Quoth hwbuckley :
What is the best way to keep pidgeons from landing on my roof?

Line the entire thing with concertina wire. Should get rid of the problem.
;-)

In all sincerity, I've seen many buildings in cities that have broken glass,
metal spikes, or some such embedded in their concrete window sills (and all
other convenient roosting points). I don't know how feasible such a system
would be for a residential roof, but it is a simple, no-hassle solution.

-D.