free standing 10' x 14' deck w/building

mclemensAugust 18, 2006

I've got the CEO's approval to build a 10' x 14' roll-off roof observatory in the back yard on our hill top.

I was wondering if this structure could be supported by six 10" sonotubes sunk to frost depth (shown as "X" below). I was thinking to stand 6x6 posts on those six concrete footings, holding up two 4x12 beams as follows:


10 ft


Then I was going to put 2x10 joists 16" o.c. on top of the beams and use 2x6 cedar as a decking from front to back.

Is this adequate support for a 10x14 freestanding deck? Even if I then build a pretty light building on top of it? (6'5" 2x6 walls all around)

Thanks for any insight


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We are considering a simular issue. We have a gideon septic, which is covered with a sand mound which is flat on top and about 20' x 30'. i don't think it is possible but was wondering if it would be possible to build a deck support system with only support along the edges and no middle supports, so none of the supports is resting on the tank field. Is it possible to find beams which are sturdy enough with no middle supports? Or does anyone have another suggestion to accomplish this?

    Bookmark   August 20, 2006 at 2:38PM
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This is not a permit store,or a free plan review. Both you guys need to hire a Deck Contractor, have Him check into the local codes, and give you a bid. John

    Bookmark   August 20, 2006 at 10:13PM
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mclemens- maybe I'm misreading but you can not put a 14' deck on 12' beams!?! beams must span the full 14'

bsd777- the only way to span such large distances would be to consider steel beams not wood...

as J Hyatt suggested...go to a contractor or code official to get more information from those in the know.

    Bookmark   August 20, 2006 at 11:05PM
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IÂll answer two of the posts here as the questions are similar.


If you have the 4 x 12 beams on hand, fineÂ..use them. If not, (2) 2 x 12Âs would be adequate for the beams on top of the posts. I say on top of the posts, not bolted to the sides. By bolting to the sides of the posts, the limiting factor is the shear strength of the wood parallel to the grain in the posts. In other words, if the loads acting on the bolts split the posts, you can imagine the result. To cut a long story short, this is the load on the bearing area of the bolt shank in the hole and is expressed in PSI. If the load in PSI exceeds the allowable for a particular species or grade of lumber, you violate the building code.

As for the joists at 16" on centre, 2 x 10 is adequate and this is factoring in a 75 lbs/sq. ft. load on the deck. This will be way above what your local code calls for floor loading but I have seen well over a foot of snow on my deck for a prolonged period. Better that little bit over than not. As an extra, it wouldnÂt hurt to put one row of bridging mid span between the 2 x 10Âs.


When you say supports along the edges, I assume you mean the edges would be the 30Â long beams. OK, depending on how many posts you want to put in would be the determining factor for the size of the built up beams. For instance, (2 or 3) 2 x 12Âs. Not that IÂm saying thatÂs whatÂs needed but If I knew how many supports the beams would have, I could determine what would be adequate. As to the 20Â span over the sand mound, it could be done with engineered wood joists such as "Microlam" or "Trus-Joists" but youÂd be getting pretty pricy in comparison to sawn lumber. IÂll e-mail you a brochure for you to look at.

The structures here arenÂt the problem though, you should both check with your local authorities as to limiting access to the septics. That could be your biggest stumbling block.

    Bookmark   August 23, 2006 at 2:59PM
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Using microlam,trus-joists,outside,is a really bad idea. J

    Bookmark   August 23, 2006 at 5:10PM
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To John Hyatt,


From an engineering standpoint, I was trying to say that what these guys want to do is not impossible...structurally.

I don't want to give anybody bad advice and I know that engineered joists are usually for interior use, but surely there is a way to weatherproof them. Any ideas?

    Bookmark   August 23, 2006 at 6:27PM
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There,,,,Is,,,,NO,,,Way,,,to use those things outside exposed to the Weather,you know that and so do I. J

    Bookmark   August 23, 2006 at 8:24PM
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John is right, engineered joists are only meant for interior use and that is all they should be used for. Im sure there is something you could coat them with but you would have to do every so often and who in their right mind would want to take their deck apart to recoat the engineered joists that should never have been outside in the first place. What he could do is use doubled up 2x12 Ipe and that would be more then adequate. Mclemens you should give advantage trim and lumber a call and they will help you out with the 2x12 that you need.

    Bookmark   August 24, 2006 at 9:25AM
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bsd777 - If you use 2x12 Ipe, better have some strong backs to help. Based on the ipe 2x8 16 footers used on my deck, I'd guess each 2x12 20 footer would be about 140 pounds or better.
Even if the Code is OK with this deck over the field, you ought to price out the alternatives, steel, ipe, big timbers, other? You might decide on something other than a deck.

    Bookmark   August 24, 2006 at 5:08PM
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