Chimney-house gap; siding repair?

We have a problem that seems to be perplexing to several contractors in our area. Any help would be greatly appreciated.
As briefly as possible, we have developed a gap between the house (25 year old aluminum siding) and the chimney ranging from the top of the house at the gable to 4 ft. down. At its widest, it's approximately 2-3 inches and narrows as it moves down the house. It's hidden behind a tree that hugs the house and went unnoticed for a period of time.
Our first move was to have the chimney checked. While the flashing does appear to be pulling somewhat from the roof, the chimney person/mason put a long level on the chimney and contends that it is straight. We have also consulted with at least one general contractor and other siding contractors. They have raised the possibility that the gable has moved inward somewhat away from the chimney, and that, at a minimum, the sheathing has "disintegrated" in that area from rain and elements. The siding does have "give" a bit in the area when poked, although there is no sign of leaks/weather damage on the interior of the house.
In general, we are being encouraged to replace the siding, which could use it anyway (we would pretty much have to switch to vinyl), with the ultimate determination as to the problem waiting until the siding has been removed. While I can see the logic to this, I'm uncomfortable with the "open-ended" nature of the fix (not knowing what the "real" problem is) and wonder if some of this work potentially is beyond the expertise of those who do siding ("warping" of wood?). Thanks much in advance for any thoughts as to what this might involve and potential pitfalls.
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Get a price or be at the job, openended is right. Ck your foundation its settled or shifted, it has probably stopped but maybe not. You will want measuremets set into the chimney with the house you ck every 6 mo.
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In my not so vast experience, you're very likely to find rotted/damaged sheathing behind the chimney. What happened to us was an area about ten feet wide up to the peak had to be resheathed, a few sisters placed on top of a new piece of footer that was added at the bottom, and new vapor barrier, insulation, etc. installed. Rain and winter weather had taken its toll. The job was pretty simply actually, and relatively straight forward to accomplish, and all done from out doors. It wasn't necessary to disturb anything inside even though the insulation was even pulled out and replaced. It stunk the high heavens, too! It was left open for about three weeks to dry everything and then closed back up and resided. Wasn't terribly expensive, but not cheap either since it happened to two out of three chimneys. I guess where it could get REALLY expensive is when sillplates, whatever they call it at the second floor level have to be worked on. Ours were OK: somehow they were protected & were "fine" for another hundred years, as one guy put it. I am mildly surprised that contractors won't tell you approximately whaty THEY think it'll cost, based on a straight forward job, of varying degrees. e.g. open wall, no problem, close, open, remove, replace, etc.. My guys all gave me four different scenarios - and their pricing was interestingly close, enough so that I snooped during the job to see just how well they knew each other. They didn't, near's I could tell, so ... who knows? I don't pretend to know how they knew what to do where, other than what was visibly rotted or still ater soaked, but I did think everything I watched them do made sense. It's interesting to watch your house wall come apart like that without the whole place collapsing! <g> Gave me a whole new meaning for "bearing wall"!
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Can't see your house from here, but it sounds like you are getting good advice so far. If a competent mason or engineer has signed off that the chimney stack is solid and vertical, the only conclusion is that part of the house has moved. Not suprising that they are reluctant to quote a repair cost before wall is opened up. You never know what the 'as built' framing/sheathing/weather seal is like, and you never know how far the rot has spread. But the longer you wait, the more it will cost. Just for laughs, check your household insurance- part may be covered as weather damage. (The framing, not the siding.) Get the contractors to run estimates based on worst-case scenarios, of demo and replace of all wood on house that appears to have moved. Hopefully, it won't be that bad once they open it up.
One last thought- A 'long level', typically about 4' or so, may not be long enough to show problems with chimney stack. If you have a ladder long enough, try using a string and plumb bob on all 4 corners and in the middle of each face. Hang it off a notched wood block so the string is about an inch out from the chimney. Any waves or lean will jump right out. Wait for a calm day, and be patient till the bob stops moving by itself.
aem sends....
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