"m35603" wrote in message
oups.com...
I have noticed something very weird about my neighbor's roof: there is
a large rectangular area on her roof that is wet most of the time. I
took pictures of it and showed it to her, but neither me nor her can
figure out what could be causing it. If anyone here knows anything
about roofing, please take a look at these pictures and see if you can
figure out what the problem might be.

The neighbor's bathroom is pretty much right there were this wet spot
is. Her ceiling in this bathroom is vaulted. We were thinking it could
be condensation, but if it in fact is, will this constant moisture not
damage the roof over time?
=
Here's my theory.
The shadows indicate that it is on the West or Northwest side of the house.
A cold roof like the surface of bridges will develop frost before a warm
roof. This area of the roof, being over a cathedral ceiling hopefully
would be well insulated from the warmth of the bathroom below. Thus the
roofing material would get cold at night and condensate moisture out of the
air as the air warmed up...like your glasses steam up when you walk into a
warm room from outside. The rest of the roof is exposed to an attic which
should be better vented and would allow the roof to warm up from below as
the outside temp rose. It should be the same temp below and above and
might not condense water out of the atmosphere while the wet section would
not warm up until the Sun got to it. Here in Northern Illinois, roofs on
the North side of a house usually show frost in the morning, until the Sun
gets high in the sky and warms the North exposure. I would suspect the
permanent dark lines are possibly due to mildew growing in the moisture.

Tom G.