When you say pork roast are you talking about a pork butt cut of meat?
I have two favorite ways:
1. Place a 9 x 13 pan with apple juice on bottom rack and place salted, peppered, and oiled roast right on rack right above it. Cook at 225 F. until done - this could take about 8 hours, depending on the size of the roast. I got one from Sam's one time and it was huge! It took 20 hours
Luckily I didn't actually needed it until the next day but I was hoping to use part of it for appys the night before...oh well...we really didn't need it anyway.
2. Oil, salt, pepper, wrap in aluminum foil, place in shallow pan - 350 for about 6 hours usually does it.
Either way you want the meat to come to an internal temp of at least 190 - 200 is even better but not 100% necessary. When it reaches the 200 degree mark, especially using the #1 choice above, you'll think you are eating pork-flavored butter
It pulls beautifully into hunks or "strings".