I am doing the design of a deck footing and in many places where I read it says that the footing (sonotube) should be set with the top end terminated 2" above the ground level
In my case I am building on top of a patio and I need to understand why 2" and if those 2" must be above soil level (which is way below the three layers -gravel, crushed stone and bricks-that build my patio) or just above the patio level
Please have a look at the diagram below and tell me which case is recommended and why
Update:The recommendation regarding the 2" comes from this book step 5 on that page
Best Answer
"C" is the way to go - it has cleanest look. The point of raising the concrete is to prevent the end grain from wicking moisture and rotting - the concrete is NOT going to deteriorate (unless its mixed poorly and not allowed to cure correctly). Generally, you don't want wood engaging the ground, nor do you want it sitting directly on concrete. We separate sill plates all around building foundation walls with foam because the sills will wick from the concrete and rot. Raising the concrete is not the only way to solve the problem... take preventative steps to limit/reduce/stop water from wicking. Raise the bottom of the wood so it doesn't sit directly on the concrete. https://www.strongtie.com/capsandbases_woodconnectors/category
We put fences in the ground all the time. They don't rot (because preventative measures are taken), or do so over an extended period of time. We don't want the column rotting, but we aren't sticking it in the ground either. Your detail shows it sitting flush with crushed gravel. I would dip the end in post preservative or better yet coat it with tar. I'd use a simpson product to keep it off of the concrete (I like the stand off post bases), but keep it below the paver.