All you require is an unshaded roof facing roughly north (any aspect between north west and north east will give good results). Even an Easterly facing roof can work without too much loss of efficiency.

The solar collector should be mostly unshaded by large trees and buildings, especially between the hours of 9 am to 3 pm.

If your roof aspect is very limited, there are always solutions. An offset frame can be installed, or in some cases, it is possible to face part of the system in one direction, part in another.

(Some houses do get too much shade for adequate solar gain. If your house has limited solar access for some reason, then consider installing a hot water system driven by a heat pump technology. These are extremely efficient and will reduce your hot water bills by a similar amount.)