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Thread: Deck Footings

  1. #1
    Join Date
    Jan 2006
    Location
    Maryland & California
    Posts
    16

    Default Deck Footings

    Issue: Best alternative for deck post footings.
    Replacing existing deck with new.
    Existing post supports sitting on 4" poured concrete patio using Simpson stand off post bases.
    Deck floor is 5' high.
    California location, in no freeze zone.

    New deck: 28'x 16'.
    13 posts are to be 6', 12' and 16' from house, 8' on center.

    Three alternatives being considered.
    1. Continue to use Simpson stand off post bases on the 4" poured concrete patio. Existing deck has had no settlement or movement issues.
    2. Use concrete saw to cut 8" openings in patio for 24" depth concrete tubes with Simpson post or column bases. Cutting 13 holes in 4" concrete seems labor intensive.
    3. Given the 4" thickness of the patio, use a jackhammer to create holes for 8" footings. The jackhammer approach would create a rougher opening and some risk of cracking surrounding concrete.

    If you have experience with this type issue, what's your best thinking?
    Thanks!
    George

  2. #2
    Join Date
    Apr 2008
    Location
    SF Bay Area (East Bay)
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    Default Re: Deck Footings

    Without knowing your soil type, you'd have to assume low bearing strength, i.e. 1500 psf.

    Then you'd need to calculate the worst-case scenario for post load (tributary area x deck load, which is often 50 psf)

    I expect you'd end up with a minimum bearing surface of 1 2/3 sq. ft. or so, so a 8" diameter sonotube is a little small. (I understand the bearing of the soil might be significantly more than 1500 psf, can you prove that it is?)

    I'd either saw cut or use a rotohammer to perforate some holes in the slab and pour footings at least 1 2/3 sq. ft. in bearing area at undisturbed soil. I'd either use sonotubes or forms to bring the pour above grade and then use column bases for the posts. I'd make sure the concrete coverage from the corner of the base to the edge of the concrete meets minimum specs.

    kevin
    “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” - Upton Sinclair

  3. #3
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    Maryland & California
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    Default Re: Deck Footings

    Thanks Kevin!
    I'll make sure the calculations are correct.
    Here in earthquake territory, we are into correct calculations.
    George

  4. #4
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    Default Re: Deck Footings

    You're welcome, George. Others may have differing methods, of course.

    Where are you working?
    “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” - Upton Sinclair

  5. #5
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    Default Re: Deck Footings

    Kevin
    We work primarily in the San Francisco area ... the "Left Coast" ... if you get our drift (pun intended).

  6. #6
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    Default Re: Deck Footings

    I work in Albany, Berkeley and Oakland for the most part. If you're ever in the east bay, I'll buy you lunch.

    BTW, can you sketch out the post locations? I'm not quite clear on how 13 posts gives the spacing you described. You might need footings of 2 sq. ft or more depending on tributary area.
    “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” - Upton Sinclair

  7. #7
    Join Date
    Oct 2007
    Location
    Sonoma County, CA
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    116

    Default Re: Deck Footings

    I am NOT an engineer but all the decks I build are engineered (they need it for the permit), and the footing is almost always 2' square, 12" into competent soil.

  8. #8
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    Default Re: Deck Footings

    Quote Originally Posted by larryh View Post
    I am NOT an engineer but all the decks I build are engineered (they need it for the permit), and the footing is almost always 2' square, 12" into competent soil.
    With rebar squares in the 2'x2' (by 12" deep) pad as well, I bet. (My engineers like beefy footings too.) But dude said 13 posts (!) for a 450 sq. ft. deck?

    I'm not an engineer either, and my advice is worth exactly what he paid for it.

    That said, if he really has 13 posts, I imagine his most loaded post has a trib. area of about 50-60 sq. ft.
    60 sq. ft. x 50 lbs/sq. ft. = 3000 lbs.
    At really bad soil conditions (1500 lbs/sq. ft.) that only means 2 sq. ft., or like 17"x17".

    It seems like an engineer would design something with way fewer posts and bigger footings. The savings on excavation and concrete/formwork would pay for the engineering and the beams... but I was just going on what he said.
    “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” - Upton Sinclair

  9. #9
    Join Date
    Aug 2009
    Location
    OrangeCounty California.
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    153

    Default Re: Deck Footings

    It is entirely possible that you could convince the local B.O. to allow you to use the existing pad as your "footing" if the pad is min. 3 1/2" thick and there will be a load of no more than 750 lbs. per post for the deck.
    This is based on the allowance in the code for porch roof covers.

    Possible and may be worth a shot really.
    I had done this once with the Brea city BD but no guarantee in your city of course.

    Andy.
    Was a GC, doing drafting & design now.
    www.draftinginoc.com

  10. #10
    Join Date
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    Location
    Maryland & California
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    Default Re: Deck Footings

    Update on deck footings decision.
    Our structural engineer and concrete consultant have decided that the best alternative is to bore the footing holes.
    Boring requires less water (an issue in California, as well as less risk that any resulting slurry might enter the drain and coagulate).
    Cutting requires over-cutting the hole in order to release the hole plug.
    Boring is 1/4-1/3 more expensive, but less mess (water is vacuumed) and creates a neater hole.
    While many engineers want over-size footings, our footings are all 6' and 8' oc, which significantly reduces both dead and live loads.
    We will bore 12" holes for sonotubes, 24" deep, on undisturbed soil.
    We have a soil load bearing report from our next door neighbor which meets our engineer's requirements.
    Rebar will be added to each footing.
    2/3 of the deck is less than 5' above grade, 1/3 is 2' above grade.
    Using the existing 3-4" patio concrete as a footing, was not an option because there is no guarantee that the soil under the pad was properly compacted, prepared and/or has not settled leaving a void between the concrete and soil – thus increasing the risks of cracking/failure under post load.

    Many thanks for everyone's input.
    George

  11. #11
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    Default Re: Deck Footings

    Thanks for the update.

    I'd be curious to hear more about the boring process- what size rig, cost, etc.
    “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” - Upton Sinclair

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