proper posts - DIY Chatroom

lun4cer · 07-02-2009, 12:23 PM

I am looking for information about loading of posts in deck construction. To be more precise, I'm not interested in how to attach members, but in how to correctly size the post itself. Looking into IBC & IRC 2006, there are specs for horizontal members (joists & rafters), but not for the posts. I have a feeling that they want to leave this to design engineers, but most of the folks I know who build these just 'wing' it. A rule of thumb would work for me, if there was supporting evidence.

Daniel Holzman · 07-02-2009, 01:13 PM

The reason that information about vertical posts is generally not included in code is that computation of the allowable load on a post is more complex than for a horizontal member. Typically, allowable vertical load is controlled by the compressive strength of the wood for SHORT posts, however once the post is beyond a critical length, the allowable load is controlled by buckling of the post. The exact length at which buckling begins to control is not simple to determine, that's why engineers go to school.

The length at which buckling becomes an issue is a function of the length of the post, dimensions of the post, end conditions (i.e. is the post buried in concrete or nailed to a beam), and whether there is any intermediate support of the post (i.e. cross bracing). This is typically beyond code, however I realize nobody wants to hire an engineer for a 2 foot deck off the back of the house, so the local building inspector can probably tell you what size you need for that. If you are building a high deck, like for example an 8 foot high deck to match the second floor of the house, you might need a structural engineer to size the posts.

Michael Thomas · 07-02-2009, 01:27 PM

It's in IRC 407:

All deck posts shall be 6x6 (nominal) or larger, and the maximum height shall be 14'-0"...

There are also diagonal bracing requirements if the deck is more than 2" above grade and not attached to an exterior wall.

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GBR in WA · 07-02-2009, 03:16 PM

Michael and Daniel, could you take a look at this and tell me if I should keep recommending it as good?

http://www.ideas-for-deck-designs.co...post_size.html

I thank you for your time. Sorry the hi-jack, Lun4cer, but maybe this will help you as well. And..... Welcome to the forum. Good question! Be safe, G

**Scuba_Dave** · 07-02-2009, 03:40 PM

I've yet to see a deck built with 6x6 posts
Most are built with 4x4 posts
Every now & then I see a few 4x6 posts

Quote:

Originally Posted by Michael Thomas

There are also diagonal bracing requirements if the deck is more than 2" above grade and not attached to an exterior wall.

Bracing over 2 inches above grade?
I've never seen a deck braced when over 4' off the ground

DecksEtc · 07-02-2009, 04:28 PM

The general rule of thumb I use is anything under 4' above grade, I use the proper amount of 4x4's.

Anything above 4' then I go with the proper amount of 6x6's.

Yoyizit · 07-02-2009, 04:29 PM

I'll agree with Mr. "Woodman".

Michael Thomas · 07-02-2009, 05:41 PM

The current IRC requires min 6x6" posts, diagonal bracing if the deck is more then 2' above grade unless it's attached to the structure, and that the deck be engineered if any post will be more than 14' high

lun4cer · 07-02-2009, 05:47 PM

Excuse this if it's a double:
Thanks for chiming in. Good points all. I too have rarely seen a 6x6, except in overly large decks,
Micahel T. - My 2006 IRC makes no mention of 6x6 in 407. It does give a minimum of 4x4, tho'.