ajk1
Structural
- Apr 22, 2011
- 1,791
I had an earlier string with the question as to what the "pull-over" strength of a drywall screw might be in the drywall. There were no specific answers received to this question, so I did an at-home approximate test of a couple of drywall screws in 1/2" thick drywall and thought people might be generally interested in the answer, although the test was not conducted precisely.
I set up the test with a bathroom scale at one end of a specimen of drywall and a wood block at the other end, on top of a work table in the garage. I placed a thin strip of wood on to of the bathroom scale so that the load would be applied to the scale only at the wood strip location. The span of the drywall was 11.5". I placed a wood block under about the centre of the drywall specimen and then clamped this wood block to the work table. Then I zeroed the bathroom scale and screwed the drywall down into the wood block below. The scale gave me the reaction when the screw head punched through the drywall. I then calculated the load that would cause that reaction, based on actual measurements of the screw location. The calculated value is I believe the pull-over strength of the screw fastener.
I repated the above procedure for a second screw, but reduced the span of the drywall to 7.3".
The results were failure loads of 12.9 pounds and 13.7 pounds respectively.
Dividing by a saftey factor of 4 gives an allowable capacity of about 3.3 pounds.
This allows me to calculate the required screw spacing when supporting a new drywall ceiling that is placed tight to an existing questionable old plaster ceiling to restrain it from falling (part had fallen).
Prologue: We actually removed the entire old plaster ceiling, and the work was completed about a week ago, but if we ever get another similar situation, the foregoing experiment will give us a bit of a "feel" for what might be doable, although more precise and extensive tests should be done.
I set up the test with a bathroom scale at one end of a specimen of drywall and a wood block at the other end, on top of a work table in the garage. I placed a thin strip of wood on to of the bathroom scale so that the load would be applied to the scale only at the wood strip location. The span of the drywall was 11.5". I placed a wood block under about the centre of the drywall specimen and then clamped this wood block to the work table. Then I zeroed the bathroom scale and screwed the drywall down into the wood block below. The scale gave me the reaction when the screw head punched through the drywall. I then calculated the load that would cause that reaction, based on actual measurements of the screw location. The calculated value is I believe the pull-over strength of the screw fastener.
I repated the above procedure for a second screw, but reduced the span of the drywall to 7.3".
The results were failure loads of 12.9 pounds and 13.7 pounds respectively.
Dividing by a saftey factor of 4 gives an allowable capacity of about 3.3 pounds.
This allows me to calculate the required screw spacing when supporting a new drywall ceiling that is placed tight to an existing questionable old plaster ceiling to restrain it from falling (part had fallen).
Prologue: We actually removed the entire old plaster ceiling, and the work was completed about a week ago, but if we ever get another similar situation, the foregoing experiment will give us a bit of a "feel" for what might be doable, although more precise and extensive tests should be done.
