The Challenges of Lath and Plaster in Winter Months

Carl Fernald

January 2005

We all know that weather affects the installation of metal lath and plaster, but do we really understand all the different challenges that we have with the winter months? After all, here in Southern California the winter is our rainy season as well as a drop in temperatures. And don’t forget that winter also means fewer hours of daylight. All of these—rain, lower temperatures and shorter daylight hours—have a substantial effect on the scheduled time it takes to install lath and plaster and our ability to install the system properly. We all have heard the phrases "time is precious” and "time is money,” but the truth is that during the winter months, for a lot of reasons, to properly install the lath and plaster system, we just need more time in the schedule.

We know that we are not going to be able to work in the rain. For most of us, that is not even an issue, but how many of us think about the day(s) after it quits raining? Are we still affected? There can be a huge impact on the schedule, or if we are ordered to proceed with the work before things dry out, there can be tremendous quality control issues.

STEP ONE: AFTER THE RAIN

The first step of the lath process is to install the 60-minute paper over the wood framing. If this is done the day after it rains, this step impacts the ability of the wood to dry out properly. And, if this is done and the wood does not dry out completely, then insulation and drywall are installed.

The result is that we install the plaster on the outside of the wood frame, which is about 12 to 15 pounds per square foot. This weight must now be supported when the wood frame is drying out. Here is a quick list of why this situation does not promote a quality installation of the plaster:

  • When wood becomes wet, it swells. When it dries, for every 4 percent change in the moisture content of the wood, the size of the wood changes 1 percent.
  • In addition, as the wood dries, it will warp and twist.
  • The oriented strand–board sheeting can be affected even more than the framing members.

The result of the above items is that there is stress placed on the plaster system. This stress can result in cracks. My recommendation is that more time be allowed to let the wood framing dry out, at least to the extent that most of the moisture is out of the wood. With the shorter daylight hours, and the different exposure of all walls, i.e., north, south etc. it could take several days to properly dry out, which is necessary before proceeding to the next step.

STEP TWO: A THICK SCRATCH

If we follow the process along, when it is lathed and ready to install the scratch coat, the rain affects our ability to install a scratch coat with the best thickness. One thing that seems very clear from closely monitoring our experience is that fewer cracks appear in plaster when a thick scratch coat is installed, but this cannot be accomplished when the wall is wet. For example, during the summer months when the wall is dry and hot, we can generally install three-eighths-inch to one-half-inch during the scratch coat. When the weather is cool and damp and/or wet, the scratch coat will probably only be one-quarter of an inch thick.

My recommendation is that more time be allowed after a rain to ensure that the 60 minute paper and lath are dry enough to accept the full thickness of scratch coat material. If the temperature is less then 40 degrees, we cannot install any plaster material. Not only are the daylight hours shorter, but we must stop work even earlier since we need about four hours of curing time before the temperature falls below 45 degrees.

STEP THREE: BONDED BROWN

The brown coat should properly bond to the scratch coat. Part of the reason that this can occur is that the scratch coat has suction and when the brown coat is installed, it is immediately attached due to the suction. After rain, when the scratch coat is wet the required suction does not exist; the brown coat should not be installed. Depending on the post-storm conditions, elevations may remain wet for days. The scratch coat must be somewhat dry before the brown coat is applied.

My recommendation is to delay the installation of the brown coat long enough to make sure that on all elevations of the home, the scratch coat is dry enough that suction can and will occur. A quick check of the exterior walls should tell you when you can proceed. Also, if the temperature is less then 40 degrees, we cannot install any plaster material. Not only are the daylight hours shorter, but we must stop work even earlier since we need about four hours of curing time before the temperature falls below 45 degrees.

STEP FOUR: TROUBLESHOOTING

There are many problems that can occur from the installation of the color coat too soon after a rain. Here are a few for you to consider:

The color itself is actually very small particles that must be properly mixed in order to get a consistent, true color. Since the color must be mixed, it should be easy to understand that those same particles can move around; they are not an integral part of the cement. The walls, prior to the installation of the color coat, need to have a little—but not a great amount—of suction. For that reason, we typically wet them significantly during the summer and only a little during the winter.

In the summer months, due to the heat and lack of humidity, the walls dry fairly quickly. Once we install the materials, the quick drying of the walls helps stabilize the color particles. In the winter, the walls don’t dry as quickly and the slower movement of the water to the outside surface (the drying process), which is through the newly installed color coat, moves the color particles closer to the surface. We all know that the plaster material is installed in different thicknesses throughout the wall due to the different plane of the substrate. Due to the different thickness of the plaster materials, more or fewer color particles are moved. The more color particles move toward the surface, the denser the pattern, which makes the color darker in appearance. This leaves a mottled appearance that is often unacceptable to the buyer. Dark colors are achieved using more color particles; therefore, in the winter months, dark colors are affected significantly more than are the lighter colors.

