Question
thanks for the service.  i live in Pittsburg, Ca.  Soil is heavy clay.  
i have a 12X20 ft section that i want to plant a lawn.  too small
to til, plus there's irrigation tubes under.  the soil is 4" below the
patio level.  can i just dig out 3" of clay.  put in 6" of some
additive.  put sod on top an have a great lawn for years?  do i
need to til if i have 6" of top soil - whatever is recommended for
the area?  should i dig deeper?  if this is ok, what type of mixture
do your recommend for a top soil?  thanks.

Answer
Gary,

What you propose is possible.

The deeper the topsoil you bring in, the fewer problems you'll have with the grass, but like you say, it's such a small area, even with shallow topsoil, the problems are manageable.

I have an area on my grounds that approximates your situation ... the baseball infield.  Every year we have to switch the field over to soccer use by sodding the 9000 sq ft of infield dirt.  That stuff is so compacted that I can barely till it...only down an inch or less.  I lay sod on top of that, and I can maintain it, but it takes about twice as much water and twice as much fertilizer as the grass on normal soil.

It takes more water, because the little bit of soil that sod brings has no place to store water so dries out fast.  (Any soil you add past an inch will only improve this).  More fertilizer for the same reason...1 inch of soil just doesn't have much nutrients, so fertilizer provides what the grass needs.

When this sod over compact soil gets watered, the drainage is bad.  The water quickly saturates the grass, and then sheets off the underlying hard surface, creating a big soggy spot where the infield ends.  Make sure your lawn area, before you add soil and sod, tips slightly away from anything you don't want wet.

Get loam topsoil if you can.  Or even a soil / compost  mixture.

For what it's worth,
Mark in Portland