One of the big problems may be when the old turf starts to decomp and
it creates quite a bit of heat. You could be setting yourself up for
burning the roots of the new turf. Not only that, but it will be much
more difficult for the new turf roots to grab hold of the soil and take
on water. If you are going to replace an entire yard of old turf -
whether with seed or sod - the best solution is to kill the existing
turf first, cut it down as low as possible and then if you are going to
sod, till the whole yard. If you do not have an irrigation system or
hoses in place to keep watering the new turf you stand a good chance of
losing the whole thing.

The Lawn and Landscape Guy

http://beautiful-lawn-care.blogspot.com/
http://lawn-irrigation-system.blogspot.com/