Question
I am about to sod a section of my yard that was destroyed during a sewer pipe repair.  The job was performed almost a year ago and over the summer a broad spectrum of weeds has taken over.  My question is:  When I till will tilling the weeds and laying the sod have a negative effect on the sod?  Do I need to remove the top growth first?

Answer
If you are talking about weed heights of an inch or two, then no, it is not a problem to till weeds under & sod on-top of the new grade.

IF the weeds are taller (like up to your waist) then the rotting matter in the top-soil under your new sod will create a heavy demand for nitrogen, so the new sod may get yellow or look poor.  This is sometimes inaccurately called chlorosis, which is actually a condition of iron deficiency.

This can be offset by fertilizing with a nitrogen-heavy fertilizer. On top of it all, there is the weed-seed issue.  You'll want to till these weeds under BEFORE they go to seed, or you're just spreading the pain around.

If your weeds are the taller type, you must hack down the top growth before tilling under*(which is what you originally asked).  Some weeds have invasive roots, like crab grass or zoysia, so you MUST kill them first down into the roots, or they will come right back up through you new sod.  The roots ARE the weeds.

Grab a clump and take it to your local garden shop if you don't know what it is, or if you think it is invasive.  If you cannot pull it up, it is a good clue that you have to kill it with an herbicide first!!  

~M