ok if u spawn at the top end (SnD)
head north west of where u spawned u will see a little room half blocked of with an iron fence
(u will need a friend to help u with this)
1 person prone in the corner facing the fence
the second person will jump on the proned players back then the prone player will stand up and u jump till you get ontop of the fence. --> from here you can run to the left and jump ontop of the building--> now u are on the rooF
hint: dont fall inside the house, u wont be able to get out :S