# Replacing Rotting Sheathing Behind Chimney.



## fredhead (Nov 15, 2010)

I am trying to figure out how to replace rotting exterior wall sheathing behind the chimney.
The roof was repaired which caused the damage but now I am faced with the damage that years of a leaky roof caused. Water was leaking in around the flashing of the chimney and running down the back of the chimney that backs the wall of the house.
When I opened the interior sheetrock wall that backs the exterior chimney, mold was inside the wall cavity and the particle board sheathing had fallen apart. And what had not fallen apart was held togther by mold. Even the tar paper between the sheathing and the cinderblock of the chimney had fallen apart. I am now faced with a 4x4 hole in my wall facing the cinder blocks that form the back of the chimney. The studs and floor sill are in good condition. 
How do I go about replacing the exterior sheathing that rotted behind the chimney? 
Can this be replaced from the inside?
Can it be replaced by removing exterior siding and trying to slide new sheathing in from the side (Not sure how to secure sheathing to studs).
Will this require demolition of the chimney to repair?


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## wombosi (Apr 22, 2008)

hey there.
sounds like a bad situation, but not insurmountable.

having just taken down a 40' brick chimney today, i can tell you you'll want to avoid this at all costs.

is the chimney on the gable end or the eave end? platform framed walls? balloon framed?

if platform framed wall on gable end (usually joists are not sitting on top), i would chop out a section of wall slightly wider than chimney. reframe and sheath on the floor, then slide it into place and fasten it on the inside. then deal with finish siding on the outside.

is it just one floor that needs to be replaced? two floors?

might be possible to sawzall the nails/screws holding the sheathing in place from the inside, then slide new boards across and fasten from the inside, with say 2" screws at an angle through the studs. or figure the chimney is narrow enough that the sheathing only needs to be fastened at the first stud to each side of it (or you can add blocking).

if you want to get into a big expensive mess, take the chimney down, fix it all, and put up a new metalbestos pipe.

good luck, sir.


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## Ron6519 (Mar 28, 2007)

If this is a masonary chimney, you can't slide sheathing behind the chimney due to the brick ties in the framing. Unless the ties rotted along with the sheathing.
You will still need to secure the chimney to the house.
Ron


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## Joe Carola (Apr 14, 2006)

Ron6519 said:


> If this is a masonary chimney, you can't slide sheathing behind the chimney due to the brick ties in the framing. Unless the ties rotted along with the sheathing.
> You will still need to secure the chimney to the house.
> Ron


If the ties are still there he can sheath the walls on the floor and put a slice in the sheathing where the brick ties are and nail the ties on the inside of the sheathing.


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## fredhead (Nov 15, 2010)

The roof is a hip roof so the chimney starts at the edge of the roof. The chimney is 6 feet wide. 

I removed more of the sheathing and I don’t see any straps holding the chimney to the structure of the home. I also don’t see any signs of straps that rusted away. Due to the large size of the chimney, could this just be a free standing structure parallel to the home with no straps?

I think my plan is to support the ceiling with temp braces, pull out three studs and reconstruct the wall from the inside and outside. I will rollback the exterior vinyl siding to nail the sheathing to studs not behind the chimney. For studs behind the chimney, I will fasten U shaped braces to the sheathing before positioning the sheathing back up and then attach the U braces to the studs once the wall studs are replaced. I hesitant to toe nail the sheathing from the inside since there is a chance that while doing so, I may pierce the tar paper/tyvek between the chimney and the sheathing.

Some more questions:

1) Can I use pressure treated plywood as sheathing behind the chimney? Are there any health concerns? Any fire concern since this will be up against the chimney?

2) Should I place tar paper or Tyvek between the chimney and sheathing? Should I use both? Tar paper against chimney and Tyvek against the sheathing. It is very tight so everything would be sandwiched together. I like the heavy duty feel of tar paper but the mold ate right thru that. Tyvek is a great barrier but I am concerned that it may be damaged against the rough chimney when placing the sheathing back up.
 
3) Since the space between the chimney and the studs is very tight, and I want to place a skim coat over the cinder blocks (due to the many gaps in the blocks), would it be okay to use 3/8 plywood instead of the existing 1/2 inch?


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## Jasonmac8181 (3 mo ago)

I’m up against exactly what you got going on. I put temp studs up. ripped out the studs, and going to put tar paper against the black tar like sheathing against the back side of chimney. Put new studs up as well. Because mine is so tight I don’t think I’m going to worry to much about fastening the sheathing to the studs. And just let it rest on the bottom stairs ceiling. If that makes sense. I know you post is old. May I ask how yours turned out, and did you end up doing anything else? Thanks, J


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