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Re: Stucco Shear Walls

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Here are two classes of problems I've seen with stucco:

1.  Old stucco without weep-screeds -- moisture has risen into the wall by
capillarity and corroded the nails.  If the nails give way so that the stucco
begins to detach, you get a warning before the earthquake.  If the nails are
almost (but not quite) rusted through, there's no clue of the danger until a
quake: when the house needs bracing, suddenly the stucco isn't connected.
Chicken-wire lath also corrodes when it gets old, and loses its effectiveness.

Nails and lath in stucco that has weep screeds may eventually corrode due to
carbonation, but that may take 50 to 70 years; there probably are no weep-
screed installations that old yet.

Stucco next to a planting area that has been well irrigated for a long time
(years) with sprinklers aimed at the house can have corroded nails and lath
even if it does have weep-screeds.  Coastal locations are also rough on lath
and nails.

2.  Chicken-wire lath (woven wire fabric) nailed tightly to the studs with
conventional nails or roofing nails so that the nail head and the lath at the
connection cannot be embedded in stucco; the un-embedded lath tends to rip
away from the stucco when a shear load is applied by earthquake shaking.  The
stucco lath nails that should be used have furring-washers -- a fiber-washer
about 1/2" diameter by about 1/4" thick.  The lath is fit between the nail
head and the furring-washer before the nail is driven so that the washer and
nail hold the lath firmly with good embedment in the stucco.  Self-furring
lath tends to hold the lath away from the framing the proper distance for good
embedment in the stucco, but if connected with conventional nails not driven
tightly, the lath tends to be loose, so that it can be pushed around as the
stucco is applied; thus the nails get driven tight to the studs, and the self-
furring advantage is lost -- furring-nails or other furring devices are needed
for self-furring lath.  

It seems to me that properly installed, with stainless, or galvanized furring-
nails, stucco should be capable of providing very effective bracing strength
with a long serviceability period.  I believe that if all stucco had been
properly installed before the Northridge Earthquake, we would still be using
the bracing values allowed by the 1991 UBC.  

However, I have not heard of any indication that the stucco industry is
interested in finding a way to regain the old allowable values.  Maybe others
know of problems that I have not heard about that are more difficult to solve.

Nels Roselund
Structural Engineer