As is well known, while the consumption of bread on Sukkos must take place in a sukkah in most circumstances, significant rainfall is an exception to the rule. If there is significant rainfall, it is permitted to eat bread outside of the sukkah. At what point, however, is rainfall considered “significant?”
The mishna in Sukkah (2:9) permits eating outside a sukkah when the rainfall is strong enough to ruin a porridge-like substance. The Shulchan Aruch seems to agree. Based on this, the criterion for determining whether rainfall is a legitimate exemption from eating in the sukkah seems to be the quantity of rain.
A sukkah covered 51% by schach (“schach coverage”) is fit for use.* While 100% schach coverage renders a sukkah invalid, the schach coverage can get close to that figure. I feel quite certain that schach coverage well north of 95% would still render the sukkah kosher.
What results is quite interesting. You can have two neighbors, one with a schach coverage of 51%, the other with a schach coverage of 96%. Both have kosher sukkahs. Yet when a light rainfall occurs, one of them must remain in the sukkah while the other is permitted to eat inside his house