Thanks in advance for any help. I'm building a house in South Louisiana with a 6/12 gable roof that changes to a 3/12 roof for about 10 ft. to cover a porch in front. Are there any special details to handle the change in pitch when using 3 tab shingles? Metal flashing? Doubling up on the felt? Roofing membrane? Should I lay out the courses to end or begin at the intersection of the two pitches? I'm doing the roof myself and haven't talked to any roofers.
Before you install the shingles install a bituthene underlayment (Ice & Water Shield, Winterguard are two brand names) over the entire 3:12 pitch and extend it up the 6:12 pitch at least 18". Then install your shingles. It will make a nicer job if you tweak the exposure on the lower roof so that the first 6:12 shingle leaves a perfect exposure on the last 3:12 pitch shingle. If you don't want to do the math on a calculator, try this trick. My explanation assumes a 5" exposure. 1. Measure the distance along the 3:12 pitch. Let's say the distance is 11' exactly. 2. Since 11' doesn't divide evenly by 5" round up to the next number that does (125"). 3. Hook the tape on the bottom of the roof and pull on enough of a diagonal so that 125" aligns with the transition point. 4. Now mark off every 5" on the tape.
Also, Certainteed has a great site full of instructions.