Why are there two layers of dry wall in my ceiling, and how can I patch it?

Question

The fan in my bathroom died. We bought a replacement that's a bit bigger. The original was nailed to a beam in the ceiling and required me to cut out some drywall to remove.

When removing the old fan, the air duct came loose. I need to open up the ceiling a little more to reattach that to the where ever it was attached originally.

I'm gonna have to put in a piece of drywall ~18"x36". How in god's name can you do more than just fill in little holes when you have no studs to lay new drywall directly onto? And how was this drywall attached to begin with!?

Answer 1

We've done a double layer of drywall to slow the spread of fire in multi-unit developments, specifically between the ceiling and attic space. However, judging by the gap, I'm guessing that you might have this done as a retrofit to block sound. There will be somewhere that the drywall is attached, and it's likely going to be metal tracks running every 16". Normally, we'd attach the tracks directly to the joists, but if this is was a decision made after the home was built, they may have elected to go over the drywall instead of making a mess removing the existing drywall.

If you want to patch the drywall, you can either install additional blocking between the drywall at this opening to give you something to screw into, or you can cut back the drywall to the current tracks and attach your patch there.

Edit: If you decide to use blocking between the drywall, some thin wood would be be easy to install, and you're not getting a lot of sound protection at this spot anyway. You'll want a piece of wood behind each sheet of drywall, and then screw through the drywall/wood/drywall/wood to give you a solid edge to work with. You can get longer drywall screws that can make it through the entire assembly in one shot. If you go with the long screws, I'd recommend using a clamp to hold it tight.

Answer 2

There is also a technique for butt joints that could work on a patch. A backer-board, about 4 inches wide and as long as the joint is placed in the ceiling or wall above or behind the sheetrock, centered along the joint. The butt edge of the sheetrock is then screwed into one half of the board. The butt edge of the next sheet is then placed over the second half of the board. The sheet is attached to the studs or joists and the butt edge is screwed to the backer.

To get a flat joint, the backer is often lifted slightly away from the back of the sheetrock by placing a shim of carboard along the edges between the board and the back side of the sheetrock. When the butt edges of the sheetrock are screwed to the board, they bow up slightly to give a small valley for taping compound that leads to a flat finish.

There is also a commercial product that is tapered in the middle to accomplish this.

On large pieces of sheetrock, this is used only on the butt ends, the sheet being nailed or screwed into joists or studs everywhere else. On a somewhat small patch such as you are proposing, this probably could be used on all four sides.