Re: Porch floor leaks into basement...
Remove the decking and put down ½" plywood,Weather Guard/Ice Guard on the plywood, nail down furring strips, leaving path way for runoff, then the decking . Insulate the underside. The weather guard will keep water from entering, the insulation can then do it's job. Weather Guard is a self sticking roofing underlayment to protect against ice dams and it self seals around any nails run through it. If you floor is 3/4" thick and the joists are 16"on center you can use ½"plywood, because you would not be putting any weight directly on it, then ½" X 4" furring strips. Then new decking, that would raise the floor 3/4" If you can't handle that rise, the joists can be rabbited out or strips sistered to the side of the joists to suport the plywood and ½" strips of plywood installed between and even with the top of the joists, the weather guard, and ¼" spacer strips and the decking.
Be sure you live your life, because you are a long time dead.-Scottish Proverb