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RE: Glass Cracking

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You do not say where the house is located--I assume in Arizona.  The time
lag between construction and onset of the problem is a bit baffling.
Without signs of settlement or foundation distress, I would generally
attribute sticking/breaking windows with wood shrinkage, which would
manifest itself within six to nine months (or less in AZ).

Last year a client came to us who had a 17-year-old house with sticking
sliding glass doors.  Glass in one or two of the doors had "exploded"
according to the owner (this would be tempered glass, no chance for a
parabolic crack to form). The house is still under construction (a vacation
home, once to the point of getting enclosed and roofed, urgency to complete
it tapered off), so repairs were easier to accomplish than in a finished
house.  Heavy snow loads and layout changes dictated replacing some headers
with steel beams and glu-lams, which was done last November.

Two weeks ago the client called to say that two of the doors were sticking
again.  They had worked fine until April, and got progressively worse since
then.  Upon visiting the site it became apparent that the problems stem from
quirks related to wood-framed construction and less-than-optimal
construction techniques.  Pond-dried wood nailers used on the steel beam
started trying to crawl back to the forest when they began drying in April
(house not occupied, wood would have remained basically frozen until then).
Joist depths varied by 3/8-inch in 16 inches under one of the doors, making
bottom track very uneven.  One side of a door opening was over a 6x12 header
that had shrunk 1/4-inch or more, while the other side landed over standard
wall framing, so the opening had slowly changed from a rectangle--one
assumes--to a parallelogram.

The worst shrinkage-vs.-window problem I saw was in a log home (also not
finaled after 10-12 years...), where the window openings had shrunk in
height so that the wood window frames were bearing the weight of the roof
and upper log courses.

It could be that your windows were loaded to the point of insipient
cracking, and an impact load (as mentioned by others) set off the crack.
Such "impact" could even include slamming doors, micro-earthquakes, sonic
booms, birds flying into the windows (although probably not for the interior
panes), teenagers' stereos, etc.

I used to do stained-glass work. Glass cracks in strange and unpredictable
ways once the crack is initiated.  I don't think the shape of the crack
would reliably indicate the forces that caused the crack in the first place.

Suggestions:  Pursue the quality of installation and window manufacture.  My
slightly cynical view:  the construction boom in AZ in recent years meant
that anyone who knew which end of a hammer to grasp became a carpenter, and
as long as the paint and carpet looked good, buyers thought they were
getting a great deal.  Properly installing replacement window units with
adequate clearances should certainly solve the problem.  At least get a
manufacturer's rep out to look at them.

Will be interested to hear more.

Thor Matteson,  S.E. (calif.)

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