I have heard of people doing this… but it is not a good idea. It may seem like an easy solution, but here are the problems you create.
Unlevel lawn: You want a firm, level surface to lay your new sod over. It may start out level, but as the old grass breaks down it will turn out very uneven.
Weeds: Your old grass is not dead and it will work to grow up through your sod and produce a weedy looking lawn.
Harder to root: You want your sod to root very quickly. You want to place it against firm level ground and give it the shortest possible distance for the roots to grow. By laying the sod over an existing lawn, the roots have a much longer distance they need to grow.
If you want a top quality job, you can’t take the easy short-cut. Take up the old sod and rototil your soil and prepare it correctly. More on installing sod.
I went over this website and I think you have a lot of good info, saved to favorites (:.
Last summer my neighbor had some exta fescue sod that wasn’t needed for there new lawn that had been, tilled, layer with dirt and finally sod added. I had recently spayed my yard to get rid of a yard 50 percent weeds ( two weeks prior) so I had a lot of bare spots. I took the extra sod, about 300 sq ft and laid it directly over my current grass (side of home) even used small remnants around tree roots that I thought would never take. I watered the sod daily for two weeks, went to watering every other day, then every three days. Laid down fertilizer for new sod and The result is my yard is beautiful! I ended up purchasing an additional 1,000 sq foot and did the same to my back yard. I like to note I am a female, that has never ever done yard work. I have the best looking yard in the neighborhood, golf course, soccer field, baseball field these are references to what my yard looks like.