Multi-story deck: What is proper method of full-height post?
I’m looking to replace an existing deck with the exact same footprint and height. Current one is well past replacement age, and wasn’t well built to start with. 6′ 6″ wide, 40 feet long. Deck at grade (6 inches), deck at second story (ledgered to house band-joist), then a 2 or 3 in 12 roof, which joins the house roof, which is steeper. House is solid logs. It’s a fixer upper! I’m at the “Drink a beer and think about it stage”, no doubt I’ll get plans and a permit, but I want to fundamentally figure one thing out before I go any further. How the heck do I attach the second story beam to the posts? So assume I’m going with 6 6X6 posts, 2X8 joists 16 OC. The deck level of the second level (story) is 8 feet, the existing posts are 15′ 6″ with a beam on top of those, and the deck roof rafters on top of that. I can only think of two ways of doing this:
- At the 8-foot (second level deck) level, notch the 6X6 3 inches for 2 2X8 for the beam. This doesn’t make sense to me, that only leaves 2.5 inches of post left, which, if I continue it up to the roof, will mean I’ve notched the same post I’m gonna use at the guardrail post, I don’t even need to ask the inspector ahead of time if that will pass;
- Build your normal, typical, two story deck (with the required attention to lateral bracing, proper ledger, footer depth and size, etc….). Deck at grade, 8 foot tall posts, deck at second level. Then, on top of this deck, 6 more 7’6″ tall 6X6 posts, beam on top of that, deck roof rafters on top of the beam. At least this is what I think is going on in all the photos of this scenario that I see online. But how the heck do you attach the posts to the second level deck (or deck beam)? Again, I’m also claiming these posts as my guardrail posts, so I can’t just toe nail them, or even use a post base, and meet the what-ever-pounds of force they must resist. The deck roof is staying in place, it’s real 2″ thick rough-sawn rafters very solidly attached to the house. Two options I see in this version is notch the posts and a typical 2 ply beam, or, 6X6 on top as the beam, which would look better imo.
Steel is not an option. THis is a rustic log cabin and I aim to keep the “look”. 8X8 posts are out to0. I know there’s a lot of other variables in a deck this high, but I’m really only interested in how you configure the posts to hold a 8′ high beam and 15′ high roof. Current deck can’t be used as an example. Not a lot of detail, but I can tell you there was no permit or inspection when it was built. I do not have wind, snow, or seismic requirements, and the county still allows 4×4 posts for any height, but if I had to over-build one thing, I’m on top of a mountain and do get some 70 mph gusts before a t-storm.
Replies
I like the full-height post for overall stability. Why don't you go with option 1 but without notching the post. I would use a concealed flange joist hanger for the band beam and frame it into the side of the post. (Simpsun HUC series or similar)
https://www.strongtie.com/joistframingconnectors_coldformedsteelconstruction/huc_hanger/p/huc
If you do not like the look of the hanger visible there are some powder-coated "architectural" versions available, or you can wrap the perimeter of the deck with an additional band member outside of the "structural" members to hide.
Good luck with the project!
"I would use a concealed flange joist hanger for the band beam and frame it into the side of the post". I thought about that too, but everything I read (code wise) about the beam to post connection is that is must BEAR on the post. If sandwich-bolting the beam to the post isn't allowed, I don't see where any other mechanical means is, either.
Because properly engineered hangers are designed for the job.
a simple through bolt is not.
This is a job for parts that have some engineering and material science behind them.
You have to have numbers calculated and understand the loads and verify the specific connectors you use meet the requirements.
Have you considered built up columns?
A 3 or 4 ply design makes it easy to interupt 1 or 2 plies to support the beams. Other plies are continuous top to bottom. Can be field assembled to ensure there is no gap between the interupted plies on either side of the beam. Allowable compression load is only about 60% of a single solid sawn post, due to reduced buckling capacity, but may not be a factor with your design.
Design rules are in the National Design Specification for Wood Constructions: Sect 15.3
http://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2015-ViewOnly-1603.pdf
Simpson has some design data when using their SDS screws to assemble the beam: https://seblog.strongtie.com/2015/09/designing-built-up-columns/
I am not positive why "sandwich bolting" is not allowed, but here is why I don't like it:
-If you bolt a (2)band nailed together on one side of the post it puts all the load too far from the center of the post.
-If you bolt one band to each side of the post to keep the load centered you end up with a gap between your band plys. If not blocked properly the plys will try to rotate or buckle on you.
-When beams sandwich a post, the inside beam can end up carrying all the load (until its bolts give way that is).
I have had my local codes guy try to force just what you are saying (that the beam must bear on the post directly). But in the end we were able to convince them of the Simpson hanger approach. Just run the loads and make sure that the hanger will carry according to Simpson's tables
1st thought is you might post some pictures of the house to get a better perspective of what you're trying to do
2nd thought is you should probably get some engineering help figure out how to transfer the deck loads - depending on where you live, the Authority Having Jurisdiction (AHJ, aka the building inspector) may require an engineer's stamped plan to issue a permit.
improperly built/attached decks injury or kill more people than most people realize, Simpson does a deck collapse demo and it's pretty impressive seeing even a small deck fail.
In my area, a guy doing some clean up work at a site was killed by a deck temporarily installed awaiting the final assembly and 3 women ended up on the downstair's neighbors' deck when their's failed. No deaths but injuries
just some thoughts, no real solutions
good luck