Waterproofing Cantilever Carport

[dreber]

So I am planning to build a wood carport by cantilevering 6x6 wood posts out of drilled pier concrete foundations.

I'm concerned about the wood, particularly at the base of the structure where it will be wet longer.

My original idea was to create a 3" tall cone on top of the 12" diameter foundation, but the customer is afraid that it will be a trip hazard.

Any thoughts on how to waterproof the bottoms of the posts?

 

[paddingtongreen]

In my experience, the weather will defeat your best shot and any other thing you try. Wood embedded in a foundation is considered to be certain to rot.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[JAE]

I would agree with paddingtongreen - concrete is just a very hard sponge. Even treated lumber will rot eventually.
Mostly where water and oxygen meet.

 

[JAE]

What you could consider is embedding a steel section in the concrete with a steel section extending up - perhaps a hollow tube of the right internal size to receive the post.

The tube could embed into the pier whatever is required for moment - then extend the tube above the concrete pier some distance - perhaps 2 to 3 feet depending on your lateral loads.
Keep the bottom of the wood post above grade with an internal bearing plate with a hole in its bottom. Then through-bolt the post to the extended tube in both directions up the tube.

 

[TomDOT]

I agree with the above two posters, though some coatings (creosote comes to mind, though the health concerns make it generally unavailable in the USA) should delay the inevitable rot. I've seen bolted metal bases used for the transition between a wood post and a concrete slab or pier. The end grain of the post should still be sealed in some way.

 

[Splitrings]

I would recommend you specify the AWPA Use Category for the pressure treatment you best think meets the needs. I would say UC4C as a minimum. If the design life is long and the components are critical, you might consider UC5x. This is over 4 times more treatment than any PT post you'll get at your local lumber yard. If this structure is residential in nature you'll be limited in the treatments available to you. The good ones; CCA, Creasote, etc are still available for non residential uses. If it is residential, Copper Naphthenate might be a good choice. Its oil based and will help minimize the shrink swell that you may see with water based preservatives.

 

[Splitrings]

I would form a slight channel at the post/concrete interface and fill it with a watertop product such as SikaSwell.

 

[dreber]

Thanks for your concerns everybody - I will certainly look into using steel in the concrete instead of wood.

There should be some life span associated with pressure treated wood. If I could get 20 to 30 years out of the posts, that would be great. Does anyone know of any literature they could recommend regarding expected life spans?

 

[msquared48]

No, but it also depends on the local chemistry of the soil and water.

In my area, the Pacific Northwest, it is wet, but not humid. I placed fence posts set in concrete in the ground that had the coniferous and deciduous overburden removed thirty years ago and they are still very serviceable. Many of the posts went into hardpan. I would expect a minimum of another ten years before they will have to be replaced, but I will not be in this house to see that.

Mike McCann
MMC Engineering