Drywall used as sheathing

Todays inspection was exhausting. My shutter finger is sore from taking so many pictures. Anyway…I was doing a termite inspection in addition to the home inspection on the property. The house was clad in aluminum siding, so I was checking to see if there was any damage to the sheathing underneith. I ran my fingers under the bottom edge to see if I could pull out any frass or some cobwebs with frass or dead insects and I felt something really strange. I took out my inspection mirror and tried to see what I was feeling. I was feeling wet paper and was getting white particles dropping on the face of my mirror. Someone sheathed the entire exterior of the house with drywall and covered it with aluminum siding. The siding job was horrible too. What was going through this idiot’s head when he did this?


Hey Scott… sheetrock is cheap, really cheap…:lol:

Doesn’t look like he splurged on green board either…:lol:


Back in the day there was a gypsum product that was made for exterior underlayment. Black paper faced 2x panels that installed horizontally, the paper may have been asphalted. The top edge was convex and bottom edge concaved so they interlocked. We just call it gyprock. I think it was USG product if my memory serves. I still find it on old EIFS and stucco jobs. I try and find a pic.

BTW: The cut end was never meant to be exposed, DUH! Once wet it turns to mush.

In Canada, exterior grade drywall has been allowed by codes for years as exterior sheathing. There are whole subdivisions built in the 1960-70’s with the stuff…houses are still standing!!

Nice catch Scott, did you recommend blue board? :smiley:

I see it was raining in your area as well … flood warnings still being broadcast up here.

They use drywall for sheathing all the time around here. Commercial work, in particular. They yellow faced drywall is manufactured specifically for this application.

I’ve never dealt with the stuff, but here’s some info I found searching for gyp sheathing.


Great find Chris.

I would have never guessed that a paper coated product could be used outside. Regardless of what the manufacturer say in their literature, I would never have a paper coated product under my aluminum siding.

I see it used more often as a base under efis.

Gypsum Board Sheathing.

Common in Philadelphia area as the Gypsum manufacturing plant was located on the river in South Philadelphia …


For years some county and cities have require fire rated drywall on the exterior of homes being built, if the houses are within so many feet of each other. At least on the exterior adjacent to other houses or buildings next to or behind the home being built. Years ago I framed some homes in Md., Washington D.C. and Va. that we had to put fire rated drywall on.


Exterior gyp rock! Not uncommon in the kc metro for a period of years either.

Dens-glass ie: the yellow colored is still use under EIFS most of the time on commercial buildings.

Carl is correct, Dens-glass is used all the time on commercial projects, whether it is behind brick, stucco, or siding. It is the sheathing of choice.

It looks like this;

Here, it is behind siding and behind brick above.

Good product.

Also used quite a bit behind DryVit, and Sto systems commercially. EIFS

Hope this helps.

Marcel :slight_smile: :smiley:

Good find Chris.

Here is the Application of Gypsum Sheathing document that goes with what you found.

LINKto American Gypsums prouct line.

I see it quite often around here. It is almost always in bad condition.

I had the same thing awhile back. Except this house had vinyl siding with only one wall sheathed in sheetrock. Reason it was sheathed with sheetrock was for fire code reasons. This development was a PUD with the homes being only about 10’ between each home. Each exterior wall that was facing the other structure was sheetrock. Front and backs were typical wood sheathing.

This link might be helpfull to exterior rated walls.


Marcel :slight_smile: :smiley:

I am doing a research project involving the use of type x gypsum and densglass being used on exterior walls of residential houses and whether or not this is the answer to “fireproofing” the house. I have had the misfortune of working with the stuff and would really like to be able to prove that it is not if at all possible. Does anyone know of any reputable websites that may contain material I can use to make this point?