I have been noticing that early in the morning when the condensation is on the roof, you can see the rafters on a finished house. They seem to be somewhat random, or not where the purlin brace is, etc. Rafters and the roof is braced inline with code. I was interested to see if anyone has come up with a solution. I’ve come up with a few theories, but would appreciate any and all feedback.
1)The shingles(mainly on three tab houses) are cheaper and thinner than they used to be, so it’s not covering it up like it used to. This could be compounded with thinner felt nowdays?
2)The crown in the rafters.
3) Todays lumber vs older lumber
What do you guys think?
Replies
it's just a temporary situation with ideal conditions
the rafters are acting as insulators and raising the roof section they are in contact with above the dew point
the open areas in between are below the dew point temperature, so you have condensation
when you get early snow storms the reverse is often true
with no snow in the open areas due to melting from the warmer air in the attics and the rafter sections are colder, so no melting
the basic answer is the rafters have a different R-value than the adjacent roof areas
I've seen the same effect on vinyl siding, with the right weather conditions you can see where every nail head resides beneath the siding.
Cool stuff.
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Wouldn't it be more consistent though, if it were only the difference in R values ( e.g. around the same height on the roof, or etc.)? These seem to be in random spots on the roof. I know you guys are right on the R value, but would in not be continuous down the rafters, or atleast around the same height or area? Down in the south lately, there has only been a 20 degree difference between night and day temperatures. What if you could still see it as the day went on, or as the rafters and decking got closer to the same temperature (hypothetically eliminating the R value variance)?
I read your post so please excuse the repitition... You do mean conventional framed stick built roofs - not trusses - right? And, are these rafters on 16" or 24" centers?
Yes. Stick framed rafters on 24" centers.
24" centers for stick framed is pretty unusual - or at least around here. Here, on modern construction, it's stick frame = 16" OC, truss = 24" OC. Anyway, with 24" centers, if you use 1/2" or 7/16" sheathing, the roof won't be perfectly flat. Like others said, temperature differentials during certain times of the year can cause that too.
Sorry for the highjack.
Hey Matt, was in NC lately and saw hickory floors. See my post Good Looking Floors and comment if you have info.
I have to give a great thank you to you.
I coach swimming in MN. I always put an inspirational quote at the top of the board for each practice. Your quote at the end of your post will be at the top of the board tomorrow.
Thanks.