Can you do a water test over just the waterproof membrane, prior to installing shingles?
There is no reason to do a “water test” on underlayment for a shingle roof. The underlayment alone for shingles is not a water proof system. And when shingles are installed about 400 nails per 100 sq. ft. of roof area will be driven in the underlayment. On a shingle roof the shingle alone is what sheds the water, the underlayment is installed as part of the system mostly to serve as a buffer between the shingle and sheathing.
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I think there is no need to test the waterproof capability before installing shingles. The shingles normally have water-resistant property