Diagnosing Cracks In Ceiling Drywall

 

Here are some key observations about drywall (aka sheetrock)

Drywall is brittle and easily fractured 

It is not as brittle as floor tile, but it is brittle enough that slab foundation distortion can cause it to crack, especially at the corners of window and door openings. The question is: can foundation distortion stress the ceiling drywall?   

When the foundation bends, it causes the first story walls to rack

The first story walls are fastened to the slab so it is easy to see how the wall sheetrock will be stressed. But, it is not clear how racking a wood frame can stress the ceiling. 

Virtually no stress is transferred from a wall to the ceiling structure

I can see how a wall that racks could pull the paper tape that is attached to the wall and the ceiling at the inside corner where the wall and the ceiling meet, but since there is no rigid connection between the wall and the ceiling, all that might happen is for the tape to wrinkle.

What is the normal cause of drywalls ceiling cracks?

The ceiling sheetrock is attached to the ceiling joists. if the joists sag more than 1/360, the drywall can, and probably will, crack. 

Why would a ceiling joist sag? It might be overloaded. Even if they aren’t, after ten years creep may be significant enough to guarantee that the joists will sag sufficiently to crack the attached drywall. 

 Consider the following from a book titled Expansive Soils – Problems and Practice In Foundation And Pavement Engineering by John D. Nelson and Debora J. Miller, page 223:

Heaving of slabs can affect stud walls, wall paneling, interior partitions, staircase walls, door frames, water lines, furnace ducts, shelves, and bookcases.          

Notice what’s missing: ceiling drywall

I realize there are equally authoritative sources that include cracking in ceiling drywall as a possible indicator of foundation distortion. And I can imagine that if a wall racks there is a possibility that the wall and the ceiling can separate. I have also seen walls that have cracked sheetrock at a door opening and the crack goes from the door opening and across the ceiling. A situation like that is likely to indicate foundation distortion.

Absent those exceptions, the default diagnosis for ceiling cracks should be that they are due to framing issues and not to foundation distortion.

The most common ceiling cracks are found in gambrel ceilings. These ceilings inevitably develop drywall cracks in the seams. The cause is not foundation distortion or overloading. The cracks are due to the fact that a gambrel ceiling is inherently unstable because there is no ceiling joists.       

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