Q. Can I open a Suka cover after it rained, if the water on the cover falls on the grass or falls on the patio stones and then on the grass? Does it matter if I take off the cover immediately while there is still rainwater on the grass?

A. Poskim rule that removing the cover of a suka is permitted after the rain even if the water falls directly on the grass, when the lawn is so wet that the additional water will not be beneficial to it’s growth and one does not have any intention in watering the grass. (Halachos of Shabbos p. 59 notes, quoting Igrois Moishe). They also permit when the water does not fall directly onto the grass or plants, but rather as a secondary act into patio stones or pavement, even if the water then streams into the lawn.
Horav Shlomo Miller’s Shlit”a opinion is that in case of need, the same may apply even when the rain water on the grass has dried, since at this time of the year, in northern latitudes people are not that much concerned with watering the lawn that has recently been rained on.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a