In article 5iEyj.3173$6R.2699@trnddc04, (gntry)
says...

what would be the draw backs to using one full piece of 12" flashing folded
to 6" instead of seperate step flashing when installing new roofing shingles
on an addition where it butts up against an existing vertical wall of the
house? The flashing will be behind siding.

Just appearance. Some people don't like the looks of the flashing,
which would be exposed on top of the shingles. You can't put the
shingles on top of the flashing,because any water running down the wall
would end up under the shingles. Step flashing avoids this problem.

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