Roof based collection system in Bangalore, Karnataka - Need horizontal, stainless steel filter for RWH

I am planning to implement a RWH system at home (in Bangalore). The objective is to collect the water from the terrace into a loft tank in the toilet so that it can be used for toilet flushing purpose. In this scenario I would like to know the filter that should be sufficient. The whole system is planned to be on gravity flow (from the roof to the lower level loft within the house).

My main constraint is the vertical clearance between the loft and the roof where the filter has to be setup - so I was looking for some kind of horizontal filter if that is possible. I am also interested in the stainless steel filter - could you let me know where can I more details - availability, dimensions etc

Jagadish S