Makeup and Uses of Stuccos and Renderings
Stucco is a plaster or cement rendering that is applied to the face of walls or ceilings to give a smooth aesthetic. Stucco and renderings also have a functional application as well to increase moisture resistance of a partition or masonry assembly. Stuccos and renderings have been used in construction going back to historic times and can be applied on top of a variety of substrate materials. Some of the main substrate materials applicable for a render coating application follow:
- Brick masonry
- Block masonry
- Stone masonry
- Cast In Place concrete
- Plywood or cement or gypsum backer board sheathing
Traditional historic building facades are generally built with brick or stone masonry. The building in the picture above was built with double wythe brick like so many of the historic buildings of Capitol Hill, Dupont Circle, and Georgetown neighborhoods of Washington, DC.
Often beautiful architectural ornature will appear to be an elaborate stone carving, but can in many cases may actually be instead built with plaster or rending stuccos. Often well built plaster or cement renderings can successfully fool the eye into believing the building facades are tediously built stone carvings. By comparison, form or track knife plaster or rendering applications are much faster and therefore less expensive to build. This work is fine craftsmanship and even the faster approach using a track mounted to a wall and a plaster shaping knife on a guide is much slower than installing wood crown mouldings, by comparison.
One of the biggest differences between wood crown mouldings versus mortar and plaster mouldings is that the size of wood mouldings is generally much smaller. Historic properties of architectural value generally had very large plaster crown moldings.
The plaster and cement type stucco materials are very similar to the same materials used for making the plaster and cement moldings. By comparison a stucco facade application will generally be flat. There are always details though at the areas outside of the field. The field is the main area of a wall that is generally uninterrupted and continuous. The other areas of the wall such as the perimeters,
terminations, edges, transitions to windows, doors, and other items will be more detailed and require additional types of work and processes.
Binders and Aggregates
Cements, mortars, plasters, concrete, gypsum, joint compounds and stuccos can include a variety of subcomponent materials. Some of those subcomponent materials are lime, gypsum, aggregate, stone dust, silica, sand, pozzolan, Portland, and more. Together all of these technical terms make up a confusing sounding list of materials. However, it is not that confusing when you look at each item closely on an individual basis.
To simplify things more, we can look at plaster, cement and concrete as really just composing two main constituent material types: aggregates and binders.
Aggregates are essentially reactively inert. They play a really important role in the building materials that we are discussing, but they do not create a chemical reaction. Examples of aggregates are the obvious things like rock and stone which are a major part of concrete and the sands and fine silica materials which constitute the majority of mass and volume in mortars and renders. These aggregates are essential for making concrete and renders work the way they are intended. Thick concrete slabs, for example, rely on a comparatively large size aggregate, often over one inch in diameter. By comparison, base coat or scratch coat stuccos use a coarse sand as an aggregate. Topping coats may use very fine sand.
A photograph of a crushed blue stone aggregate, used as an underground pipe bedding follows below for reference. This aggregate could also be used as a concrete; however, it is too large for use in most stuccos and renderings.
By comparison, sand is similar but used in thinner renderings, where larger stone aggregates are used in thicker applications. This is one of the principle differences between concrete versus stucco and renderings. A picture of masonry sand follows below for reference. In this case, this large batch sand is shipped and moved on construction sites in large cubic yard sacks.
Binders though, on the other hand, make these building materials stick together and, in some cases, stick to substrates. Binders are generally chemically reactive. Examples of active binder ingredients include cements, plasters, lime, and polymers.
Substrate Adhesion and Bonding
The most important part of building construction quality control and therein durability and performance of good plaster and rendering is substrate preparation. Good and proper substrate preparation leads to strong adhesion and will translate to longevity.
There are several important considerations in planning for bondability.
- Material compatibility
- Substrate preparation
- Temperature control
- Cure rate
- Material application and mixing controls
A lot of construction work can be done through muscle and strength, but by contrast planning for bond strength requires analysis. It is good to consult an experienced contractor who really understands material science, when planning this type of work. Experience and education is needed to understand the technical elements of compatibility. For example, polyurethane type materials bond well with natural materials because urethanes are often hydrophilic. On the other hand siliconized materials often fail to bond well to most carbon based materials and often performs better on synthetic materials that are vitreous, metallic, or polymer in nature because siliconized materials are often hydrophobic. To build for good bond strength, it’s important to analyze the substrate and then also understand the material that’s being applied. We recommend consulting an experienced professional and an engineer for large scale or critical applications to request a design specification.
Substrate preparation is mostly about two things: profile and removal of bond breakers. A profile of a surface is the amount of texture of the surface. Very smooth surfaces like glass do not have much profile. Rough surfaces like many natural stones have a high level of profile. Surface applications’ bond strength generally benefits from a greater profile with a coarse or rough surface. In some cases, application materials will require that the substrate surface be abraded or even scarified to allow for better adhesion. When applying stucco directly to wood or or impermeable surfaces, it’ll often be required to first install a high textured metal fabric such as expanded metal lathe or even a stucco hardware fabric which resembles chicken wire.
Dirt, oil, grease, laitance, and saponic materials are forms of impurities which act as bond breakers. Essentially, bond breakers are materials on the surface of a substrate that prohibit the bond of an applied material. To understand this concept, consider a greasy surface. If you bake cookies directly on an ungreased pan, the cookies will stick to the pan and be very hard to remove or clean. On the other hand, Greasing the pan before applying the cookie batter, will allow the cookies to be easily removed. In this case, grease is working as a bond breaker. The same concept applies to building materials. If you apply a stucco, cement, or rendering on top of a dirty, oily or greasy wall, you will also have a problem related to lack of bond strength.
