deck post clearance

[JLSE]

California Residential Code 317.1.4, states a deck post shall be pressure-preservative treated or naturally resistant to decay, unless the post is supported by concrete piers or metal pedestal projecting not less than 1 inch above a concrete floor or 6 inches above exposed earth.

For the condition of a typical deck, using concrete pier footings;

My interpretation has always been, that the postbase requires a 1in standoff pedestal to keep the wood above the concrete pier. So, I would typically specify a PBS post base, or some other Simpson base with a 1in standoff. I would then set the concrete height to a minimum of 6in above grade.

My interpretation that using a post base without a standoff, setting the bottom of post base flush with the top of concrete, would not conform.

But as I read it again, where it says ".. 1in above a concrete floor OR 6in above exposed earth.", I question it. It now sounds like the 1in standoff is not required, if the wood is 6in above grade.

[This is a part of an evaluation of an unpermitted, existing, deck.... ]

Whats your opinion? I could not find any commentary.... if you have a reference, please provide.

Edited: Section 2304.12.2.2 reads; they shall be treated, except: posts or columns that are not exposed to the weather, are supported by concrete piers or metal pedestals projected at least 1in above the slab or deck and 8in above exposed earth and are separated by an impervious moisture barrier.

Well.. they are exposed to the weather.

 

[LuK13]

JLSE

The code language switches from pier to floor, so I interpret that as a literal floor, separate from the pier. Say the piers were surrounded by a slab on grade, then the piers need to project 1 inch above that slab. I think the or in the concrete floor or exposed earth statement reinforces this interpretation.

Your standard practice of PBS post base is more of a concrete pier and exposed earth reasoning. FYI, I think a standoff post base is best practice. How many contractors treat end cuts? A standoff post base is an easy way to minimize wicking concerns. But I would also say the standoff post base is not require by code. For evaluating an existing deck I would probe the bottoms of the posts to check for decay if I had any concerns.

 

[JLSE]

Luk13. Im envisioning a 12" or 18"dia pier, with an unfinished surface, trapping water... and I cant imagine it was intended to be allowed that the post rests at that elevation.

To ask it differently.... At what point is the pier large enough, that it becomes an effective floor?

Ive looked for definitions of pier and pedestal, thinking the code section would make sense if they are defined as having a footprint no larger than the column. Ofcourse, thats not my understanding of a pier, but it is my understanding of a pedestal.

There's no rot. The construction is new. The owner just bought and realized there was no permit for it, so she wants to get one...

 

[LuK13]

I will be honest, in my mind I was picturing in my head a 4x4 post on a little 6" or 8" sonotube pier.

To ask it differently.... At what point is the pier large enough, that it becomes an effective floor?

That is what I call a great philosophical engineering question that I could burn an hour or two on the phone discussing with a few like minded engineers I know. But you started your post with "Im envisioning", so what size piers are the actual posts on before we get into a longer, more philosophical discussion? If you are going to help this owner acquire a permit after the fact you kind of just need to take this deck as it is and look at bare minimum code. Have you reached out to the AHJ for their interpretation?

I agree this, and a lot of other things, would be easier if the code offered up more definitions.

What I was envisioning is the advice I once got about parking a car on a paved surface instead of grass/dirt in an effort to minimize corrosion of the frame. I pictured a concrete floor hopefully draining water away from the post more effectively. I pictured below-ground moisture taking longer to diffuse up through a concrete floor than just evaporating from saturated soil. I think the spirit of the code is that a concrete floor is a situation where the risk of exposure to moisture is less, so they give you a "credit" as a break on the minimum clearance requirements.

I am also thinking of sill plates. We put those in direct contact with concrete all the time, they just need to be treated and have a capillary break. Having a 1" gap between the foundation and sill plate would be rather silly. Are the posts bearing directly on concrete, or in some kind of metal bracket that just lacks the 1" clearance? A bracket might give you some peace of mind if you think of it as the capillary break under a sill, granted its not the end grain of the sill in contact with the foundation...

 

[JLSE]

LuK13
Sorry. I should have posted a pic originally. I thought I could verbalize it.

It is not quite 12in dia., with a 4x6 post. But in an attempt to try and understand it.. and to highlight something that doesnt sound right, I thought I would use something a little larger, but still common.

Also, I found the code section on exterior decks, R507.2.1. In my mind, there is no question now. Those need to be pressure treated posts.

I got an interpretation from a code genius I know, he says 317.1.4 requires the 1in standoff if its not pressure treated.

I havent heard back from the AHJ yet. But that is good advice.

Thoughts?

Thank you for everything

 

[LuK13]

JLSE,

I missed the part in your OP about the posts being untreated. There I go assuming things again...

The post in the foreground appears to be discolored around the base. The post in the background is set atop a 2x on flat as if it rotted at some point in the past and someone trimmed the rot off and slipped the 2x in to make up the difference. Or maybe it was just an attempt to prevent wicking like giving the post some kind of shoe, but they stopped short of putting peel and stick on the end cut of the post? Eyeballing it you seem to be within 6 inches of exposed earth, so you don't qualify for the exception.

My first impression is that means these posts should be naturally decay resistant or treated since you don't meet any of the exceptions. Maybe a 1" standoff gets the post to be 6 inches above the ground and gets you in compliance with the letter of the code? I could also see the AHJ accepting the 1" standoff as meeting the spirit of the code, and I would be comfortable with it myself. I just don't know if I'm willing to say it meets the letter of the code yet. I might change my mind after lunch.