I’m converting the attic of our small cape into living space. How does one build kneewalls in an attic conversion without transferring roof (and snow) loads to the unsupported ceiling/floor joists below?
—Kris L. Johnson, Madison, Wisc.
Peter H. Guimond, a structural engineer from Pawtucket, R. I., replies: Depending on your roof pitch and on the size and span of your existing ceiling joists, there may not be a problem with transferring some of the roof load, but to be sure you would need to have an engineer check out the structure.
The trick to adding kneewalls without transferring roof loads is to connect the kneewall to the rafters without the rafters actually bearing on the kneewall. This way the rafters can deflect under snow loads without affecting the kneewall. Below is a sketch of a possible solution to the connection problem. It employs a slotted metal connector called a roof-truss clip (Simpson Strong-Tie, 1450 Doolittle Dr., P. O. Box 1568, San Leandro, Calif. 94577; 510-562-7775).
Basically the kneewall is built short of the rafters—how short depends on the expected deflection. Blocking is then nailed to the top plate between the rafters. You can place blocking between alternate pairs of rafters to save lumber and labor. And finally, the roof-truss clips are nailed to the blocking and to the rafters. The slots in the clips allow for rafter deflection.
With the framing connection made, the next question is how to keep the drywall joint between the wall and the ceiling from cracking when the rafters deflect. The answer is not to have a drywall joint at that intersection; use molding instead. The molding should be tight against the kneewall but attached only to the rafters. This covers the gap in the drywall and allows the rafters to move.