HOW STRONG IS DRYWALL?

About 10 years ago I organized a field trip to a drywall factory. I took my employees, my father, the owner of the local drywall yard, and that’s about it. Turns out visiting a drywall factory is not at the top of most people’s list like it was on mine.

The most interesting thing to me was that after the gypsum core is poured and sandwiched between the two layers of paper the drywall has to set up (cool off) before it can be cut to the desired lengths. I believe that it is this process of pressing the paper faces flat with the liquid gypsum sandwiched between along the length of the drywall is what gives drywall its grain. Just like the grain in wood which gives wood much greater strength with the grain (typically the long direction) than against the grain. Drywall is approximately 3 times stronger in the long direction. Consequently, drywall hung perpendicular to the framing members is stronger than drywall hung parallel. (It is less likely to sag between joists).

Did you ever notice that the recommended on center spacing of ceiling joints is quite often closer when attaching drywall parallel to the joists? see chart below form USG Handbook:

Photo Above: JLC LIve Show: Demonstration on the strength of the lighter weight drywall, which is rated as more resistant to sagging than regular 1/2″ drywall. As a result the lightweight held a lot of weight and still did not break.

In the photo above the strength test was done using regular 1/2″ drywall. Not much sagging occurred before the drywall cut off the end of a panel broke.

Parallel versus perpendicular attachment;

If attached perpendicular to the ceiling joists lengths can be cut so that the butt seams fall where desired (floating, see blog “Eliminating Problems With Butted Seams). Yes we are always trying to avoid butted seams, but an occasional butt seam is better than ridged seams running the length of the ceiling or sagged drywall between joists. Note: The ridged seam could be caused by the beveled edge seam being attached for its length on a low ceiling joist. This happens when hanging drywall parallel to joists. another problem when hanging parallel is less shear strength is added to the assembly compared to hanging perpendicular. In addition drywall hung perpendicular to joists allows the seams to float over slightly uneven joists, making them less conspicuous. (See ridged seam above) .

Can insulation case drywall to sag?

Sagging insulation in ceilings can make properly attaching the drywall impossible. Either staple the insulation up higher along the inside edge of the framing or fur the ceiling. I know this will get a response from some readers, but I don’t like to see the paper flange of Kraft insulation stapled over the face of the framing. The thickness of the paper edge along with the protruding staples creates a problem for fastening. Sagging drywall, poorly attached drywall, and a real increase in fastener pops are a direct result. Make sure the drywall is rated for the framing span. Error on the side of caution by using ceiling board or 5/8 thick drywall. Note: Most drywall manufacturers now make a light weight drywall which is more resistant to sagging that regular drywall. Consider weight of insulation, ½ drywall can support 1.6psf,1/2” ceiling bd can support 2.2 Psf. (Loose fill cellulose is a little over 2 lbs. a sqft for an R 49 value) Note: Other factors to be concerned about is the temperature and humidity of the interior. High moisture content of the air is absorbed into the drywall which can lead to drywall sagging between joists. Also high moisture textures can contribute to problems. So if I haven’t confused you too much you can see that there is a lot to consider when hanging ½” thick drywall on ceilings. But to me the most interesting thing I learned from my trip to the drywall factory was about the grain of drywall and how it affects decisions I make when hanging drywall.