Expert: Bruce E. Johnson - 3/19/2009

Question
QUESTION: Hello,
I am having a concrete patio made (23ft by 12ft) attached to my backyard. I want to later add a wood and shingle cover held up by posts (4x4 or 6x6 not sure yet). My contractor wants to use #3 rebar to make 5 piers-2ft wide by 3ft deep under the patio to support the cover. I was told be someone else that they should use #5 rebar which one is correct?

ANSWER: Hi Cheryle, #5 would be my choice, however if he uses enough #3 it would be okay I suppose.  #5's in a grid of 6" apart each direction or #3's, 3" apart each direction would theoretically accomplish the same thing. I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: I am comparing this contractor with another that tells me that instead of making the piers they would make a 12in wide by 18in deep beam all along the outer edge of the patio using a #5 rebar for the same patio size (23ft by 12ft). Which method would you use piers or the beam? Also, which posts size would you use? 4x4 or 6x6?

ANSWER: Hi again Cheryle, if your soil conditions are loose and you are building on a sloping piece of property where a shifting foundation is a concern, then the piers in the ground are required, however I would then top those piers with a continuous footing at lease a foot thick and a foot wide.  This gives you the best of both worlds, piers/pilings for lateral movement and a footing for bearing capacity. A thickened edge alone can be used if the soil conditions are stable and good compaction of the fill material and footings prior to pouring concrete. Of course the continous rebar is a given. As far as posts go, 4x4's are capable of holding a lot of weight but if they are too tall they can begin to bow out and eventually fail.. I say 4x4 up to 8' post length, 6x6 up to 12' post length, 6x6 with knee bracing, up to 18 foot post length...I hope this information helps, feel free to write again regarding this or other matters, sincerely, bruce e johnson..bejohnsonconsulting.com

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QUESTION: What exactly is a footing? Is it a boxed in area of concrete that is a part of the patio(that you can see on the top part of the patio) or is it the concrete underneath the 4" concrete patio? The soil is stable here and there is no slope.

Answer
Hi again Cheryle, yes the footing is the bottom, supporting part of the foundation. In a monolithic slab the footing would be directly below the slab thickness around the perimeter.  If the soil is stable then you really don't need the pilings/ piers.  A monolithic slab foundation will work just fine.  I hope this information helps, feel free to write again regarding this or other matters, sincerely, bruce e johnson..bejohnsonconsulting.com