Go to St. Mere Elise and out to the cemetery.
Look back at the church and turn slightly to your left, you should see some pillars.
Jump on the one furtherest to the left. Then turn left and you should see a ledge. Jump onto it.
Then turn to your left again and jump on that ledge.
Turn to your right and then jump up as high as you can.
Throw a grenade asgianst a wall and count to 5 or 6 seconds and jump.
It should blast you up high enough so that you are on the roof.