I recently purchased a 20+ year old house with a brick front porch and vinyl siding. Shortly after moving in we had a period of heavy rain and I noticed a small damp spot in the wall of the basement. After doing some homework I found that water is running around the door, behind the siding, behind the brick front porch and into the sheathing of the 2' high wall behind the porch. It appears that when the house was built they did not terminate the siding correctly around the bottom of the door or around the porch. There is no j-channel around the bottom of the door or porch and the siding is just jammed in behind the porch.
I'm confident this sloppy work is why I have a leak but I have no idea how I'm going to fix it. I can't access anything easily behind the porch and replacing the water damaged sheathing is going to be a challenge. Since I don't have good access, the only thing I can think of to fix the problem is to run a large piece of flashing behind the porch and up to the bottom of the door, and a few pieces of flashing about a foot or so up the sides of the door to where the channel for the siding ends at the bottom of the door. My reasoning is that the water will still run behind the siding, but it will flow along the flashing and out to the ground without getting to the sheathing.
Is this idea sound or is there a better way to attack this? BTW, I'm attaching a very crude diagram to explain the issue a little better.