My recommendation is to delay the installation of the color coat long enough to make sure that all elevations of the home are dry. Also, the color coat is a portland cement–based material and will be affected by the temperature. If the temperature is lower than 40 degrees, we cannot install the material. When the temperature is high enough for us to work, we still need to provide approximately four hours after the material is mixed before the temperature can be 45 degrees; therefore, the daily production is less.

Another typical problem that occurs in the winter months is the appearance of a crystalline deposit of salt on the surface known as efflorescence. Salt is a normal part of the raw materials used to produce portland cement. When there is water, such as from rain during the winter months, this salt can become soluable and is then transported to the surface as the result of evaporation. On the surface, it crystallizes as a whitish material. The obvious question is that if salt is a normal part of the material, and if water can make it soluable and then transport it to the surface, then why is this seemingly a problem only in winter? The answer is that during the winter months, there is more water, and that water does not evaporate as quickly as it does during the summer months. Due to the slower evaporation, the water remains in the plaster longer, and the salt has time to become soluable.

My recommendation is that the literature you provide the homeowner explains what efflorescence is and how to clean it off of the wall. While effloresence can be a very unappealing aesthetically, it can be fixed fairly easily.

STEP FIVE: THE PICKUP PROCESS

Pickup work brings on all of the above problems, but due to the nature of pickup, meaning that we have to match and blend in to existing textures and colors, the challenges are more enhanced.

To perform the patching, the scratch and brown issues are typically eliminated due to the use of a different mixture of materials that will set much quicker than normal plaster will. That does not mean that we can violate the temperature issues, however.

Color and texture are the real challenges for the pickup process. The appearance of the texture will change due to the dryness of the materials when it is installed. Since winter slows down the drying process, it can be very difficult to achieve a texture match. While this is not a problem on small areas, it can be a real problem blending the texture on large areas.

Matching the color is another problem. As mentioned above, the movement of the color to the surface darkens the color. Since the pickup work is typically performed a month or so after the color coat has been installed, it is easy to understand that the factory mixed fog coat material is not going to match. Therefore, the plasterer must job mix his materials. To see if the material is a match, the installer must install a small amount and literally guess how it will look when it dries. And to make this whole process even more difficult, if it rains after the fog has been installed, the color particles in the fog coat will move toward the surface, once again changing the appearance of the final color.

My recommendation is to never install any fog coat materials within four days after a rain or seven days before a rain. As you can see, during the winter months, with these criteria, it is very difficult to complete the work. If we have a wet winter, then no fogging will be able to be done until the middle of March or April.

All told, winter places many challenges to the plastering subcontractor, and these challenges affect all facets of our scope of work. For us to perform our work in an acceptable manner, we need the help and assistance of the builder. We need to be provided more time when it rains. We need to be provided more time during the season when the low temperature begins to drop below 40 degrees.

What are the reasons that a builder should help the plastering subcontractor? Cracks are not the byproduct of a normal plaster installation. They are typically the result of stress and movement that exceeds the absorption ability of the plaster. Here is a quick list of examples:

  • If the wood is not dry before the installation of the plaster, when the wood does dry, the movement causes stress on the plaster, which will result in a higher frequency of structural cracks.
  • If the paper is not dry when the scratch coat is installed, the scratch coat will not be as thick as it could be. The result will be a system that is not as strong and resilient as it could be.
  • If the scratch coat is not allowed to dry properly, then the installation of the brown coat does not get as good a key into the scratch, and the system will not be as resilient.
  • The typical cure time between scratch and brown coats is one day, and the cure time between brown and color is seven days. The purpose of this cure time is to allow the plaster materials to cure and harden—in other words, to get strong. During most of the year, the plaster has a 24-hour per-day curing process, but in the winter the curing process is limited to only the hours per day that the temperature is above 40 degrees and really above 45 degrees since the curing process is significantly slowed between 40 and 45 degrees. The result is that during the normal construction process, at identical times, the plaster is not as strong and resilient as it is during most of the year.
  • Homes that are plastered during the winter months have a more mottled appearance then those constructed during the summer months. Depending on the darkness of the colors, this will cause problems with the homeowner’s acceptance of the home as well as future customer service problems.

Construction cannot stop during the winter months. As a specialty contractor, we need to understand and communicate these challenges to the builders. We also must let them know the repercussions of their decision to force the installation of the plaster by not allowing enough time in their schedule to compensate for the challenges of winter.

About the Author
Carl Fernald is president of Precision Plastering, an EIFSmart Contractor located in Ontario, Calif.