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I am building a 20' x 20' foot deck with a 10' x 12' Pavilion in one corner, away from the house.I need the concrete footings for the pavilion to be flush or just above the surface of the deck. There are four 8"x8"x8' posts for the pavilion, (it was a package deal), so each of the concrete footings will be a different height above grade due to yard slope. Three of the posts will have a footing of 20 inches or less above grade, but that fourth one will need to be 34 1/2 inches. Will a 12"x12" Footing at 3 feet deep and 34 1/2 inches above ground be sufficient? The wood, connectors, and roofing for the pavilion weigh in at 2300lbs. This is my wife's dream deck for sitting back and people watching but I want it to be safe. we are in the KC metro area in Missouri. Any help will be appreciated. Steve

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  • Maybe this is something that a lay person could safely and properly construct given well prepared plans. Properly accounting for wind snow and seismic loads as well as soil and site conditions and existing construction make this something beyond the scope of normal homeowner design. You probably need a permit anyway.
    – user23752
    Apr 22, 2015 at 4:28
  • Is buying a longer post out of the question?
    – Steven
    May 21, 2015 at 23:50

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If properly reinforced and if the actual footing (which the 12" column isn't, unless you have no flare or flat pad on the bottom, which you should reconsider unless you have stunning bearing) is of adequate size to support both the imposed load and the weight of the longer column, it should not be a problem.

But it's difficult to accurately assess such things from afar, or to be certain that it will be safe in your particular soils and configuration. A few hours of a civil engineer's time to establish exactly what would be safe is probably money well spent on peace of mind.

This article might be inormative even if it's not all that DIY oriented. The sidebar suggests that the minimum cover (concrete between surface and steel) should be 3" on the bottom (earth) side of the footing, and 1-1/2 inches on the sides of the column, assuming you are using #4 bar which is pretty typical for small projects. So you might use 6 #4 bars in a 8-9" circle, bent outward on the bottom and tied together every 8-12" vertically. If you really want to be the oddball overkill homeowner you'll figure out a way to bend a bar into a spiral to tie the vertical bars together, just like the big boys do. The rest of us normally compromise with wire, other than the ones that hope fibered concrete is all the reinforcing they need.

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  • Sorry, I knew I would forget something-I am using readi-base on each corner. 8 inches deep and 24 inch diameter. I am currently trying to find DIY articles on rebar "how to"
    – Steve Harris
    Apr 21, 2015 at 20:19
  • Without fussing too much about the exact volume of the redi-base, the column looks like 707 lbs of concrete (at 150 lbs/cubic foot) and perhaps another 300 or so for the base - plus the load from above (600 for optimists, 1200 for pessimists) spread over 452 square inches for a very comfortable loading of 5 PSI or so.
    – Ecnerwal
    Apr 21, 2015 at 21:41
  • Even a pretty massive snow load (neglected above) would still leave you in very comfortable soil loading range. "clay, sandy clay, silty clay and clayey silt" is figured at 2000 lbs/sq ft, so your 24" base would hold 6278 lbs even if your soil is that bad, and you have 4 of them. Better soils will bear more.
    – Ecnerwal
    Apr 21, 2015 at 21:59

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