Replies

1. 
   cedarguy | Feb 12, 2004 11:52am | #1

   If you do repaint. Where the stains are I would use a very good primer.

   You may even want to look at something like Rust-oleum

   What I would guess is going on-- the screw is rusting and bleeding through. I know this is all hindsight but I like the screws that are for outdoor use.

2. 
   JHOLE | Feb 12, 2004 02:57pm | #2

   It's a thermal / vapor short. Yes you have a vapor barrier and insulation but the screws go from the warm side of the room THROUGH both of these and into a cold rafter. So, the screw is transmitting the cold, is slightly cooler than the material around it, condensates faster, airborne impurities are attracted, dark spot.

1. 
   crt2cu2 | Feb 13, 2004 08:54pm | #7

   JHOLE,

   A thermal/vapor short makes sense I guess. I just wonder why I have never seen this before in the many remodels I have worked on?

   Mike S.

3. 
   mlbfreestyle | Feb 12, 2004 05:38pm | #3

   The primer to use may very well be Zinsser tinted shellac base.  This has several great properties.

   1. Best adhesion to problem surfaces.

   2. Excellent (if not best) ability to prevent bleedthrough.

   3. One of the best vapor barriers.

   4. Low cost, fast drying, ...

4. 
   JohnWW | Feb 12, 2004 05:55pm | #4

   I too believe that the problem is due to the screws rusting.  A couple of questions: What climate do you live in, do you do anything to reduce the humidity in the house, and do you recall how long the screws were or, more specifically, how far the screws went into the joists?

   John W.

   
   

   ---

   Edited 2/12/2004 12:19:31 PM ET by JohnW

1. 
   Arby | Feb 13, 2004 07:32am | #5

   The primer of choice for me would be Zinnser for sure!

1. 
   CUEMASTER | Feb 13, 2004 05:43pm | #6

   Try "Kilz"  nothing comes thru that stuff.

   Art(cuemaster)

2. 
   crt2cu2 | Feb 13, 2004 08:57pm | #8

   John W.

   To answer your questions: I live in a cold, dry part of southwest Montana. We have had a long and constantly cold winter this year.

   I don't do anything about reducing humidity other than running the bathroom exhaust fan. The screws are standard black drywall about 2 1/2 inches long I think which means penetration into the rafters about one inch.

   Mike S.

1. 
   JohnWW | Feb 13, 2004 09:42pm | #9

   The reason I was asking about the screw length is that the further the screw goes into the rafter or stud, the more heat it will conduct away, leading to a colder screw head and a greater chance of condensation and rust.  Sheet rock screws should only penetrate 3/4 of an inch into the framing, mostly because this reduces the incidence of "pops" over the screw heads when the framing shrinks as it dries out but it also prevents the problem you are experiencing.

   I doubt that the extra 1/4 inch of penetration is the primary cause of your problem.  A 2 1/2 inch screw through 1/2 inch rock would penetrate far enough to cause trouble though, how is it that a 2 1/2 inch screw is only penetrating 1 inch into the framing?   A second question, how wide are the rafters and are they actually rafters, that is are they supporting the roof deck?

   John W.

   Edited 2/13/2004 1:48:44 PM ET by JohnW

1. 
   crt2cu2 | Feb 14, 2004 08:32pm | #10

   John W.

   I guess I need to be more clear. Screw penetration is one inch due to two layers of rock screwed into 2x trusses 24" o.c. What I don't fathom is why the dark stains on ceiling sheetrock and not the wall frieze board? This seems to negate the thermal/vapor short theory suggested by JHOLE earlier. I guess I will have to embark on the noxious task of going into the attic and crawling through insulation into the sharp angle of a 4/12 roof and check the insulation and vapor barrier over the top plate. Maybe there are some air infiltration and/or insulation "leaks" near the soffit/wall planes.

   Also- if this has any influence on the screw stain problem- on the interior non-load bearing walls I am having some trouble with truss lift. These interior walls are about 12 feet from the exterior walls which have the stains.

   Namaste',

   Mike S.

