The Making of a Ceiling Collapse
We have seen ceiling collapses that have been caused by pressurizing of the attic with high winds after a gable end wall failure, from someone stepping on the drywall rather than the ceiling when in an attic, or from excessive or heavy contents being stored on the drywall between the ceiling joists rather than on the ceiling joists. These causes are usually rather obvious based on the eyewitness reports and/or the physical evidence such as, the resting place of the fallen components and/or contents. But, on many occasions we evaluate a ceiling collapse that appears to have no obvious cause. Eyewitnesses will often guess or propose that these collapses were due to high wind, excessive weight of ice and snow, earthquake, roof or pipe leak, etc.
When the physical evidence and/or weather reports eliminate the obvious sudden causes for the collapse, we examine the construction of the ceiling and compare it with industry standards and/or the requirements in the appropriate building code.
There are numerous requirements depending on the number of plies of gypsum board; the application or the lack of a textured finish; the direction of the board relative to the span of the ceiling support members, that is, the joists or trusses; the spacing of the ceiling support members; the use of single or double nailing; the application or lack of construction adhesive; the use of screws or nails; the requirement for a floating condition at interior partitions; etc.
For the most common condition where a single layer of the gypsum board is to be textured and is installed without an adhesive to ceiling joists or trusses spaced at 16 inches, industry standards recommend and codes usually require a ½ inch thick gypsum board placed with the long direction perpendicular to the span of the joists or trusses, fastened with 1-3/8 inch long nails spaced at no more than 7 inches on center (Figure MCC-1). Nails shall be driven slightly into the paper without cutting the paper or damaging the gypsum core.
If the finish is changed, the joist or truss spacing is changed, the thickness of the board is changed, or the fastener type is changed, the fastener length and the fastener schedule may also change. Fastening recommendations for other conditions are specified in Recommended Specifications for the Application and Finishing of Gypsum Board.
The construction related damage producing causes that we have seen with gypsum ceiling construction have been excessive nail spacing, inadequate nail length, inadequate gypsum board thickness, damaging of the gypsum board during installation, weakening of the gypsum board from repeated wetting, application of excessive weight from additional texturing, and loosening of the fasteners due to excessive shrinkage of the wood framing.
We have seen ceilings with these types of construction defects remain in place for weeks, months, years, or sometimes even decades. When the separation of the ceiling from the structure has been present and progressing for an extended time there is often an accumulation of dust on the topside of the ceiling directly under the ceiling member from which it has separated. Further examination may also uncover other evidences regarding the history of the ceiling collapse.
Our objective is to examine and take note of what the structure has documented.