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fangarang_10 05-04-2012 04:32 PM

Help/Critique my deck foundation design
Hi guys,

First time deck-DIYer looking for some help.

I'm looking to build a deck off the back of my house, and I have a bump out which is cantilevered from the house. As such, I don't want to put a ledger on it, so my deck is going to be free standing but sit right beside the house.

The deck is going to sit 14" off the ground, so my plan is to have the beams sit directly on the cement footings.

With respect to the attached design:
- Green = beams made of 2 2x10's
- Pink = 10" cement footings in sonotubes, dug 4' down,
- Red = existing fence
- Blue = existing 6x6 fence posts (set in cement footings)
- Black = bump out from house

My other question is what should i use for joists? Are 2x8's 16" OC okay? should it be 12"?

Thanks in advance for your help guys.

nealtw 05-04-2012 05:05 PM

I wouldn't set the treated posts into the concrete, they often bend twist and crack and sometime need to be changed , so setting them in saddles is a better idea. I prefer 2x10 floor joists myself. With out it being attached to the house is a good idea but you will have to pay attention to cross bracing the posts. If your just throwing a tube in a hole I would inlarge the bottom of the hole to give you a footing of sorts.

fangarang_10 05-04-2012 05:10 PM

sorry, if I wasn't clear. Given that the deck is only 14" off the ground, I wasn't going to use posts.

My beams are going to lie directly on the footings.

nealtw 05-04-2012 06:27 PM

My mistake I read feet not inches.

BridgeMan 05-05-2012 11:33 PM

First things first--use concrete for the footings instead of cement. Cement (Portland cement) is a fluffy gray powder that has no strength, and is one of the ingredients of concrete.

The deck will last much longer if you use galvanized steel brackets between the footings and carrier beams. Less likely for the wood to suck moisture out of the concrete and rot prematurely (even if treated, moisture will degrade the wood over time).

I'm curious as to why you chose to use imbalanced spans, both for the joists and carrier beams. Normal design practice uses equal-length spans to maximize the performance of members. Meaning your joists should split the 13' total width with 2 equal 6.5' spans, instead of the 5' and 8' like you've shown. Same for the beams--place the footings symmetrically instead of imbalanced. Also, cantilevering the ends of load-carrying members would result in shorter span lengths and better load distribution. And don't forget to properly anchor the railing posts so they don't wobble.

Continuous joists, vs. simple span, is the way to go. They will cost a bit more, but perform a lot better (stiffer, less deflection under load, etc.). Using 2 x 8s at 16" centers, continuously spanning the 13' (less if you cantilever) should easily carry most normal deck loads. Go with 12" centers if you plan to have large parties with lots of heavy people.

I'd suggest some heavy black plastic to prevent weed growth, too, before framing out the deck. That or soil sterilizer. Your access for cutting weeds will be severely limited, and they will grow.

fangarang_10 05-06-2012 05:52 AM

Bridgeman, thanks for your help/suggestions.

The uneven spans had been suggested to me to cut down on the # of cuts and work required. Make sense though, so I'll revised my plan to even things out.

I'll also scrap the middle joist, and span the full 13' with my joists.

AndyGump 05-06-2012 08:50 AM

Try THIS link. It is for the Prescriptive Deck Building, and is very useful.


fangarang_10 05-06-2012 11:01 AM

thanks for the link. that looks awesome, i'll be reading it over.

One other question, we're now re-thinking the design, cause we want to run the decking parallel to the house. The thought would be to rotate everything and have the beams running parallel to the house (at similar intervals to the original design), with joists in between.

If we were to go this route, any thoughts on how to provide support to the areas of the deck on either side of the bump out; since the footings would be close enough to the house to support it

AndyGump 05-06-2012 09:30 PM

Sorry but your question is just too involved to answer over a talk forum.
You should design or have it designed in accordance with the Prescriptive deck guide I linked to.
There is lots to consider even when building a deck like this with this many nuances.
You also need to consider how high the deck will be in relation to the door of the house that will access it and other things.
Will you be getting permits for the deck construction or are you even required to get permits in your area?


nealtw 05-06-2012 09:44 PM

The idea of not attaching the deck to the house, is to protect the house from water dammage. To turn the deck 90 degrees I would still run the beams in the same direction. Atach hangers to the foundation for the beams below the siding. Your deck will be a few inches lower than the door. If you are going to use a solid surface you should have a 4 to 6" step down anyway.

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