Point 1 of my 12 point list of methods for reducing
cracking in portland cement stucco




Point one - sheathing and framing


Stamp on plywood says space 1/8" at ends and sides.
Next to stamp is joint butted so tight you can't stick
a razor blade in it.

   A large source of cracking in stucco is the movement, warping and expansion of plywood or OSB sheathing.  The American Plywood Association  recommends spacing of 1/8" at ends and sides of plywood and OSB panels. "If severe moisture conditions are anticipated, increased spacing may be required."  The buckling caused will crack the stucco, not necessarily at the joints, but anywhere. If this wasn't important, the plywood manufacturers wouldn't waste the money on stamping every sheet. All I can do is insist that the plywood be spaced 1/8".

    The plywood is easy to space during installation by driving nails as spacers into the studs. The lack of spacing can be remedied later by cutting "kerfs", or grooves with a circular saw. The saw blade can be set to a 1/2" depth and the joints traced.

      Lack of spacing is hard to detect because usually the sheathing is covered immediately with Tyvek. We don't discover this condition until we build a scaffold and tear off the Tyvek. The delays of waiting for the sheathing to be corrected or the disputes created are too expensive, so usually we proceed with unspaced plywood. All I can do to enforce proper spacing is to insist on spacing before the plywood is installed.

      We have had fewer cracks on jobs with OSB than with plywood. I like OSB a lot better. When you  try to drive a nail (or roofing staple) into plywood, it bounces like a basketball. OSB has a nice solid thunk, indicating it is stronger, and more resistant to
vibration, a cause of cracks.

When plywood or OSB gets wet due to inadequate flashing or drip caps, the swelling and buckling is increased, cracking the stucco. Water infiltration is the subject of a future point on my list.

                  driven through plywood
Nails are driven through plywood, indicating pressure is too high on the nail gun.
    Another typical condition is the nails are driven through the plywood. The plywood is then only supported by the bottom layer of wood. Bear in mind the metal lath is attached to the sheathing, and not necessarily the studs. This creates a lot of movement which obviously cracks the stucco.
  There is a lot to be said about driving nails by hand. Not only don't the nails go through the wood, but the beating and banging pull the sheathing tight to the studs. (Remember wood studs aren't straight).

       A great method we have used to limit cracks (but not cheap) is to cover all the plywood sheathing with durock. We did a house this way and a year later there were only 2 tiny hairline cracks over windows on the whole thing. The reason is that the layer of durockIn the old days, sheathing was done by applying 1x6's diagonally to the studs, leaving a space of about 1" between the 1x6's. This seems a superior method to modern plywood or OSB.

     We stuccoed a three story townhouse once where the framing was so flimsy you could shake the whole wall easily with one hand. If I find this condition again, I'm leaving.


    Wive's tales