1. 
   mitch | Feb 15, 2004 05:44pm | #11

   totally off topic- but where in sw montana?  i used to spend a lot of time around dillon fishing.

   m

2. 
   crt2cu2 | Feb 16, 2004 06:52pm | #12

   Mitch,

   Just north of Dillon in Butte, MT. I guess you must have fished the Big Hole River?

   Mike S.

3. 
   mitch | Feb 16, 2004 08:12pm | #13

   the bighole- various stretches from wise river down to buffalo jump but mostly the canyon section above melrose, depending on the water level, weather, crowds, etc, and the beaverhead below clark canyon dam.  also spent time on the madison, gallatin and others.  floated the smith a couple times, too.  the bighorn was fun before it became so overrun.

   i was a real screw-off in my younger days- 30+ days/yr fishing in montana and wyoming wasn't unusual in my 20's.  not to mention that much or more spent hunting...

   m

4. 
   crt2cu2 | Feb 17, 2004 01:52pm | #16

   mitch,

   hey buddy! You are sounding like an "old guy" Better watch out, soon we will be telling the world all our old outdoor stories and fishing lies! Where do you hail from now? BTW, I just returned this early morn from a call at the Dillon Hospital. Roads were wierdly icy.

   Mike S.

5. 
   JohnWW | Feb 16, 2004 09:24pm | #14

   The screws are condensing moisture on them because they are extending too deep into the truss components and the trusses are probably poorly insulated especially out at the eaves where it is difficult and unpleasant to work.  

   The eaves are probably ventilated, they should be, and the cold air is running through gaps in the insulation along the sides of the truss components, cooling down the wood and the screw shank which is just a fraction of an inch inside the wood's surface, especially if the screw wasn't driven into the centerline of the truss member.  The walls aren't giving you a problem primarily because there isn't a source of flowing air to carry away the heat and the insulation may have been packed in better.

   The type, condition, and location of the vapor barrier has no bearing on the screw problem, since the screw penetrates the barrier and the screw head is just a fraction of an inch away from the moist air inside the house with no effective barrier between the screw head and the interior air.  Some specialty paints can block water vapor penetration to some extent but ordinary interior paints aren't at all effective as a vapor barrier.

   The insulation in the attic space should be tight against the sides of the truss and the insulation should also go over the top of the truss components so that the bottom chord of the truss is totally enclosed in insulation, something that probably wasn't done by the builder since it takes some time to cut and fit the batting between the truss braces.  If you add insulation it should be unfaced.

   There should be air flow through the attic space from the soffit vents up to a ridge or gable vent.  Check that there is a clear space between the soffit vents and the insulation out at the eaves so that the rising air goes above the insulation rather than being forced to go through it. 

   Once you take steps to prevent heat loss through the screw shanks, the condensation should stop,and you should be able to use a stain blocking primer over the spots and then repaint the ceiling.

   John W. 

    

6. 
   crt2cu2 | Feb 17, 2004 01:42pm | #15

   John W.

   Thank you so much for your thoughtful replies. Being an ex-framer and finish carpenter I know exactly what you are referring to regarding insulation in the rafters and appropriate airflow through the soffit vents and then along the bottom interior of the roofline. We used to call that a "cold roof" back in my hammer swinging days. I am pretty sure the insulation and vents up in my cramped attic space are as you mentioned, but not having inspected that part of this house personally, I guess I had better check it out. I dread the job. In other parts of the attic there is about 20 inches of blown cellulose to wade through. Whoever built this house 100 years ago neglected to put an attic access where I need it now.

   I am still stumped however. Having built quite a few new houses and more than I can remember remodels- why have I never ran across this problem before?

   Mike S.

7. 
   mlbfreestyle | Feb 17, 2004 05:39pm | #17

   The stain blocking primer to use in this case is Zinsser BIN.  This is a shellace based primer that is several times more effective at stopping water vapor as latex paint.  Most latex paint has a perm rating of 2 or 3.  BIN is rated at 0.4.  I just finished putting 2 coats on the ceiling in several rooms of my house to stop a similar problem.