Dormer wall gap- repair

Hi. I have a 1 1/2 story Cape Cod style house. In 1984 I built a shed dormer across the back. The wall-ceiling joint in one room has kept breaking apart. At first I just retaped it and repaired it. It would only last till the weather would change. In the winter time everything contracts and the gap would gone. However in the summer time the roof, wall everything expands and the gap will reach at least an 1 1/2".

The side walls are built in from the roof edge abvout 8"- 12". The wall is built from the floor to under roof rafter. The drywall is attached to the ceiling joist untill it gets about 3 feet from the back wall. There it is attached directly to the rafter. Due to roof pitch restaints we had to lower the back wall. The worst part is where the where the drywall is attached to the ceiling joist. The angle part (last 3') is cracked but dosen't expand that much. The ceiling in the center, right under the peak (about 18" - 24" between ceiling and ridge) is fine. It's really hard to paint a picture of this but I am trying. I hope it's clearer than mud.

I have seen the cause of this adressed years ago in a newspaper colum. Gary Sullivan I believe. Of course I forgot. I need help figuring the cause and a solution. I remember the a proper repair was quite involved. Please help me solve this problem

Can you post a picture of the crack?

No not at this time. I think the problem is "truss deflection". I posted it backwards. The gap is wider in the winter and closes in summer. The ceiling joist is warmed and the rafter it attaches to is cold. The guy at the local lumber yard had the answer for me. Thanks.

And what was the answer??
A piece of molding nailed to the joists only will solve your issue.

The answer was as you suggest is to put moulding up and just nail it to the ceiling so it will move up and down the wall. Or........ rip the end off unnail the sidewall from the rafter. Reattach with metal plates that allow for expansion. Moulding sounds good.

Glad it works for you.
This is what is usually done on a truss roof building also. When a house has trusses, they get truss uplift and the ceilings almost always crack at the wall joints....moulding solves the problem before anyone see's it.

Look at figure 2, floating corner at the ceiling: http://bestdrywall.com/files/ReduceCallbacks.pdf

Instead of nailing with-in 7" of the wall, install a 2"x2" metal flashing on the wall only, then screw the ceiling drywall to that. No screws into the corner ceiling backing or framing. It moves with the wall, especially if the next rafter is 2' away. More room to flex more.

Be safe, Gary

Great Idea. That was a very informative article. I need to do something soon while the gap is at maximum warp factor.