rolled roofing product

Hi, there. I am replacing a very low-pitch roof on a building. I have called a couple of building suppliers and been advised to go with rolled roofing. They both suggested the same product (a rolled roofing material made by a company called Tarco). Where I got confused is that one said it was a 3'x36' roll covering about 108 square feet, and the other said it was the same size roll (3' x 36") covering only about 54 square feet. I understood from the second supplier that the reason was the selvage (?), basically meaning that you have to lap one 3-foot-wide strip over the next 3-foot-wide strip by 18", meaning that you are doubled up, and lessening the possible coverage of each roll.

Now, I am more inclined to believe the advice of the latter supplier, since they are local and were generally more informative, but I also don't want to buy double the product if I don't need to. I have looked at some rolled roofing tutorial online that said you only need to lap the strips by 3-6", not a full 18".

Anyone have any thoughts on this? Thanks!