seeking advice on leak: chimney chase

Built a house 5 years ago. The chimney chase was framed up with OSB, then had a facade of Centurian Stone applied. This is cast concrete pieces shaped and colored to look like natural stone, with mortar between all pieces.
When the chase was complete, but before the facade was applied, it leaked like hell. The idiot homeowner (that's me) asked the idiot builder, "whiskey, tango, foxtrott!??" Builder replied that fake stone facade would be the waterproofing. Fake stone facade was appplied during midst of summer, and as luck would have it, during a drought. The rainy winter that followed taught me otherwise.
Manufacturer of Cent. Stone expressed opinion similar to my own of builder's competence when asked about his claim of waterproof nature of facade.
Anyway, still have an intermittent leak during the wettest of weather. Presuming that the facade becomes saturated and passes water to OSB underneath, can you recomend a sealant that would prevent this. Such as a silox sealant?
Thanks
Tim
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riverkeeper wrote:

/snip/
Are you sure that the leak is coming through the OSB and not at the base of the chimney where the flashing is? I would think OSB by itself would be pretty watertight (at least, until it decomposes and falls down...).
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Travis wrote:>

The OP didn't say anything about chimney flashing... Get a good roofer to check it out. Tom Work at your leisure!
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Take a close look at the whole chimney, flashing and failing mortar joints come to mind. A thin break in mortar joints could be the problem., so thin as only a knife point will enter but will apear solid and sealed. He may have messed up the motar job and grinding and tuckpointing is the way to go. Sealers can do more harm as they could affect future mortar bond and actualy trap moisture in the wrong area causing big freeze failures. You best way is the old fashioned way, Good mortar and proper flashing.
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Probably needed a cricket at the connection with the roof. http://home.pacifier.com/~tomleach/Cricketpage.html

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In this situation, the chimney chase is attached to the gable end and is outside of the main house roof line. It does pass through a porch roof. The house has a wrap-around porch.
The leak is through the porch roof at one corner where the chase passes.
Thanks, for the input.
Tim
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