Actually are getting into a drought again. Yesterday before the storms southern Maine stream gauges were mostly at all time low flows for the date.
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Washout is likely actually washout from the trucks. Leftover cement aggregate from truck cleaning that is a mix of various sizes from pea gravel up. Since its mixed and they probably don't have a screen at the plant they can't reuse it in new batches and have to get rid of it hence the cheap...
Whatever you do carefully price it out. Trusses and sheathing are cheap and if you need to reroof it anyway...
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