Most simple roof trusses consist of a flat horizontal member called a bottom chord. The sloped portions of the truss that form the roof surface are called the top chords. The bottom chord in your house and many others like yours is probably buried in insulation. During the heating season the warmth from the heated ceiling below causes this portion of the truss to remain dry. Meanwhile, the upper portion of the truss is exposed to cooler attic air. This air almost always has a high relative humidity. Since wood is a hygroscopic material, the top chord members absorb the available moisture in an effort to reach an equilibrium with the surrounding air.

The differences in moisture content cause the truss to bend. The top chords expand and pull the bottom chord up. The greatest amount of upwards movement is in the center of the truss. As the truss moves upwards, it takes the drywall ceiling with it. This might explain why your cracks are larger as you get closer to the center of the house. If, for some reason the interior walls are nailed more securely to the truss than the floor, the truss will actually lift the entire wall. This will account for gaps you might see at your baseboard trim.

There is a way to prevent this problem. If you are building a new house, don't allow the carpenter to nail the roof trusses directly to any interior walls. Instruct him to use handy metal L-shaped truss clips. These simple, inexpensive devices attach directly to the top of interior walls. The long leg of the clip contains a slot. A nail is driven through the slot into the side of the truss. The head of the nail is driven so it is just short of touching the metal clip. This allows the truss to move up and down independently of the wall.

The drywall hangers must cooperate as well. Do not allow them to nail or screw the drywall directly to the trusses within 16 inches of an interior wall. They can fasten the drywall to pieces of wood that fit between the trusses or metal clips that attach to the top of the interior walls. Because the upward truss movement occurs over a period of weeks or months, the drywall will flex at the corners. Cracks should become a thing of the past.

If you have an existing home that suffers from truss uplift, it can be corrected with some effort. You need to climb into the attic area and get to work before it gets too hot. Use a cat's paw tool to remove nails which connect trusses to interior walls. Install the L shaped clips in their place.

After the truss clips are in place, nail a board on top of each interior wall which overhangs the wall by at least 1 inch on each edge. This will act as a bumper when the truss attempts to lift the drywall next year. When this occurs, you will probably see nail pops develop near the interior partitions. Take a large nail punch and drive these nails completely through the drywall. This will release the drywall from the trusses at each partition. Patch these holes with regular joint compound and hope that the problem disappears.