# What might cause drywall seams on the ceiling to become darker?



## RustNeverSleeps (Sep 26, 2015)

In a few months I plan to remove this popcorn ceiling and paint. I do not expect to have any serious issues, but I was curious why all the seams appear to have shadow cast across them? It is hard to capture the effect in a photo and is only visible at night in low light. Since the joints would have been taped and finished before the popcorn texture was sprayed I might expect the seam to be lighter instead of darker. This ceiling is a little over 30 years old, could the joint compound underneath changed color? Does it somehow trap/absorb dust from the air? It is not visible if standing beneath and shining a light straight up.


Second photo is to more explicitly highlight the area in the first photo.


----------



## mark sr (Jun 13, 2017)

If the drywall is getting wet from above it will darken but I don't think that is what you have going on. The ceiling joist isn't always insulated as good as the cavity between joists. This temperature difference can cause the area directly below the joist to attract/hold airborne dirt/dust.


----------



## XSleeper (Sep 23, 2007)

Generally those shadows are caused by poor drywall finishing. The joint was not full before they textured it. On your butt joints the shadow is likely a hump that was not feathered out far enough. You will have some work to do after the texture is scraped off.


----------



## Nealtw (Jun 22, 2017)

They cheat here and don't do what would be the last coat of filler when they apply popcorn. 30 years ago, did they use a primer first, they didn't on a couple I have removed.


----------



## RustNeverSleeps (Sep 26, 2015)

mark sr said:


> If the drywall is getting wet from above it will darken but I don't think that is what you have going on. The ceiling joist isn't always insulated as good as the cavity between joists. This temperature difference can cause the area directly below the joist to attract/hold airborne dirt/dust.


Interesting thought, as temperature difference would mean slightly different moisture levels in these spots depending on the season. Which might possibility cause more dust to be attracted or stick there. 



XSleeper said:


> Generally those shadows are caused by poor drywall finishing. The joint was not full before they textured it. On your butt joints the shadow is likely a hump that was not feathered out far enough. You will have some work to do after the texture is scraped off.


This is my concern. Before starting I think I need to be prepared to spend some extra time with prep work, to widen out the joints. I had not planned to skim coat the ceiling but maybe I need to consider doing that. I would not be surprised if there is a small hump, though it looks to be a slightly darker color. 



Nealtw said:


> They cheat here and don't do what would be the last coat of filler when they apply popcorn. 30 years ago, did they use a primer first, they didn't on a couple I have removed.


I have taken off the wallpaper and popcorn in one bathroom and there was no primer used. Not sure if that makes the job harder or easier, but that must contribute to how the drywall finishing step is shining through decades later.


----------



## mark sr (Jun 13, 2017)

Unpainted popcorn is a lot easier to remove than painted popcorn. Ceilings that are textured with popcorn are rarely ever finished to the same level a slick finish would be. It's fairly common to need to skim coat the ceiling after the popcorn is removed.



I've painted a lot of new construction houses [as an employee] where we didn't prime the walls under wallpaper. That makes wallpaper removal extra hard!


----------



## RustNeverSleeps (Sep 26, 2015)

mark sr said:


> ...Ceilings that are textured with popcorn are rarely ever finished to the same level a slick finish would be. It's fairly common to need to skim coat the ceiling after the popcorn is removed....


 Thanks for the input. Getting me mentally prepared to skim the whole ceiling when I tackle that job.  



This is borderline evidence/conspiracy but to add more credence to the temperature differential theory I looked carefully at a few other rooms (the whole house still has original popcorn ceilings except the one bathroom). 


I could not find any visual evidence of seams in a formal living room with cathedral ceilings. It probably used nearly full sheets of drywall on each side, so there would be less seams but I can't detect them. From what I remember from being in the attic is that one slope follows the roof line while the other doesn't, but construction/insulation is different using kraft paper backed fiberglass and extra plywood reinforcement on the rafters. While the rest of the attic is loose fill pink fiberglass. It could also be that the person who finished it did a better job, or the angle and height of the ceiling hides it.
In a bedroom I could find one seam but it was not dark, mostly visible because of the joint hump.
Also of note is that the photo was taken in the part of the home where the roof above is the highest, so there would be a bigger column of cold air above in the winter. And this area is where people congregate more frequently to talk, eat, watch TV, etc. So there would also be more "dust" and more stirring of the air in this location.


----------

