You can shingle a 3/12 pitch roof, but special application measures should be taken to prevent leakages at some point down the line. Some kind of protective, waterproof membrane should be put down underneath the shingles, as well as using shingles that are recommended by the manufacturer for that slope. You can find this information on the shingle packaging. This issue with these low sloped roofs comes from the flow of water not being quick enough to prevent water from flowing under the shingle and into your ceiling. Read more