Porch foundation options
I'm planning to add a screened porch to the rear of our home, and am wondering about the best type of foundation to use. The porch will be 12' x 15' with a gabled roof.
Right now there is a concrete patio in the area where the porch would sit, but I believe it's just a thin slab with no footers to support a structure. The issue is that the finished floor of the house is only about a foot above grade, and the patio area has little or no slope. The sliding door that opens to the patio has a 6" high, 4'6" square concrete step that splits the difference between the height of the interior floor and the patio slab.
My first thought was a concrete slab foundation, poured with integral footers along the edge, that would also act as a concrete floor. However, to bring the floor of the porch up to the same level as the finished interior floor would require 12" of concrete above grade (and presumably would need to extend below grade?), which, along with footers extending below the frostline (we're in New England) seems like a lot of concrete and an expensive pour.
However, with the floor of the house so close to grade, I can't picture how a post-and-pier foundation would work. I've read that concrete piers should extend 4-6" above grade to avoid rot and insects, which doesn't seem to allow enough room for a beam and joists (let alone any kind of posts!).
It seems like this must be a pretty common issue, but all the examples I can find either have the floor of the house much closer to grade and use a slab, or much further from grade and use post/pier.