Technical Aspects of Hardcoat Stucco

In Moisture Intrusion by Morgan McClure

Cracks, Control Joints and Drainage (ie: barrier and drip or weep screeds)

Stucco cracks are without a doubt the most asked about questions associated to stucco systems. Understanding that and without an indepth study on the matter, the following information hopefully will answer your questions and give you information typically not provided in most general inspection reports.

Although cracks are the result of many different factors, some natural and some the result of construction or application errors, typically they are cosmetic only, common in hardcoat stucco and not associated to damaging moisture infiltration. In some cases cracks can be of grave concern and need to be understood so that proper modifications and/or repairs can be made. Unfortunately due to building codes that adopt ASTM (The American Society for Testing and Materials) standards there are some conditions associated to cracks that can be difficult to avoid. Specifically, ASTM standards that relate to stucco systems, both ASTM C1063 & C926, do not appear to contimplate real world conditions and are therefore, in my opinion, incorrect and should not be used in the application of stucco systems. I believe through a better understanding of these problematic code requirements one can provide for better application and repairs details.

ASTM C926: Under A2. Design Conditions section A2.2 establishes design requirements for Drainage Behind Exterior Plaster. In this section it provides for the following requirement: “a drip screed and throughwall flashing or weep holes or other effective means to drain away any water that may get behind the plaster should be provided”. The problem with this requirement is it suggest water will drain behind the stucco, a condition of which I have never seen. In every case that I have found where water has entered behind the stucco, long before it ever gets (drains) to the base of the wall, the water has damaged the barrier and entered into the structure. Plainly put there is no such thing as Drainage Behind Exterior Plaster and I do not believe anyone should ever consider it as part of a functional design element of a stucco system.

ASTM C1063: Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster section # defines the standard for installation of Control (expansion and contraction) Joints. Looking at it from an engineering prespective, installation of control joints may sound quite logical, but my field examinations reveal that in the real world control joints do not eliminate field cracks. In fact, cracks at control joints are often the cause or source of very eggressive and damaging moisture infiltration.

It is very important you understand that information I provide comes soley from what I call “real world” conditions. Its weight is on what I have observed as the result of the inspection of thousands of stucco applications and takes little consideration to either code or ASTM standards. This is not because I beleive in all cases these codes or standards are incorrect it is rather that I find actual conditions fact. Below are just a couple examples of my findings:

One of the major problems I have with control joints is if their intent is to be a design point of stress relief then doesn’t it make sence that relief would be in the control joint itself. If that is the case then the question begs to be asked: Is a crack in a control joint better or worse than in the stucco panel itself? The answere is, it depends. Due to the shape of the control joint, if a crack on a horizontal joint is on the upper side it normally would be of little concern (typical of general field cracks) although if the crack is on the lower side of the control joint it can result of excessive infiltration. The attached photograph shows the control joint and what appears to be a very small crack at the lower side of the joint. The clay mold is used for the purpose of testing the crack for infiltration.

This photograph was taken after the stucco was removed to expose the barrier material. Note the damage to the barrier begins just below the control joint.

This photograph shows the entry point of the water. Note the following important findings:

  1. The point of water entry is the low side of the control joint.
  2. Two layers of building paper failed to keep water from entering into the structures substrate.

This photograph shown the interior side of the wall with the drywall and insulation removed. Although I do not believe this picture needs an explaination the evidence demonstrates water was not able to drain behind the stucco.

In an attempt to determine the cause of some moisture related damage to some interior drywall we conducted some water tests at different points of the exterior wall and discovered the source of the water appeared to be at the control joints within the stucco system. This photograph shows an area where the stucco was removed to expose the substructure and barrier.

This photograph shown the back side of the removed stucco panel. The horizontal line dividing the 2 stucco panels is the horizontal control joint. The panel is as it was on the wall with the upper being the upper portion of the panel and the lower being the lower panel which was below the control joint. Note the following:

  1. The upper panel has not been exposed to moisture related rust in the lath and staples.
  2. The lower panel, just below the control joint, has been exposed to moisture infiltration and related rust to both lath and staples.