sure which way to go.
Since these are supposedly treated already, should I be concerned about weather proofing more?
The longevity goal is forever. Is that too much to ask lol. In all seriousness this will be our forever home which for us would hopefully be another 25-30 years. If all goes well the land and structure would be left to our kids. I'm not sure if any of the timbers will need replacing at some point if that could even be done.

Not sure if relevant but I am thinking of doing 6" walls or maybe a double wall of 2x4's so the rim boards would need to be 6"-8" wide.

If this is your forever home, why not use sonotubes and concrete pillars?  Then there's nothing to worry about?  On my cabin I did cement from 4' below grade to just above grade.  Then I used post base brackets and had 4x4s from the cement up to the rim joist/beams.  Maybe that would be a way for you to avoid rot and still use some of the posts you have?

Well, I think for maximum life of these timbers, don't let them be in contact with the soil. If they are, it's only a matter of time before they rot. Could be 25 years, maybe even 50 or more in your seasonally wet but mostly arid climate.

They can be replaced in the future, it just requires a hydraulic pumping truck and a bunch of jacks and steel beams. House gets lifted, posts replaced, and house gets set back down. It's just a matter of money really.

Hope this helps!

Mike Jay wrote:If this is your forever home, why not use sonotubes and concrete pillars?  Then there's nothing to worry about?  On my cabin I did cement from 4' below grade to just above grade.  Then I used post base brackets and had 4x4s from the cement up to the rim joist/beams.  Maybe that would be a way for you to avoid rot and still use some of the posts you have?

I was planning on using the tubes at first until I came across these timbers. Based on the quantity of piers it was getting rather expensive for the tubes. Plus, we have to hand mix and haul in our water so I was hoping to save some $ and work :)

Cant you use a concrete mixer, either petrol driven or electric. Hand mixing is a hard way to go about it.

John C Daley wrote:Cant you use a concrete mixer, either petrol driven or electric. Hand mixing is a hard way to go about it.

Poor choice of words on my part. We do have a mixer. I just wanted to stress we can not get a truck up to where we want to build.

If your building to last forever put a rebar reinforced 2'x2'x4" footing six inches below the frost line,
Using sonnotube pour a rebar reinforced concrete pillar at least 8" diameter, 6 inches + higher than finish grade.
Embed brackets in the top to engage your pillars. Cut your pillars to length, soak them in fungicide and insecticide.
as an alternative to brackets an 1 1/4" bolt with nut and a 1/2" steel plate will give you adjustability to deal with frost heaves / minor land slippage and more options as the house settles
At the top put a rodent shield between the top of the pillars and joists extending 6" from the pillar and 30 degrees down from horizontal.
Put flashing as a rodent / insect shield around the perimeter also.

Personally I would try my best to avoid ground contact with the wood if I could.  Another suggestion is that you are planning on a 3 ft crawlspace, I would honestly suggest going another foot.  Four feet will really be a nice thing as you get older and even now if you have to do much work under there.  Since you are in the planning stages now would be the time to do it.

You can pour half a bag of concrete premix dry in the bottom of each hole. It will cure with the moisture in the soil and stop any settling. You can also back fill the holes with washed gravel which will keep the post mostly dry. My concern is using the backhoe the holes will be too large to really support the posts side to side.

Well since winter is still here all I can do is just wait and second guess all my options
At this time I am leaning toward using the tubes and have a solid column of concrete in the ground, then place the timbers on top. I'm thinking about having the tubes stick up out of the ground a few inches above the ground. I will place the timbers on top of the concrete for a total rise above ground of 4'. This will give me more room for the crawl space. *Thanks for the tip
Anyway, taking into consideration all of the above, does anyone see any problems with this?
Can I go 10' between piers or keep it at 8'?
Best way to mount timbers on top of piers?

Thanks again!

Since your soil sounds very solid, I think the post spacing depends mostly on what the structure above will be. The wider the spacing, the more important each footing's bearing is. If the frost line is 18", I don't see a benefit to going more than say 8" deeper than that with your footings. What kind of framing are you planning? Some of your spacings are hard to read, but I expect you could go slightly wider and more uniform in the top and middle rows, and possibly eliminate the intermediate posts on the sides. With the jogs in the bottom row, you are probably stuck with your 7' spacings, though you might be able to eliminate the two posts at the inner corners of the jogs.

Of course, all this is dependent on knowing your actual framing plans.

The spacing will dictate part of the amount of floor deflection. The other part will be what the floor is made of, be it trusses, timbers, dimensional lumber, etc. Having piers closer together means smaller spans for the joists and you can use smaller material without a lot of deflection because it is well supported by the piers. Considering that you already have those posts for the above ground portion, it might be cheaper and less work to use more piers and less massive, less costly floor joists. This would be highly dependent on what you are using to build the floor. An engineered floor truss system wouldn't need it, and would likely come as a package complete with instructions for the piers.

This would also be an important consideration for settling and any large loads. A water bed or rocket mass heater would definitely be more stable with more support underneath. Any unusually large loads that are well supported will have less settling, and will save you from problems related to that down the road. Understanding these types of loads in advance would be much easier to account for now versus trying to crawl under a house and reinforce things after the fact.

There are a few ways to attach the posts, but the easiest might be to get a spool of strapping and cut pieces of an appropriate size to set in the concrete. This way once it is set up you can just screw the straps to the post. There are also ways to do it after the concrete is poured, but that requires drilling which would be unnecessary if you can set it in the concrete beforehand. I have seen some places with the straps and some places using J-bolts in the cement and buckets on top afterwards. I'm not certain if it was just what the engineer called for if there was an inspector that is picky one way or the other. They can come up with really strange and nonsensical ideas sometimes.

