Wind-effect on Rain

The roof catchment area needs to be adjusted because rain is usually accompanied by wind, which can increase the catchment area. There are various formulae and slope factors that can be used to determine the specific wind drift effect for each building, some of which are shown in AS/NZS 3500.
The formulae quoted in this C.O.P. assumes the worst scenario and provides a conservative answer because shielding would decrease the catchment area when the wind is in the opposite direction,

The sloped roof catchment area for all sloped roofs which has a pitch of more than 10° and is freely exposed to the wind, must be increased by 10% to allow for the wind drift effect.