There is a right way and a wrong
way to lap and install metal lath
With out a doubt the installation error that I have witnessed
the most often in the last few months has been lath installation
errors involving paper back lath laps and expanded metal
lath orientation errors. As I stated above, there is most
definitely a right way and a wrong way to install metal
lath and the wrong way will usually result in structural
cracks that will allow moisture intrusion.
All manufacturers produce what is commonly known as offset
paper backed lath. This product would be best described
by describing the paper orientation on each sheet. Originally
paper backed lath was oriented with the paper edges even
on all sides. This required the installer to sandwich the
succeeding sheets between the paper and metal of the previous
sheet. Since this greatly slowed the progress of production
the paper was usually torn or folder under the lath and
this often resulted in a gap in the paper coverage and a
source for water intrusion.
Over two decades ago some one decided that an easier method
would be to let one end and one side edge of the paper overhang
the lath and the opposite edge and long side the paper would
be recessed from the edge. In this way all the installer
has to do is lay paper over paper and metal over metal to
achieve the proper lath and paper orientation. You might
think that this is a “no brainer” and you would
be wrong.
It seems that even this method of installation is too
complicated for some installers in the field. The author
has seen many examples of lath and paper laps that are as
much as five to six inches over the adjoining sheets. In
fact if you walk up to a stucco wall and witness horizontal
cracks spaced vertically approximately 22 to 24 inches on
center you can bet that the paper is overlapping metal below
the stucco. The author offers several pictures to depict
bad paper laps and the resulting cracks in the stucco.
So Why Does Stucco Crack at These Locations?
There are several theories as to why stucco cracks at these
locations including the variation in stucco thickness, the
lack of bonding afforded to the two sheets of metal lath
and the susceptibility to vibration due to a lack of bonding
and a resulting weakened plane in the stucco at that point.
The correct answer is likely a combination of all of the
above although the author leans toward the stucco thickness
variation as the main cause of the cracking. In any case
the cracks are structural in nature, large enough to invite
water intrusion and often result in the wrong paper orientation
and easy moisture or water access to the sheathing. Once
the sheathing gets wet and starts to expand at the edges
many, many other structural problems can result.
This simple error in the installation of metal lath can
result in the overall deterioration and failure of the entire
stucco system. The final word is that a paper to paper and
a metal to metal ship or roof lap orientation must be maintained
for the system to work properly. Too large of a lap can
and usually is as detrimental to the system as too little
lap. As the old saying goes, “the devil is in the
details.”
The accompanying photos depict the improper lapping techniques
and the resulting cracking and damage that almost always
results because of this installation error. I see this error
on nearly every job that I inspect and on every one that
has cracking problems; almost without exception. Often times
the supervisors are aware of the proper installation techniques
but fail to properly instruct or enforce the rules with
those that are actually doing the installation.
Don’t let a lack of training or failure to properly
supervise your installers; cause you untold dollars in repairs
or worse yet, law suits. This is an avoidable problem.
|
