Untreated post on concrete slab

[da20captain]

Hello everybody. Been a while since I have posted but I have another question for the gurus of GJ. Just finishing up a 40x50x14. 40x30 of which is enclosed. On a gable end, inside, I would like to construct a loft that will give me a floor at 10’ high. For posts my brother in law has some fresh milled rough sawn 8x8 untreated posts. (The barn is built with 8x8’s). Given the fact they are free I would like to use them. For the posts on the open floor, I plan to use a post base ( https://milspecanchors.com/full-8x8...amental-made-in-usa-black-powder-coat-finish/ ). For the posts (4”x8”) that will be at the back wall, I plan to scab them on with bolts. My question is I would like to avoid having to buy posts bases for all posts. So can I just place some sort of moisture barrier between the bottom of the post and slab? What material would be recommended? There is a plastic moisture barrier under the slab. Thank yall for any input

 

[duneslider]

I have used PT 1/2" ply under posts before when it is interior like you are doing. The bigger issue is weather or not you feel like the posts need to be secured to the floor. The post base makes that easy to do.

Frankly, if you have vapor barrier under the slab there isn't going to be much moisture coming up. I have seen basement walls that were built with regular bottom plates that are 20+ years old and not showing any signs of issues when VB is under the slab. Now, I do live in the desert so water coming into basements is not common. If you area is a lot more humid or wet I may not feel the same way. I know a guy who just puts a rubber sheet material under his wood touching the floor.

 

[larry4406]

I am thinking you are stuck using a metal post base with minimum of 1" vertical height.

IRC Code excerpt below (R317.1.4) does not specifically call out application to a detached shop. That said, it talks about wood columns in basements which I think would be comparable.

On basements and foundations, there used to be an exception (buried in a foot note) for sill plates of framed walls on slabs (not columns) that allowed non-treated materials if there was a vapor barrier below the slab (basement walls) or if there was 8" or more separation between sill plate and grade (foundation wall framing). However when this was done you had to provide a membrane between the sill plate and slab/wall. I can't find this at present and not sure if still permitted.

R317.1.4 Wood Columns

Wood columns shall be approved wood of natural decay resistance or approved pressure-preservative-treated wood.
Exceptions:

1. Columns exposed to the weather or in basements where supported by concrete piers or metal pedestals projecting 1 inch (25 mm) above a concrete floor or 6 inches (152 mm) above exposed earth and the earth is covered by an approved impervious moisture barrier.
2. Columns in enclosed crawl spaces or unexcavated areas located within the periphery of the building where supported by a concrete pier or metal pedestal at a height more than 8 inches (203 mm) from exposed earth and the earth is covered by an impervious moisture barrier.
3. Deck posts supported by concrete piers or metal pedestals projecting not less than 1 inch (25 mm) above a concrete floor or 6 inches (152 mm) above exposed earth.

 

[FredWanaker]

get a gallon of cooper-green, put it in a small bucket and soak the bottom of each post. It is essentially the same thing they inject into the wood as a preservative these days.

 

[Hank11]

Since these are freshly milled, they are going to shrink, so think about an allowance for that. Otherwise, you could glue a piece of sheet metal on the bottom of each post. Or a low cost base of some kind. How about this?

Simpson Strong-Tie CPS Composite Plastic Standoff for 8x8 Nominal Lumber CPS7 - The Home Depot

Simpson Strong-Tie - 4 ft.

www.homedepot.com

Or a piece of PVC trim board or deck board? (both will likely creep and mash to some extent). No downside to fastening to the floor however you use these posts.

 

[NUTTSGT]

I've seen people use nothing more than a piece of tar paper.

 

[rayra]

Nevermind what Larry the Librarian said. That sort of pedantry isn't helpful, unless you too live under an authoritarian nightmare of code enforcement.
Personally I'd suggest a physical mounting of some sort, but I spent most of my adult years in earthquake country.
But anything that will serve as a moisture barrier or sacrificial piece would work for you.

 

[MushCreek]

The problem is that under certain conditions, concrete will sweat. Sweat means moisture, which means potential rot. When you get warm, humid weather after a cold snap, concrete will sweat. In the finished great room I built in my barn, I have 1X6 trim boards against the floor, and didn't want to use PT or preservative. I cut strips of tar paper and stapled them to the bottom edge. I hope it will be OK, but the trim pieces would be easy to replace. For a structural element, I would either use post bases or preservative on the bottom of the posts. If you don't use post bases, how are you going to secure them from moving?

 

[Skiff Builder]

Your posts are bolted to the wall. Maybe just a piece of P.T. 2x, cut and fastened as a foot on the bottom of each rear post in question.

 

[da20captain]

Thanks yall.