SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Doesn't sound like anything I've read, especially if you are using poly. The only "rule" I can think of to follow is that you don't want to install sprinkler heads directly into the lateral line with risers because if something ever runs over the head, you could bust the lateral line. To protect your lateral line, you want to tee off with either swing joints, funny pipe, or something similer to minimize damage to the lateral lines.

But other wise, the only reason I could think that you would need to tee off of 1" to 3/4" is because (when working with PVC) the most common fittings you will find will go from 1" slip to 3/4" threaded and 3/4" slip to 1/2" threaded. While they exist, 1" slip to 1/2" threaded is less common.

The thing I did was to forgo special slip x slip x threaded fittings and just used standard Tees and elbows, building my own threaded fitting using slip to threaded fitting inside the 1" fitting. That way, I could custom build each point a sprinkler tied into the lateral line.