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-   -   Adding support for new hot tub on an existing deck. (http://www.diychatroom.com/f19/adding-support-new-hot-tub-existing-deck-149159/)

hammerandnail 07-04-2012 02:11 PM

Adding support for new hot tub on an existing deck.
 
2 Attachment(s)
Hi,


I will be putting a 5200 pound (with water, people, etc.) hot tub on my existing 12 by 30 deck (circa 1990). It weighs more my truck. I know I need to reinforce the deck. I just wanted to run these initial thoughts past the group. Here are some specifics on the existing deck:
  • The deck is about 9 feet above grade.
  • The deck is all pressure treated wood. It has 2 x 10 joists at 16 on center.
  • A 2 x 10 ledger board is attached to the house with lag bolts spaced about every 16 through the rim joist.
  • The joists are attached to the ledger board with joist hangers.
  • The edge of the deck away from the house is supported by 5 unevenly spaced 4x4 PT posts set on, and anchored to, 8 diameter concrete piers of unknown depth.
  • The surface is 5/4 x 6 PT wood.
I need to build a sub support under the existing deck. The hot tub manufacturer said I need four 4x4 PT posts attached to concrete piers. The posts are to be sandwiched between two 2x10 PT beams, spanning the 8 feet between them.

One complicating factor is the presence of a cellar door underneath where the hot tub will go. Putting posts in front of the door would cause great difficulty. Because of this the span of the new beam needs to be about 11 feet. There is also little room (7?) between the top of the cellar door and the bottom of the existing joists. So between the narrow headroom and the increased span, wood is pretty much out of the question. I hired an engineer to tell me that a S6x12 steel I beam would easily support the weight over that span. I would also upgrade from 4x4 posts to 4x6 posts.

I envy those of you who have helpful building inspectors. I long for that. Our current one has a bit of a Napoleon complex and is confrontational to put it nicely. Unfortunately this attitude causes a great many residents to complete projects quickly when nobody is looking, but that is a whole separate thing. I will likely hire an architect or engineer to come up with final plans, but want to make sure I am thinking correctly at this point in time. I would rather overbuild and sleep well. Any thoughts or ideas?

I have attached a rough diagram of what I am talking about. The blue portions are the proposed.

Thanks,
Hammer

Bonzai 07-04-2012 02:30 PM

If it were me I would go with 8x8 posts ... Easy to come by here and that's what has been required on most of the tub decks we have done. Here we are often dealing with considerable snow load as well as hot tubs and people, etc. Even if the engineer says 4x6 is enough, that's still not a big post and square just makes more sense to me as it is just as strong in both directions. However ultimately we can speculate all you like as it comes down to what the engineer who has been on site determines. I presume he spec's to double up in the joists too??

tony.g 07-04-2012 02:31 PM

4" x 6" posts, 9-foot high, supporting 1300lbs each? Hmmmm.
Wonder what Napoleon would have to say.

AndyGump 07-04-2012 03:06 PM

Let your engineer design this for your specific location.
He will take into account all of the loads (seismic, wind, etc.) that will be associated with doing something like this (which is completely out of the conventional light frame construction requirements).
Your city plan checker will appreciate you hiring someone else to take on the liability of this beast.

Andy.

hammerandnail 07-04-2012 04:16 PM

Quote:

Originally Posted by tony.g (Post 957819)
4" x 6" posts, 9-foot high, supporting 1300lbs each? Hmmmm.
Wonder what Napoleon would have to say.

My understanding, and I am not a PE or architect, is that as a freestanding unit that is crazy, but under an existing attached deck that is already built to code (as a deck, NOT as a hot tub holder), lateral movement is mostly already covered. The concern was more that the beams and posts support it's weight.

And Bonzai, that is my thinking too. I would rather overbuild than worry. And he did not spec doubling the joists although I probably would anyway. But this was just a prelimiinary sketch.

Thanks

tony.g 07-04-2012 05:12 PM

Quote:

Originally Posted by hammerandnail (Post 957894)
My understanding, and I am not a PE or architect, is that as a freestanding unit that is crazy, but under an existing attached deck that is already built to code (as a deck, NOT as a hot tub holder), lateral movement is mostly already covered. The concern was more that the beams and posts support it's weight.

Agreed; it would never work free-standing. Much depmds on the end-fixity of the posts and you/your SE would need to be confident the the existing deck is already rigid enough. I still feel 4" on the narrow axis to be a little too slender.

daveplot 07-04-2012 05:41 PM

I have a similar setup, except with the Hotsprings Envoy tub. Orig deck used 4x6 posts. My father is a civil pe, so he spec'd out adding another post, on a 18x18x18 high psi concrete footer, plus lagging the ledger into the house floor joists. The house cantilevered past the concrete block foundation, so one side of the deck isn't directly supported by posts. I can add details and photos this weekend, but a pe should be involved for liability purposes.

In Maryland there isn't much of a snow load to take into consideration, so your region may vary.

Joe Carola 07-04-2012 07:05 PM

Quote:

Originally Posted by hammerandnail
Hi,

I will be putting a 5200 pound (with water, people, etc.) hot tub on my existing 12’ by 30’ deck (circa 1990). It weighs more my truck. I know I need to reinforce the deck. I just wanted to run these initial thoughts past the group. Here are some specifics on the existing deck:
[*]The deck is about 9 feet above grade.[*]The deck is all pressure treated wood. It has 2 x 10 joists at 16” on center. [*]A 2 x 10 ledger board is attached to the house with ” lag bolts spaced about every 16” through the rim joist. [*]The joists are attached to the ledger board with joist hangers. [*]The edge of the deck away from the house is supported by 5 unevenly spaced 4x4 PT posts set on, and anchored to, 8” diameter concrete piers of unknown depth. [*]The surface is 5/4 x 6 PT wood.
I need to build a sub support under the existing deck. The hot tub manufacturer said I need four 4x4 PT posts attached to concrete piers. The posts are to be sandwiched between two 2x10 PT beams, spanning the 8 feet between them.

One complicating factor is the presence of a cellar door underneath where the hot tub will go. Putting posts in front of the door would cause great difficulty. Because of this the span of the new beam needs to be about 11 feet. There is also little room (7”?) between the top of the cellar door and the bottom of the existing joists. So between the narrow headroom and the increased span, wood is pretty much out of the question. I hired an engineer to tell me that a S6x12 steel I beam would easily support the weight over that span. I would also upgrade from 4x4 posts to 4x6 posts.

I envy those of you who have helpful building inspectors. I long for that. Our current one has a bit of a Napoleon complex and is confrontational to put it nicely. Unfortunately this attitude causes a great many residents to complete projects quickly when nobody is looking, but that is a whole separate thing. I will likely hire an architect or engineer to come up with final plans, but want to make sure I am thinking correctly at this point in time. I would rather overbuild and sleep well. Any thoughts or ideas?

I have attached a rough diagram of what I am talking about. The blue portions are the proposed.

Thanks,
Hammer

You are never going to get your answer here. Your answer is like you've already.said....higher and architect or engineer.

mae-ling 07-04-2012 07:55 PM

Engineer will not under spec anything, they want to make sure they are well covered so as not to get sued me thinks!


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