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My family currently purchased a new home and the rear deck (about 6ft off the ground) currently is held up using 4x4s. The joists are about 12ft tall and the deck is about 25ft wide.

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Unfortunately, this did not pass the city inspection and the seller suggested putting 6x6s on the bottom the replace the lower section of the 4x4 and then leaving the existing 4x4s on top (we have a metal railing that would need to be cut/adjusted if it was 6x6 all the way up.

Is this safe? Would the load bearing work the same way as if it was 6x6 all the way up?

Edit: to answer some questions- this is new construction and the city did not sign off on it. They said either an AOR letter from the architect is required saying it’s ok, or a change to the 6x6 posts as it shows in the original blueprints.

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    Pictures will help. Would need to know how the 4x4s posts are supporting the deck now. If going with the shorter 6x6s, will need to add bracing or something to support the upper 4x4s, that will be lost by cutting them. Imagine the upper 4x4s are just giving support to the railing and not a roof.
    – crip659
    Aug 23, 2022 at 11:32
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    6ft / 2m sounds like a fairly large height for beams of both of these sizes to hold up anything. This is not so much related to loadbearing capacity, but stability. Is there currently any cross bracing, or are they (deeply) embedded in concrete or something? As @crip659 said, photos would help.
    – MiG
    Aug 23, 2022 at 11:52
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    You said this was a newly constructed house, right? That begs the question of why the city signed off on the building permits if it didn't think the deck supports were sufficient.
    – SteveSh
    Aug 23, 2022 at 13:24
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    Echoing the request for pictures or drawings. Should note that the seller’s suggestion means squat to the City official who will accept or reject the final design. Talk the the City early and see what they require. Aug 23, 2022 at 13:34
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    If I were the city, I would be more concerned about how the ledger board is fastened to the house, and how the joists are attached to the ledger board. Deck failures I've seen involve the ledger board pulling away from the house.
    – SteveSh
    Aug 23, 2022 at 14:26

3 Answers 3

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I am understanding your question differently than others are. What I get is that you have a new house which did not pass the city inspection. The approved plans upon which the building permit was issued show 6x6 posts and for whatever reason your house was built with 4x4 posts.

Aside from the question of "are those 4x4's enough structure to hold up this deck" I see the problem with getting approval and sign off from the city. In this case the city is correct, you need to get the Architect of record to issue a letter (and possibly an amended drawing) showing his approval of the as-built structure you have (with 4x4 posts). Otherwise you need to change the posts to 6x6's as specified on the plans.

If city sign off and approval are your goal, then I do not see other options for you than the city's options stated above. It seems first choice would be to get architect's letter of approval. You can then ask them about either approving the 4x4's as is or just replacing the 4x4 under the deck to 6x6 and leaving the rail posts as 4x4. It is really the architect's call on all this- they are the responsible party.

By the way, I think the 4x4's are plenty for that deck- :)

I do not like all those joists hanging into the single rim joist- usually there is a beam of some sorts which the joists either sit on top of or are hung into- that is more of a concern to me than the deck post size.

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It may depend on jurisdiction, but for a 6' deck height I'd consider 4x4 to be adequate post support at the spacing I see in the photos, given of course they're fixed to the concrete (cannot see that in your photos, but the concrete slab below should have adequate footings beneath the posts and some kind of bracket to hold the posts).

I'm more concerned about the apparent lack of beams and proper joist-post connections. It looks like they ran continuous posts from the base through the deck joists to become the rail posts, and bolted the joists and rim joist to the post.

In my area, that's not allowed. Posts are notched to allow beams to rest on the post, or at least support brackets must be used to provide positive post-beam connection so the beams/joists aren't just held by friction and bolt shear strength. Obviously 6x6 can be notched further than 4x4 since there's more material.

It's a good idea to contact your AHJ to find out what the options are before you decide how to have the deck brought up to code. This is why deck permits and inspections exist, to prevent problems during home sales.

Source: I built a deck without a permit but researched and followed code precisely, inspectors when I sold that home had no issues okaying it.

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I guess the direct answer to the questions is YES, you can use 6x6's for the posts supporting the deck, and 4x4's (or something else) for supporting the railings.

No matter what you use for the railings and their vertical supports, it is not sufficient to just bolt those supports to the rim joists as was done in the past. It is recommended that those posts be firmly tied into the deck sub-structure using tensions ties (see the Simpson catalog) or something similar. Picture below is from the Simpson catalogue:

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