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Think of it as a 1" drop per four feet of run. For guesstimates, there are six "four-feet runs" in 25' so make it a six inch drop. For precision, add the 1/4" for the extra foot (25 - 6*4 = 1) for a precise answer of 6-1/4"
1/4" per foot is basically flat. That's also, as I recall, the minimum slope for drain pipes making it the bare minimum angle to encourage the flow of water.
I assume you are not using shingles since the min rise to run for shingles is 3:12 (3 inches per foot).
What is the roofing material?
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