Room addition pilings question?

Float Pilot

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My old house is a series of add-ons to the original small cabin. We have lived in it for the last 21 years. The current living-room was a 18 x18ft addition that sits of 6x6 pressure treated posts set into holes with concrete. ( all skirted)
I want to add another room past that addition. I was thinking about more pressure treated 6x6s but my contractor neighbor says I should use 8x8 treated posts. The new addition was originally going to be another 18 x18 section although I may cut that down since I am going broke with other remodel in the older section of the house.
So I am looking for options.
1. 6x 6 posts in concrete with close spacing between posts
2. Concrete filled sauna tubes with brackets.
3. 8x8 posts in concrete
4. Pressure treated and back-filled posts with deadmen on the bottom and no concrete. ( I can let them settle until next summer)

Any ideas?
 

NRick

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Need a bit more information. How high off the ground is the new addition going to be? One story or two? What type of soil?

In general, encasing a wood post in concrete isn't the best. Wood swells with moisture and contracts when it dries out. Concrete, not so much. Those differences can cause the wood post to lose adhesion with the concrete and make it much more susceptible to frost jacking.

In straight compression (the house pushing straight down on the post) a 6x6 can hold many thousands of pounds. The problem comes in when a side load is applied, hence the question on how high off the ground.

If it were me, I'd lean towards option 2, concrete piers up to the beams that are going to support the house.

Does it need to be on piers? Can you step down to a slab on grade?
 

Big Bend

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6 x 6 Treated post sitting on a concrete pad in the bottom of your post hole. Wrap the post with 10 mil plastic stapled to the post then wrap with another layer this one is not fastened tight. This will allow Ground movement without the soil being able to jack the post .
 

iofthetaiga

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6 x 6 Treated post sitting on a concrete pad in the bottom of your post hole. Wrap the post with 10 mil plastic stapled to the post then wrap with another layer this one is not fastened tight. This will allow Ground movement without the soil being able to jack the post .
Yeah, if you're hell-bent to stick wood posts in the ground this is the way to do it. Put a footer pad under the post and wrap it in a couple layers of plastic extending above the soil surface. Package wrap the plastic over the bottom of the post, too. Not only does the plastic prevent frost jacking, it protects the post from ground moisture induced rot and increases its lifespan. Filling the hole with concrete around the post is a waste of concrete.
 

Akgramps

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Sono-Tubes are hard to beat, there is no advantage to backfilling around a AWW post with concrete and likely will add to its demise as it will potentially hold water against the post, and for that reason I would not wrap with poly as there is no possible way you will keep water from getting in between the post and the poly. Any poly above grade will deteriorate from UV and there is no way to seal the poly to the post for the lifetime of the post. Compacted D-1 with water and some tamping will work, a concrete pad under the post to increase the "foot print" of the post can be good depending on weight and type of soil. If using sono-tubes we would typically "bell" shape the bottom to increase the foot print. Gravel backfill allows the water to drain and not hold water at the post.

I have removed plenty of AWW posts that have been buried and usually the "in-ground" portion is sound, at the dirt line can be a issue and if not sealed well at top can cause a shorter life, that's for fence posts, for supports where the cut end (top) is protected they will last a long time.
 

NRick

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The other thing to note, if you are going to go with wood post into the ground, you want to get wood that has been treated to foundation grade specs. This stuff has a lot more of chemicals that resist rotting than the pressure treated wood you'll pull off the rack at home depot. SBS may have it in stock and HD can usually order it.
 

Akgramps

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The other thing to note, if you are going to go with wood post into the ground, you want to get wood that has been treated to foundation grade specs. This stuff has a lot more of chemicals that resist rotting than the pressure treated wood you'll pull off the rack at home depot. SBS may have it in stock and HD can usually order it., all weather wood
YEP, SBS has it, often called AWW for ground contact, typically incised and green
 

momspond

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I was able to get some utility poles that were being replaced with a highway project. Cedar which should last a long time with a protective coat. Some were 14" across and used in the corners. I drilled holes with a 2 man auger set the poles n backfilled with premix cement. And make the top crowned to shed water. They stay dry n appear to be working great. I put a level on the floor last spring, still right on. Being remote it was easy to get stuff there. And the poles are easy to level. I cut a shoulder into the top and bolted the 6x12 beams on. BCIs over the beams.
Our cabin floor is 4' . Good luck n have fun
 

Daveinthebush

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I copied this below. Yes, there are different grades of pressure treated. I remember back in college helping to build an underground basement using pressure treated. But that was many moons ago and those products may not be available anymore.




"The recommended type of treated wood for in-ground use is UC4B and UC4C; UC4B has a higher retention level than lower graded types, and UC4C has even more.

Marine Grade​

Wood treated with water-based preservatives for saltwater immersion has significantly higher retention levels than other types and categories. UC5A has almost double the weight of chemical protection as UC4C, and UC5B and UC5C both have more than three times the amount found in UC4C." (https://plasticinehouse.com/pressure-treated-lumber-grades/)
 

iofthetaiga

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The plastic tag stapled on the end of the treated lumber displays the % (strength) level of the treatment chemical used. You can't judge by the color, nor whether they call it "all weather" or "ground contact", etc. The stuff sold at the box stores tends to be lower % treatment. If your application dictates a higher level of treatment, check the label to know what your buying.
 

NRick

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"Foundation Grade" is what I've heard the stuff called that can be buried and not rot for a long, long time. From my understanding, it is most often treated with CCA (the "good old" stuff that is now banned for most uses) to 0.6 lbs per cubic foot of wood to be called foundation grade. That's what I got from SBS to use for posts for the deck on my house.

CCA used to be what almost all treated wood was treated with. That made it easier to tell how well the wood was treated by looking at the lb/ft3 number. After the EPA banned it for most uses a plethora of other preservatives came out. It's apples to oranges now trying to figure out what you are really getting as far as preservation.
 

iofthetaiga

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All the treated wood I've been buying is CCA treated, and that's the CCA % tag is what I was previously referring to. Can't speak to other chemicals being used, as I've not encountered them. You can still buy CCA by the gallon, too, in green and brown. Point being, tho, you CAN tell what you're getting simply by reading the tag. That's what it's there for.
 
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NRick

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All the treated wood I've been buying is CCA treated, and that's the CCA % tag is what I was previously referring to. Can't speak to other chemicals being used, as I've not encountered them. You can still buy CCA by the gallon, too, in green and brown. Point being, tho, you CAN tell what you're getting simply by reading the tag. That's what it's there for.

Are you sure? From an EPA website:

Effective December 31, 2003, chromated arsenical manufacturers voluntarily canceled virtually all residential uses of CCA, and wood products treated with CCA are no longer used in most residential settings, including decks and children’s playsets. EPA has classified chromated arsenicals as restricted use products, for use only by certified pesticide applicators. It can be used to produce commercial wood poles, posts, shakes, shingles, permanent foundation support beams, pilings, and other wood products permitted by approved labeling. Read more about CCA.

Here in Anchorage you won't find CCA treated wood off the shelf in Home Depot or Lowes. What you will likely find is alkaline copper quatenary (ACQ) treated wood. And, there are A,B,C, and D versions of it. Determining how well wood has been treated has gotten a lot harder. The green and brown stuff you buy by the gallon isn't CCA, it's copper naphthenate.
 

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