I opened a wall for an addition. The span is about 19′ (lvl). The existing ceiling was vaulted, the new room that it ties in with is flat. The joint keeps cracking. What up?? Can I fix it??
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Excessive deflection on the (long) 19' span is likely the cause. On a span this long the acceptable deflection criteria for drywall (NOT to crack) is l/360 (length in inches, or 228/360) = about 5/8". It's very easy to undersize a header this long and you may be looking at 3/4" deflection or more. Replace drywall with wood, reduce span or replace header with steel as your budget permits. A column right in the center of the span (aesthetics aside) is usually the cheapest solution.
Jeff
Edited 11/22/2008 12:37 am ET by Jeff_Clarke
Seems like at least a flitch beam would have been required for that span.
WHat's a flitch beam??
A steel plate sandwiched between two wood members, all bolted together. Alternatively a a steel plate (flitch plate) can be added to an existing beam on one or either sides to strengthen it.
If you're sure that the construction is structuraly sound, you may consider a drywall expantion joint.
http://www.hamiltonnw.com/AP_TrimTexExpansionJointsRevealBeads.asp
Lengthy article, leaning towards commercial situations;
http://www.usg.com/resources/architect/pdf/02AugustMovementStress.pdf
If it is cracking at a horizontal corner, I would use some ultraflex. Notice the logo with the baseball bat.
http://www.no-coat.com/NO-COAT/ARCHITECTS/Products/NO-COAT_Drywall_Corner_System/
Mike
Small wheel turn by the fire and rod, big wheel turn by the grace of god.