Why does my porch roof keep leaking when the rest of the roof is fine?
Because it is probably shingled and its pitch is too low for shingle. Shingles shed water; they do not hold it back. Below roughly a 2:12 pitch, water moves slowly enough to work under the laps, and the section needs a membrane system rather than another layer of shingle.
Pitch decides the system, and shingle has a floor
Roof pitch is described as inches of rise per twelve inches of run. Above about 4:12, shingle behaves the way it is meant to. Between 2:12 and 4:12 it needs a doubled underlayment and careful detailing. Below 2:12, it is the wrong product and no amount of care fixes that.
The sections that fall below the line are the ones nobody thinks of as roofs: the porch, the addition, the flat piece over a garage, a dormer cheek. They get shingled because the rest of the house is shingled.
Ponding is what actually kills them
Water that sits after a rain rather than draining is water with time to find a seam. On a low-slope section that usually means the fall is wrong, or a drain or scupper is blocked, or the deck has sagged. A new covering over an unchanged fall will leak again.
So the fix starts with where the water goes, not with what is on top of it.
What replaces the shingle
A membrane or rolled system rated for low slope, with the seams and terminations detailed for standing water, and a proper transition where it ties into the steep-slope roof above it. It will not look like shingle. On most houses the section is not visible from the street anyway.
It is a small job in area and a fiddly one in detail, which is why it is often quoted as if it were part of the main roof. It should be priced on its own.
Related questions
Can I just put another layer of shingle on it?
You can, and it will leak again. The pitch has not changed.
Is this covered by insurance?
If a storm caused it, possibly. If it has been leaking slowly for years, that is a maintenance argument you are unlikely to win.
Can it be done separately from the main roof?
Yes, and it often is. It gets priced separately either way.
