What is causing the cracking in my ceiling?
We have looked at numerous single-family ranch style dwellings constructed in the 1960s where the ceiling exhibits cracking between the dining room and the living room. It’s a crack that emanates from the corner of two walls. Those of you who have seen this cracking know what I speak of. See Figures CC-1 and CC-2.
Looking from the attic side of the ceiling one can see immediately what has happened. For gravity loads, the ceiling joists over the dining room and the living room are usually supported on the front and rear walls and at the center of the dwelling by a beam comprised of two members, commonly two 2x8s. See Figure CC-3.
Unfortunately the ceiling joists also provide a tie back to restrain the roof rafters from thrusting the top of the front and rear walls outward. Where there is no beam, the ceiling joists are usually supported on and lapped over a center wall. The ceiling joists are commonly face nailed together at the lap so that the outward thrusting from the rear wall is resisted by the approximately equal outward thrusting of the front wall.
Where the ceiling joists frame into the side of the beam (Figure CC-3) there is no lap and no face nailing of the opposing ceiling joists. Rather, the tension from the ceiling joists on each side of the beam is transferred through the beam by nailed connections (Figure CC-4.
When the nailed connections loosen over the years from the seasonal shrinkage and swelling of the lumber, the connections loosen and the beam or the connection of the ceiling joist to the beam pull apart (Figures CC-4 and CC-5). The separation at the beam also stretches the drywall or plaster ceiling and causes it to crack (Figure CC-2).
The risk of this separation and cracking may be reduced by providing an adequate tie between the opposing ceiling joists with 2x4s or metal straps. The addition of ties for ceiling joists is not for the inexperienced. We recommend that a qualified contractor be hired for the work and that permits be applied for. A Professional Engineer experienced in the structural discipline might also be required to properly size the ties and their connection.