Major cracking in stucco

Ask the pros- January, 2002


                     major cracking in stucco 
 I recently saw your web site and have a couple questions about stucco finishes. I am a structural engineer and have no experience with stucco. I was recently called out to look at a structure that had major cracking in the stucco siding that is less than 6 months old.  The reason for my site visit was to do a post earthquake evaluation on the structure after we had a mild (4.0) earth quake in Spokane Washington. The structure has many cmu partition walls between concrete moment frame columns, the cmu wall showed no signs of damage at all. The exterior of the building is brick with a band of stucco that wraps the upper half of the building. The brick shows no signs of movement or indications of cracking. The main portion of the building was constructed about 6 years ago and shows evidence of long linear cracks running horizontal or vertical in different panels. the following picture however is on an addition that was finished about 6 months ago. As you can see it has cracks running in all directions, most of these are
either hairline cracks or are not showing through at all. I looked at the
surface with a 30x magnifying scope and noticed that some of the cracks look like they are behind the finish coat, i.e. you can see a trowel mark/ stucco that runs over the top of a hairline crack with the stucco finish actually appearing to bridge the crack. I am pretty sure that this was never repaired and is still a "virgin" job.

   The building engineer is convinced that the cracking is from the recent
earthquake, and I have never seen a stucco finish after a mild quake but I
would expect to see more patterned and diagonal cracking, would this be the case? Most of the cracks in the main portion of the structure has vertical or horizontal cracks and no diagonal.

The worst spot I saw here and due to faulty application is the white curtain wall:
Major cracks on stucco panel
What apparently happened here is the finish coat is applied in 2 coats-the scratch and the double-up coat. The scratch coat was allowed to sit too long before the double allowing the mortar to shrink and crack. Or the wall was not wet down
enough. Either way, the double up coat wasn't enough to kill these cracks.
This not only looks bad, but there is a real danger of pieces falling off due to failure to bond.
This I'm sure existed before the earthquake. They look terrible because they are white and probably in the bright sunlight most
of the day. You can always expect some cracks in stucco.
I have been able to limit cracks in recent years but not prevent them.
Major cracks on stucco panel

Major cracks on stucco panel
The pictures above are pretty much typical cracks. These are at where the lath overlaps-the weakest spot. These should not be repaired or filled as it would detract from the appearance ofthe stucco. Industry standard is to overlap lath 1" at the edges. We overlap ours 3". This is what I learned so why not.
Major cracks on stucco ceiling
The ceiling probably has always had these cracks. I know these-the ceiling wasn't
suspended but attached directly to the structure. How do I know?
First of all there is no room for hanger wire. Second, we did a big ceiling 7 yearsago at Andrews Air Force Base. We tied the channel iron and lath
directly to the structure, in this case, Heavy gage metal stud trusses. Fortunately, and old lather I know told me that the ceiling would crack at every joist. A suspended ceiling would have more "give" do to building movement.  Still, a suspended ceiling always tends to crack around lights and other openings.
I was able to write a CYA letter to the government before we applied mortar
to the ceiling explaining the ceiling would crack at every joist. The inspectors
were unreasonable Nazis and I would have had trouble getting the ceiling approved
if it wasn't for the letter and a meeting with the architect. A big vibrating machine vibrating the parking lot aggravated the cracking more.

I think these cracks may have been there before the quake.   The really don't look
bad and there is nothing much you can do.

The picture with the straight-edge is bad-the stucco has buckled or delaminated
and should be repaired one day. If not it could lead to spalling.
Major cracks on stucco panel
The white stucco with the penny probably shouldn't have any water infiltration
and could be ignored.
Major cracks on stucco panel

Since I received this question, we has discovered new ways to prevent cracks
by using acrylic additives and the discovery of different kinds of sand.

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