Hydration is another important consideration. Some materials, as explained above, perform better on dry surfaces and others perform better on dampened surfaces. For example, when applying a rendering coat to a masonry substrate, it is generally important to dampen the surface before application of the new material. That might sound counterintuitive, but if a surface is too dry before a cementitious rendering is applied, the substrate may wick water out of the new material similar to a sponge. By dehydrating the newly applied material, it can cause premature drying and prohibit a full chemical bond.
Similarly, temperatures that are too hot can also dry out materials too fast and cause premature dehydration. On the other hand, newly applied cements, stuccos, renders and limes may be damaged by temperatures close to or below freezing. Water molecules expand or increase in size when frozen. If the water that activates concrete or cement freezes before it is fully cured it can critically damage the application, so environmental controls are extremely important. Temperature ranges are extremely important in cold places like Washington DC with its bitter cold winters.
Deterioration of Stucco and Renderings
With the average stucco application being somewhere between 40 to 100 plus years old, we have a tendency to see them in states of significant aging. Some of these applications are deteriorated and have both issues of cracking and/or delamination. The issues are comprable and have similar root causes, but we can classify them as 2 different types of issues. Lime acts as a decent binder. It works well with cycles of hydration and can withstand decades of exposure to the elements. However, lime deteriorates, in most cases, faster than Portland cement. Portland cements are generally much stronger, and have a higher level of cohesion and adhesion.
The word cohesion and adhesion are often used interchangeably, depending on the context. In certain contexts though, those 2 different terms can mean different things. Cohesion, for example, can describe a material’s integral ability to stay together to itself. In the case of using the term cohesion to describe a material’s ability to stay together, integrally, we are talking about the strength and durability of the material’s intrinsic binder.
On the other hand, the term adhesion can be used to describe a material’s ability to stick to a substrate. When a material fails to stick to its applied substrate, we call that delamination. A material’s level of adhesion can degrade and fail over time for a variety of reasons, as well.
In the image below, you can see an example of a historic exterior stucco wall with significant deterioration.
Delamination is an effect of deterioration, but more specifically delamination is when an application such as stucco or rendering loses its adhesion to its substrate. For reasons similar to deterioration, delamination happens when the microscopic structures which hold mortar or stucco in an adhesive nature, degrade.
More than anything else, when a material fails to adhere or delaminates from its applied substrate, the root cause can be traced back to one or two principal causes:
- Poor substrate preparation way back at the time of the initial installation or application.
- Moisture at the substrate level, causing dissolution of a binder.
To really diagnose the issue and prescribe the appropriate treatment, the area should be sounded with a hammer. Hammer sounding is the process of tapping the wall surface with a hammer and carefully listening to the resultant noise. It is actually a simple process, but a beginner might not be able to discern the difference between the sound of an application which still maintains substrate adhesion and one that has failed. When there is delamination, a hammer lightly striking the surface makes a very hollow sound. The sound is louder and deeper. When hammer sounding an application which still has integral adhesion, the resulting sound is quieter and higher in pitch.. An experienced tradesperson can recognize the difference.
Integral cracking is similar but different than a limited and isolated amount of cracking. In the comparison below, we show two photos side-by-side.
Cracking is not always indicative of delamination; however, the cracking at the left side image above has a pronounced shadow. That dark shadow is likely not only related to shrinkage, movement, or separation, but in this case also related to delamination. Therefore, in this case we would also recommend hammer sounding the area. If delamination has occurred, removal of delaminated stucco might be the best approach.
If the hammer sounding proves there is still substrate adhesion, the crack can be possibly scored and filled. That type of repair might last for years, but won’t necessarily stop future movement, shrinkage and cracking.
Amelioration of Failing Historic Applications
Both of these issues, delamination and integral cracking are similar in some ways, but they manifest in different ways and require different treatments. One possible method of treatment is complete removal of the stucco or rendering and wholesale replacement. Wholesale replacement should be treated as a last and final measure if absolutely no other options are feasible or plausible.
Historic plaster preservation, upkeep, and repair can be difficult for several reasons.
- It is hard to access the area of delamination, in a limited extent. If partially delaminated coatings cover the substrate yet lack adhesion, it can visually undetectable.
- Thick applications require the use of traditional approaches for repair: the build-up of several coats of new materials. This complicates identification of delaminated coatings and it complicates the repair.
- Water infiltration sources are often a cause of accelerated deterioration. Water sources can be hard to identify in render and stucco surfaces.
More practical but obviously less extensive measures include the following limited approaches
- Hammer sounding the surface to identify limited delamination points.
- Spot repairs can possibly add years to the life of render and stucco surfaces if repairs are done early.
- If adhesion is largely intact, limited cracks can be addressed individually by scoring and repairing.
Stucco facade element’s are very common in many details of the historic buildings of the Capitol Hill, Georgetown, and Dupont Circle and many other neighborhoods of Washington, DC. The topics of stucco and render makeup, binders and aggregates, substrate adhesion and bonding, deterioration, delamination, cracking, and amelioration provide an important foundation for understanding stucco and renderings on historic buildings.
In this article we talked about the following terminology and concepts, follow the links below for more related information from the IDS website:
- Cement rendering
- Moisture resistance
- Backer board
- Stone carving
- Concrete forms
- Plaster track knife
- Joint compounds
- Material science
- Siliconized materials
- Surface profile
- Material bond
- Bond breaker
- Expanded metal lathe
- Environmental controls
- Hammer sounding
- Material shrinkage
- Water infiltration sources
- Accelerated deterioration
- Scoring of cracks
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair,
The links in the list above will take you to other articles with more information on defects, failures, preservation and repair of historic masonry. You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.