Been there. My problem wasn't rain, but melting snow. On a sunny day in winters, the snow on the roof would melt, drip onto the plane through holes the roof and freeze...frozen water droplets all over the wing which had to be removed before flight until I started covering the wings. Additionally, as it dripped off the roof in front of the hangar, it would create a large puddle in the hangar, which would freeze, as well as in the low spot where the door closed, and freeze the hangar door shut. It's a 70 year-old T-hangar that was originally dirt floor and taxiways, then asphalted over and over again (poorly) through the decades with resultant increasingly poor drainage. It's a block of 20 T-hangars and the airport didn't (still doesn't) want to undertake the huge project of fixing it. Good news...I ultimately decided to bite the bullet and just upgraded to the newest hangar block, and all of those problems were solved. Bad news...the new hangar cost me about $25/month more in rent.