Hi,

We have a "built up" (I think that is the term for it -- technically,
it has a "membrane" but I suspect it is closest to "built up"...
layers of roofing felt, flat, etc.) roof.

Above the back patio, there is no "ceiling" so the underlayment (plywood)
for the roof is visible. It is obviously rotted in one section though
there are no visible signs of wear from above (except this small portion
of the roof doesn't bear weight as well as the rest of the roof).

[It's a *small* area -- smaller than the size of a shoe!]

First question: I assume this is caused my water getting under the
roofing material (membrane/felt)? I've not discovered any means
by which that could have happened (everything looks "sealed", "intact")
but water is a sneaky devil!

Second question: is this a self-limiting process? Or, will the damage
continue to spread (even though there are no signs of a water path)

Third question: suggested remedy? I figure I could cut out a portion
of the membrane/felt, remove and replace the plywood, then "patch" the
cut out portion of the roof to restore its integrity. But, I assume
I should first have a "smoking gun" that indicates exactly where/how
the damage has been caused in the past?