A lot of folks confuse plaster and stucco in old houses. While they are similar in a lot of ways they are different enough that understanding when and where to use them, especially in renovations, is imperative.
Their differences are not readily apparent to the naked eye. Plaster and stucco can both be applied much the same way with the same textures and thicknesses, but the main ingredients in each are what make them different animals.
Today’s post might be a little more building geek than hands on preservation, but I think that understanding how and why your old house is the way it is makes you not only a wiser human but a better homeowner. So let’s get our geek on!
Plaster vs Stucco
Like I just mentioned the ingredients are what make the difference in these two similar building materials. Both are made with an aggregate of some kind, usually sand of various sizes and textures. Both need water to mix everything together and activate the curing process.
The big difference is the binder. The binder is what holds everything together and cures into the rock hard consistency we all know and love/hate.
I’m going to preface this post for all you plasterers out there by saying that this is a purposely oversimplified explanation of the topic. Plaster, stucco, render, etc have been around for millennia with so many variables and formulas that a 1000 page book couldn’t cover everything. My purpose is to help homeowners get their feet wet on the topic.
Depending on what kind of plaster you have you will have one of two different binders, lime or gypsum. Let’s looks at both:
- Lime – Prior to the early 1900s, lime was the primary binder in any plaster. Up until this time it was cheaper to acquire than the expensive gypsum plasters. Lime plaster is a rather laborious process of mixing water and lime and waiting for the lime to “slake” before it can be added to the sand for application. Lime also needed the addition of fibers like horsehair to help strengthen the wall. Lime plaster also takes an extremely long time to fully cure, sometimes taking as long as a year before walls could be painted or wallpapered.
- Gypsum – You’ve probably heard of Plaster of Paris? Well, that is gypsum plaster. It got the nickname because in the 1700s Paris was built on top of some of the largest natural gypsum deposits in the world at the time. Most gypsum plaster came from Paris and so the name stuck. Unlike lime plaster, gypsum cures fast (only a few days) and sets even faster which is why we make casts for broken limbs with the stuff. It typically didn’t require horsehair to strengthen it either like line plasters do.
Gypsum plasters are interior plasters typically not meant for exterior use because they do not do well with repeated exposure to water.
Like plasters however can excel outdoors under most weather conditions and with certain additives to ensure they have a long useful life.
If your house was built before WWII then your walls and ceiling are usually a combination these two materials. Pre-1900 homes likely have all lime plaster walls, while homes in the first half of the 20th century may have walls with a scratch and brown coat of lime plaster and finish coat of gypsum. Slowly but surely gypsum has replaced lime for interior plaster due to its faster drying time.
That doesn’t mean gypsum is a better materials simply that the building industry has gone to gypsum. Personally, I prefer an old like plaster any day even if most of the plastering I do these days is with gypsum.
So you know what plaster is and what it isn’t now, right? So, what is stucco then? Stucco, sometimes called “render” by our neighbors across the pond, is an exterior coating that historically wasn’t much different from lime plasters.
Like lime plaster is was made up of slaked lime (or a variety of other less typical natural binders, sand, and water but it was also given numerous additives to help it withstand the elements. According the National Park Service Preservation Brief 22 stucco sometimes contained any number of these unusual ingredients:
Mud, clay, marble or brick dust, sawdust, animal blood or urine, eggs, keratin or gluesize (animal hooves and horns), varnish, wheat paste, sugar, salt, sodium silicate, alum, tallow, linseed oil, beeswax, wine, beer, or rye whiskey. Waxes, fats and oils were included to introduce water-repellent properties, sugary materials reduced the amount of water needed and slowed down the setting time, and alcohol acted as an air entrainer.
All of these additives contributed to the strength and durability of historic stucco. In 1824 a new invention changed the way stucco was used in buildings.
Portland cement was invented in 1824 in England and named as such because its inventor, Joseph Aspdin, thought the cured concrete looked like Portland stone which was a popular building material in England at the time.
Portland cement mixed with sand and water creates a very strong stucco much harder and more brittle than like stuccos. While Portland cement gave us the ability to create harder stuccos than lime and natural cements typically did what we gained in strength we lost in repairability.
Portland cement made its way into all kinds of things like mortars too and it creates such a hard material once cured that it can do serious damage to old buildings if installed where lime plasters, stuccos, or mortars should be because it’s rigidity is so dissimilar to many historic materials that Portland cement can result in further damage to historic elements.
Unlike lime it cannot be renewed as time and Mother Nature slowly wear away at the softer lime coverings.
So should you ever use Portland cement in your stucco? Well, that depends on whether it was in your original stucco to begin with. I always recommend replacing materials with like materials. If you have Portland cement stucco then by all means patch and repair with a similar formula stucco, but if you have lime stucco then don’t even think about repairing with Portland cement unless you want more repairs down the line.
So there you have it! Stucco and plaster. Plaster and stucco. The same but totally different. I hope you enjoyed getting you old house geek on with me today. Until next time fight the good fight and save that old house!