Yes you can install a roof top antenna on a town home , here is the Federal Law that over rules , HOA's , housing covenets and other regulatory bodies and any one else that will seek to prevent you for installing and using a antenna.
http://www.fcc.gov/mb/facts/otard.html. This Federal Law has been in effect for 15 years. HOA's are Well Aware of this Federal Law , however they keep there mouth shut about it , so they can scam you in to thinking that you can't install and use a outside roof top antenna. They will try and scam you by saying , well Ok you can have a out side antenna but you have to put it where no one can see it - this is not the truth. Or they will say , well Ok you can have a outside antenna , but it must be very small - this is not the truth. Or they will say , well Ok you can have outside antenna but you must mount it down low - this is not the truth. Or they will make up some other not true rules.