Roof "dry rot"?

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?
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In alt.home.repair, on Wed, 19 Aug 2015 12:49:45 -0700, Don Y

What's the top layer of the roof?

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

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

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On 8/19/2015 3:28 PM, micky wrote:

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

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.

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!)

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!)

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On 8/19/15 3:49 PM, Don Y wrote:

Me too! (not a sneaky devil, wouldn't want that made public)

Dry rot progresses whenever the moisture content of the wood, related to relative humidity in the air, is above a certain point. If it's dry rot, solar warming can reduce the moisture content enough to halt the process. Drying it further will kill the fungus, but it could return if the wood gets wet.
Where I find rot or see that it might start, I use a garden sprayer to apply a mix of sodium borate, glycol, and water. Currently, Bora-Care and Shell-Guard are two such products, if you don't want to cook it in your kitchen.
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On 8/19/2015 8:00 PM, J Burns wrote:

Ha! It never occurred to me to consider what the actual "cause" was; thanks, I've learned something today! :>

So, I guess the first step is to get rid of the current damage (and figure out its cause); then, watch to see if it wants to return (perhaps there is some aspect of the roof in this area that leads to more frequent "failure")
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On 8/21/15 3:01 PM, Don Y wrote:

If the current damage is tolerable, you can keep it from progressing with a borate spray. The glycol helps it penetrate wood deeply. It will kill fungus and bugs and keep them from reappearing, as if you'd built with pressure-treated wood.
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On 8/21/2015 4:20 PM, J Burns wrote:

Even if the wood surface is painted? I.e., does the solution permeate the paint *or* do I have to rely on it gaining access via the "rotted" portions?

Thanks! I think I have to open things up to see what's really going on, there. It's a small portion -- but exactly coincides with my first footfall when leaving the ladder. I.e., it could be that mechanical stress is what has weakened the roof *seal* in that area (despite the fact that no cracks or tears are visibly evident).
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On 8/22/15 5:24 AM, Don Y wrote:

In preparing to repaint a shed, I sprayed exposed edges where there were signs that rot might be starting. I don't remember how many days I waited to paint, but the paint stuck.
When mixing, more water means less penetration. The standard is 1:1, but the instructions says you can use 2 parts water if the wood isn't over 2" thick. So two coats of 1:1 should penetrate more than 2" beyond where you can spray.
The glycol (ethylene or propylene, the same as antifreeze) is intended to draw the borate into the wood. I've read that the borate won't go through paint, but the glycol will. I've read that if you spray a painted surface with antifreeze, it will go through the paint and give the underlying wood some protection. Not having tried it, I don't know if it could loosen paint.
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On 8/22/2015 8:42 PM, J Burns wrote:

OK. Did the painted surface *prior* to your adding that new coat (of paint) show visible signs of having been sprayed? I.e., if I opt to spray it, would I then have to paint it afterwards?

So, for plywood underlayment, this is a lead pipe cinch! (?)

<frown> I think the best course of action is for me to remove that portion of the underlayment and replace it (then paint it and repair the roof covering).
Then, I can examine the damaged piece more thoroughly -- to see how far the damage had progressed beyond what was visible. And, spray it to see how the paint behaves -- as well as cut the treated board open to see how well the stuff penetrated.
I.e., learn specifics in case I encounter something similar in the future.
Thanks!
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On 8/24/15 1:20 AM, Don Y wrote:

I don't recall. That's the only time I've sprayed paint, and it was just at the edges in certain spots. If I'd seen and worried about visible borate, I guess I could have wiped it off with a wet cloth.

It should soak in a long way.

AFAIK, antifreeze doesn't damage house paint, but I don't know. If you replace the damaged underlayment, you won't step through the rotted place one day!
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On 8/24/2015 5:05 AM, J Burns wrote:

It's unlikely that I ever would "fall through". The spot is located on the edge of the roof (so there's at least a ceiling joist adjacent). And, the roof "covering" is probably an inch thick. All of that would have to tear for my foot to go through.
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On 8/22/2015 8:42 PM, J Burns wrote:

On a related note...
We have "decorative" 4x10 (or 12?) lintels on the exterior of the house. Over the years, these have also failed (moisture, etc.) Of course, as they are purely cosmetic, you'd never know it unless you went poking around!
Rather than using a composite to replace them (expensive!), the cheap fix is just buy more lumber, paint it and hang it.
Would this sort of "treatment" be as good as (better?) than buying pressure treated lumber? Consider that this *is* a cosmetic issue and if it takes another 30 years for the replacements to deteriorate...
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On 8/24/15 1:30 AM, Don Y wrote:

I faced a similar question seven years ago when I replaced my cellar hatch (a door sloping to the ground at 45 degrees) because the pressure-treated lumber had failed.
I had scrap pallet boards, scrap shingles, and leftover borate, so I built it that way as an experiment. The bottom of the door is submerged in rainy weather, and borate is supposed to leach out from long submersion.
Since then, I have once sprayed a little more borate on the bottom ends of the boards when I happened to have it in the sprayer for another job. I haven't seen any rot.
Borate would probably protect lintels from rot, but if it's plywood, maybe the glue failed.
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On 8/24/2015 5:37 AM, J Burns wrote:

OK. Growing up, we had a "hatchway" but it was a shallower angle (~30) and made of metal on a poured extension of the foundation. I.e., the stairs beneath the hatch existed in a portion of the "basement" that protruded beyond the edge of the house.

Cool!

They are painted 4x12's. There's no way for water and "yard waste" to clear out from on top of the lintel so it just sits there and rots the wood. (we have a similar problem with pine needles accumulating on the roof, holding water and eroding the roofing)
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On 8/25/15 10:26 PM, Don Y wrote:

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On 8/26/2015 10:58 AM, J Burns wrote:

At least you weren't thinking CUPOLA! :>
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wrote:

The cause of the leak is obvious. You used your smoking gun and shot a bullet hole in it.
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