I would suggest checking with your local Planning and Zoning department.  Not only would they know what the local soil requires, but they probably have rules about what is acceptable.  Violating the rules can result in them requiring you to remove/rebuild the entire structure.

Thanks for the suggestions. We are off grid and not involving the county so no code/permits but I still want to build correctly. The design is stick frame, 1 floor. I'm thinking of either using 2x6's for wall framing or a double wall of 2x4's. I was also planning on using 12" wooden engineered I beams for the joists with spacing at 16" centers along with 3/4" t&g subflooring. The rim board dimensions I guess will be based on the pier spacing.

How does all this sound?

You can trade off pier spacing against rim joist strength; I can't give you figures for what will work without knowing complete information on the bending, shear, and deflection capacities of your joists, and the actual dead and live load per square foot of floor and roof plus any extra concentrated loads. 16' (which is what most of your common joists will span) is quite a bit, and I would anticipate a possibility of bouncy floors even if they bear the weight fine.

Glenn Herbert wrote:You can trade off pier spacing against rim joist strength; I can't give you figures for what will work without knowing complete information on the bending, shear, and deflection capacities of your joists, and the actual dead and live load per square foot of floor and roof plus any extra concentrated loads. 16' (which is what most of your common joists will span) is quite a bit, and I would anticipate a possibility of bouncy floors even if they bear the weight fine.

The joists will be run top to bottom based on the above pic of pier layout. This is why I have the center row of piers so I can support the joists. I do not have the specs on the joists in front of me but the building store had their engineer confirm these joists will work fine. Yes the spanning will be between 14' and 16'.
Other than normal furniture and appliances, the only dead weight will be the adobe wall with a masonary heater box (we like to see the fire) and I plan on building a block foundation specifically for this.

Adobe walls? Are you planning this all around the exterior? How thick will they be? If any real thickness, this will be a dead load that requires serious support, and you will definitely need piers at least as much as you show all around. For that matter, piers under an adobe wall just don't make sense to me. I would want at least mostly continuous support, so piers that settle don't cause big cracks.

Or is this an isolated feature inside the house? In that case, nevermind

Jim Grieco wrote:Thanks for the suggestions. We are off grid and not involving the county so no code/permits but I still want to build correctly.

Off-grid or not, I don't think you can legally avoid the county, at least not completely.

I don't know what the rules are where you live.  I live south of you in Cochise County.  Here, if you have at least 4 acres, you can file for an exemption to county inspections, but you are still required to get a building permit and submit your design for review, and of course you have to follow the relevant building codes (electrical, plumbing, structure, etc.)  You just don't have to have the inspectors come out and check your work at every stage.

I'd seriously recommend checking with your county's zoning and planning to find out what the requirements are.  If you're required to have a permit and build without one, you can end up in a legal battle with the county and possibly have to tear down the structure.  It's really not worth it to try going guerrilla.

Besides, when they review your plans they will tell you whether or not your posts need concrete, how deep they need to be, etc.  I don't know what it costs where you are, but here a permit is only around $100, well worth the money for the professional building advice if nothing else.

Another incentive for going through the county.  

Arizona currently offers a tax rebate(up to $1000) for rainwater collection and/or grey water systems.  I assume you are planning on including one or both systems?  The rebate will more than pay for the permit fees.  
However, if you file for the rebate without pulling a permit, I pretty much guarantee the state revenue folks will notice and start asking questions.

Peter VanDerWal wrote:

Jim Grieco wrote:Thanks for the suggestions. We are off grid and not involving the county so no code/permits but I still want to build correctly.

Off-grid or not, I don't think you can legally avoid the county, at least not completely.

I don't know what the rules are where you live.  I live south of you in Cochise County.  Here, if you have at least 4 acres, you can file for an exemption to county inspections, but you are still required to get a building permit and submit your design for review, and of course you have to follow the relevant building codes (electrical, plumbing, structure, etc.)  You just don't have to have the inspectors come out and check your work at every stage.

I'd seriously recommend checking with your county's zoning and planning to find out what the requirements are.  If you're required to have a permit and build without one, you can end up in a legal battle with the county and possibly have to tear down the structure.  It's really not worth it to try going guerrilla.

Besides, when they review your plans they will tell you whether or not your posts need concrete, how deep they need to be, etc.  I don't know what it costs where you are, but here a permit is only around $100, well worth the money for the professional building advice if nothing else.

Nope. Not even close up by us. Coconino county may be the worst county in AZ. They hold you to the same standards as if you lived in the city. Majority who live here do not involve the county. Many horror stories from the few who have tried.
I appreciate your advice though. Thank you

Glenn Herbert wrote:Adobe walls? Are you planning this all around the exterior? How thick will they be? If any real thickness, this will be a dead load that requires serious support, and you will definitely need piers at least as much as you show all around. For that matter, piers under an adobe wall just don't make sense to me. I would want at least mostly continuous support, so piers that settle don't cause big cracks.

Or is this an isolated feature inside the house? In that case, nevermind

Yeah it's isolated. I just want some thermal mass for the fire box. I know there is more to it than this and I need to research more. Where the firebox will be I plan on using block as a foundation so it will be one solid support.

Thanks

Have you looked into the Coconino county sustainable building program?  Supposedly that encourages alternative building practices designed to "benefit from reducing the use of energy, water, and resources, as well as increasing healthier indoor and outdoor environments."

I don't really know anything about it, I just came across a reference to it once.

Best of luck to you.