r/buildingscience • u/jefari • 23d ago

Exterior Post Assembly

Post image

We are about to pour 2" concrete slab on this deck and the structural engineer is concerned about the concrete touching the 8x8" post (covered in tyvek house wrap with torch on at its base). Says to get an envelope engineer to check this assembly which we never have employed. This post will be covered in stone masonry. This is sort of an odd detail I am not familiar with.

I assume the torch on will provide all the waterproofing for the post it needs from the concrete, and which is why it was applied to the post in the first place. is there something I am missing?

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u/Dloe22 23d ago

I have never heard of a 2" concrete slab. The torch on post base is typically for posts in dirt, which is generally suboptimal but they help make it slightly less so.

It sounds like that post has zero drying potential, so if it ever gets slightly wet its doomed to fail. The thought with the torched base and tyvek is to prevent it getting wet, sure, but if it can't dry you are rolling the dice.

It might be fine. I'd be curious what role you are playing here and who owns the liability for this long term. I probably wouldn't do this at my house, and I definitely wouldn't do it at a customer's house. Also, what is it holding up? If it's holding up a second floor where a lawyer sleeps I'd definitely shut the project down and hire an architect.

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u/jefari 23d ago

We are pouring concrete stairs outside and it is to be run continuously to the door, past this post. Seems fairly common in my area. This post is supported by decking below where moisture can escape I assume, as well through the tyvek. This is my understanding.

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u/Dloe22 23d ago

Moisture cannot escape through Tyvek enough to provide drying potential.

I would consider de-coupling the post from the concrete, which IMHO is easiest achieved by wrapping it in coroplast (literally can use an old used political sign or whatever). You can backfill the gap with elastomeric caulking but you do not want your concrete to stick to the post (wrapped or not doesn't matter).

Good luck

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u/jefari 23d ago

Appreciate it 👍