— Lou, Newport News, VA
A: Tom Silva replies: It's always tricky to pinpoint the exact location of a leak. It may appear to originate in one area, but water is sneaky and moves in unexpected paths. So before you jump into this job, try to determine where the leak is coming from. If the porch has a finished ceiling, for example, remove a few boards and check the underside of the roof sheathing for water stains — they may help pinpoint the leak's location. In the case of a low-pitched roof, you may have no alternative but to remove all the roofing just to track down the problem.
Chances are, the leakage is probably due to the flashing, not the roofing material itself. Damaged, corroded, or improperly installed flashing is a common problem at this location (and a lot of other locations, too). If the roofing material is in good condition, you may have to remove an area of siding as well as some of the roofing to replace any damaged flashing. (Generally, the only reason to remove all of the roofing would be if it's nearing the end of its useful life anyway.) A capable roofing contractor should be able to make this repair for you.
Resist the temptation to turn the problem over to a handyman with a bucket of tar. You might save some money in the short run, but slathering roofing tar on the flashing doesn't really fix the problem, and can actually accelerate corrosion by trapping moisture between the tar and the metal.
If it does turn out that the roofing is shot and has to be removed, you should have the flashing replaced at the same time. There's no point trying to protect a new roof with old flashing.