If you look at this diagram www.extraordinaryimage.com/bathceiling.gif you can see a diagram of my bathroom ceiling and joist placement.

The length of the room is 9', and I initially put up an 8' long sheet of drywall which ends between joists at the spot shown on the diagram. I had planned to try back blocking that seam to make the butt joint disappear.

After I cut the remaining one-foot piece I realized that I would only have the one true joist to nail it to, plus the nailers on the top plate of the partition wall which aren't terribly sturdy. They have some give to them.

My question is this: Is the above plan bad? Or is the very short piece of drywall ok with just one joist and the end nailers? Should the smallest drywall sheet be nailed to at least two joists? If so, I could cut the large piece back to either the second joist away from the wall, or a full 16" and still make the seam fall between joists so I can back block.

My main thing is I just don't want the seam to crack due to poor installation technique.

Thanks

 

Ideally you would cut the 8 foot sheet back to the center of the last ceiling joist it crossed. The cut part would be against the wall leaving the factory edge on the joist. Then you cut the other piece and put the factory edges together on the joist. The factory edges are tapered somewhat to allow you to tape and mud the seam and end up level with the rest of the sheet. That is what makes the seam disappear.
Nail or screw the nailer better to prevent any movement. Any movement is what causes cracking.
It is always best to end on the center of joists with the sheets. It is never a good idea to use blocking to bridge gaps. Unless you add a joist at the 8 foot measurement.
Remember to stagger the joints on the next piece at least 1 joist different. That is unless the 4 foot sheet covers the ceiling but I imagine the room is wider than 4 feet. So make the next piece 1 joist shortewr than the one you did 1st. then the next piece would be 1 joist longer than your 1st short piece.

 

Hmm... The drywall contractor
(aka That Drywall Guy), Myron Ferguson, does suggest back blockers in his book and in this article from Fine Homebuilding available as a PDF.

For clarification, the actual shape of the entire bathroom is "L" shaped. My original diagram only shows the 9'-long side. The width of the bath is about 7', so there ARE two sheets total, with their tapered, factory edges together. The piece in my diagram is, in reality, a bit "L" shaped itself. The area of concern is over the toilet alcove where it's only 36" wide. So I basically have cut edges on all three sides where the sheet ends and seams with the final small piece.

 

If you have not already picked up the drywall you can get a 10' sheet and do it in one piece. Anywhere you can eliminate a joint you should. Otherwise cut the 8' sheet back to the last joist it hits as badeyeben described.

 

There may be some code issues I don't know about with regard to using nailers between joists. For a really good butt joint use the nailer between the field and then use the ButtTaper (google it) or "V" notch the joint and finish it. There are some other options: Put a piece of cardboard drywall shim on the sides of the nailer so that there is a slight recess where the butts actually meet and treat it like a tapered joint. It won't be as tapered as the factory sides but it will end up flatter.

Or use lots of screws and some adhesive to hold the nailer in place then the joint won't move and you can "V" notch it and fill the gap with no tape. It usually works. Unfortunately if it doesn't work it usually doesn't show up the crack until the lid is painted.

I like nailers in the field. it gives a whole lot wider target to screw the rock than only half a joist.

BUT! It's probably better not to have only one good joist holding the short piece.

 

This is what I was referring to in my post - a back blocker. This is what I ended up using. It worked quite well, although was fairly time consuming. I reinforced the nailer at the end of the ceiling so that it's really about as sturdy as a joist. Screwed to that nailer, the one joist and the back blocker which pulled the drywall in very similar to a factory taper. It should be just right for a seam which will disappear.

The whole thing seems very solid.

I made my back blocker from 8" wide chip board with 1/8" thick x 3/4" wide strips I ripped myself. I tried 3/16" thick first, but that was too much - especially since I'm using 5/8" drywall and it's pretty stiff. It was hesitant to bend in enough with the 3/16" thick strips.