In practice if the installation is to a good standard it works so close to 100% it doesn't matter. It must be quite a few years since my Freesat was affected by rain, and then it was only for a few minutes. We marvelled at the phenomenon, such was its rarity. Then, as I remember, water came in at the top of the kitchen window. The next day I found a crack in the brickwork just above the window.
Bill