On 8/19/2015 3:28 PM, micky wrote:
In alt.home.repair, on Wed, 19 Aug 2015 12:49:45 -0700, Don Y
wrote:

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)?

What's the top layer of the roof?

The top *manufactured* layer is a membrane. Of course, there's
many layers of paint on top of that...

I've not discovered any means
by which that could have happened (everything looks "sealed", "intact")
but water is a sneaky devil!

Water was several times investigated by the House Unamerican Activities
Committee, and the majority agreed that it was sneaky.

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)

I don't think it will spread very fast, but you can keep your eyes open.

My concern is for things that I'm *not* seeing. E.g., the interior
of the plywood. Hence the thought that I may want to remove and replace
now before it becomes a bigger problem.

I have shingles and tarpaper, and when my roof was removed, there was a
little damage to several sheets of plywood. He replaced one sheet and
used sheet metal to cover a couple corners I guess so they woudln't be
soft spots underneath the new shingles, because the sheet metal woudln't
keep any damage from spreading. I assume that he thought, and that
it is true, that when it doesn't get wet anymore, t he damage won't
spread.

It is very dry here -- 11 in of precip annually. So, the fact that
there is *any* damage has me wondering where the water infiltration
occured (or is occuring!)

So I'd just wait until the roof has bo be replaced, whatever is on top,
and replace seriously damaged plywood then. And patch with some sheet
metal slightly damaged wood.

Roof is 20 years old (average for replacement/repair around here would be
closer to *7*). I intend on getting another 10 out of it! :

(The "membrane" system is no apparently longer used/available. I credit
it -- plus my vigilance -- with the roof's longevity. Neighbors who have
replaced theirs -- with felt -- have been regularly replacing them ever since!)

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?