Whether wet or frosted the area in question is cooler than the rest
of the roof. When this section gets cooler than the dewpoint then
either dew or frost will form. The majority of the roof usually never
gets cold enough for frost or dew to form because the majority of the
roof is warmed by the escaping heat from the house.
The area in question does no receive as much escaping heat as the
majority of the roof. This small section is insulated or sealed from
the rest of the house better than the majority of the roof.
Yes, that is what happens. I suggest you take a
course in what happens to roofs by looking around
at your neighborhood roofs after it snows and as
the snow disappears and after frosty nights; note
the aspect (direction of the roof slope). You
might want to note whether there is or is not
frost or condensed water on the windshields of
cars pointed north. Note that water can't pass
through windshields, so whether is water or frost
on the outside windshield surface has nothing to
do with ventilation in the car.
We had a moderate frost here this morning and I looked at a lot of roofs
on the way to work. Nearly every one had a frost pattern similar to the
pictures. There may well be problems with the neighbor's roof, but the
frost neither proves nor disproves anything. The roof-- and the house
itself-- don't look that old, so my guess is that this is much ado about
|I posted a question last week or so about my neighbor's roof having a
| wet spot on it. Two days ago, I took a few more pictures. It was below
| feezing overnight and at 7:30 am the neighbor's roof looked like this:
| What do y'all think?
| The main suggestion under the other post were that it is caused by a
| possible ventilation problem. The bathroom that is right there where
| this spot is.
| Thanks so much for everybody's input!
first of all
this house is a hip roof that has no ventilation on the hips
therefore the middle vented area takes longer to defrost.
this area is over a bathroom that may have poor insulation in the roof
allowing moisture to escape and causing condensation to freeze on the
Without an actual inspection, one can only go by the pictures you posted of
just one side of the roof.
Initial observations are:
Two power vents located on other areas of the structure, which don't come
on during winter. I see no other forms of hip/ridge ventilation, and no
other pod type vents. The bathroom soil stacks are noted, but no vents for
exhaust fans in area (In my area, codes prohibit routing fans directly to
attic area, and to soffit areas.)
Just some thoughts. Since it's a cathedral ceiling, the insulation is
packed in cavity, keeping very well insulated, allowing the frost to remain
on roof in this area, vs more heat loss in other areas.
It looks wet when no other areas are, probably because it is. It could be
improper installation of insulation (no baffles) and/or lack of proper deck
venting, and/or lack of proper routing of bathroom venting to the exterior.
Also noted is what appears to be buckling of shingles (curls) in this area,
indicating lack of proper ventilation.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.