I have a covered patio area attached to my house, it is approximately 28ft long x 14 feet wide, and slopes in the short direction. This roof meets the house about 2ft below the eaves of the main house roof, and is 3-tab asphalt shingles over plywood (assuming 1/2"). The roof framing is 2x6, 16" OC. Due to a lack of general maintenance, plus the age of this roofing, there are numerous leaks all over. The pitch of the roof is pretty shallow, it only drops about 2 to 2.5ft over 14ft slope.
As a patio, this doesn't really matter, but I'd like to use the area as a workshop and don't want anything to get leaked on. Because it's difficult to figure out where the leaks are, and I don't want to find more leaks "the hard way", I'm looking for a repair solution that I can apply to the entire roof surface that will last for 2 to 4 years until I can replace the workshop with a larger / separate structure.
I'm not a roofer by any means, but I'm a competent DIYer so I'd entertain anything that doesn't require me to buy a bunch of additional tools.