I have a 100 year old porch floor, under a roof, but rain does blow on to the floor. Over the years, the tongue and groove boards have shrunk, leaving gaps of 1/16 to 3/16 of an inch between the surface of the boards. There is a living space underneath the floor, so it needs to be rain- tight. There is no tarpaper or other barrier underneath the boards. I am seeking suggestions on how to seal those gaps, prior to priming and repainting.
Bigstretch Caulk would be my first though. Nothing is going to be the idea fix unless you pull the board up and do the whole thing the right way. Bigstrech is designed to stretch with movement up to 2". It should hold as well as anything for a down and dirty fix.
Even a tight T&G floor will leak, it is not watertight. There must be something under that floor that is working, or you would see signs of leaks. Felt definitely would not be my first choice. A rubber membrane under the floor would be.