Plaster (gypsum and Portland cement) directly over clay tile units have been done for centuries in Europe and decades in America.


Plaster and masonry are simply made for each other and belong together. This love story between stucco and masonry is not new and did not start in North America. Like Romeo and Juliet, these materials are well suited for each other. The intimate bond between the two started before any piece of paper or code was needed to sanctify their union. The attraction was strong right from the start, long before they ever came to North America. The Romans and Egyptians observed this love: they embraced and even celebrated it. They knew the two belonged together and that the intimate bond the two shared should never be broken.

Unfortunately, like any good love story, there is always a third party that can come between the two lovers. Building paper, aka water-resistant barrier, was the third wheel in this love affair. Couldn’t building paper see how happy the two were?

Plaster and masonry are so compatible because they are cut from the same cloth, or stone in their particular case. Dually, building paper has its own perfect partner in framing. But apparently, building paper wanted to broaden beyond framing, and it set out to join the harmonious bond between stucco and masonry.

Building paper is used over framed walls to provide additional water protection. While great for framing, plaster bureaus do not recommend this practice over concrete/masonry walls.

HISTORY

In all seriousness, the above description is not as far off as you may think. Building paper and housewraps are needed for framed walls but are not recommended for masonry/concrete substrates. Cement plaster and masonry/stone substrates have been used for centuries all over the world. That changed when cement plaster came to North America.

America did not have the afforded luxury of an over-abundance of stone masons around every village. What America did have that Europe did not was an abundance of wood and lots of it. Building with wood is faster, cheaper and easier than building with masonry. Plastering over wood framing presented challenges and was quite different than plastering over masonry. It did not take long for Americans to realize that cement plaster over framed walls did not work like it did over traditional masonry.

Plaster over masonry in the early days of American history was used on Mission San Juan Capistrano, built in 1775.

Unlike the mass of the masonry wall that would typically prevent incidental moisture from reaching the interior, framed walls needed extra protection. Asphalt saturated felts used on roofs were used as the secondary moisture barrier to collect the minor or incidental moisture that found its way past the cement plaster. They worked OK, but long-term vapor issues created the need for a breathable felt, thus asphalt saturated building paper was invented and has proven to work admirably in all parts of North America. In fact, over time it became apparent that Europeans are the experts at plastering over masonry and Americans at plastering over framed walls.

So a natural conclusion would follow: If adding building paper between framing and exterior plaster worked so well, why not put it between cement plaster and over the masonry or concrete as extra protection-it’s a no-brainer.

I do not pretend to be a building science guru; I do not have a degree in engineering, and nor am I a scientist. But, I do know stucco. I have applied it, supervised it, detailed it and inspected more stucco than I can possibly remember. I have seen stucco that has worked and stucco that has failed. I am not the only expert on stucco; there are many other experts and I would venture to say that 99 percent agree with me on the following point.

Building paper or housewrap between a cement plaster and a masonry/concrete substrate is just not a good idea.

Worker attaches metal lath to concrete walls before cement plaster is applied.

WHY NOT EXTRA PROTECTION?

Why would the plaster bureaus say that? It seems the extra moisture protection would be a great idea. However, the fact is you will never get that extra protection by adding a building paper. Here is why:

Reason 1: The only feasible way to attach paper to the masonry or concrete substrate is to use paper-backed lath and shoot pins into the masonry. Paper-backed lath are sheets of building paper glued or adhered to the lath. This allows paper and lath to be installed in one operation. The problem is the power-actuated pins make holes in the paper that do not resemble the tight seal achieved with staples, nails or screws into a framed wall. The same sealing quality is simply not there. You will have significant holes and tears in the paper, through no fault of the installer.

Reason 2: Consider the fastener attachment of the lath/paper onto the masonry; it is preferable to hit the mortar joints to avoid spalling masonry. How do you see the grout joints through the solid paper? You have to guess; guess wrong and you can blow apart the brick. This leads to the next problem. How do you know you have driven the fastener “home” into the cement or masonry as the manufacturer requires for a proper attachment? The answer, you don’t. Concrete nails do not fair much better. No matter how careful the installer is, the seal is not going to be what the designer was looking for. The extra moisture and the attachment of the lath/paper to the substrate will always be a guess.

Reason 3: Adding building paper will defeat the desired bond between the cement substrate and plaster. With no bond, the lath/paper system must now support the full weight of the stucco membrane. Remember, we do not know how well it is attached. The plaster becomes its own diaphragm and any movement in the structure can result in the plaster being pushed away from the substrate.

Tapping along the concrete/masonry wall with paper/lath and plaster will reveal where the hollow spots are, and they will be there. This is not a good thing and typically results in excessive and unnecessary cracking.

This does not happen with framed walls, partially due to the fact framed walls move and act quite differently than monolithic masonry or concrete wall. With concrete and/or masonry, we want that intimate bond between the two; it is critical to the success of the wall. Europeans know this from centuries of experience.

Reason 4: The questionable bond for the lath/paper combo provides less moisture protection. In freeze/thaw climates, it can be catastrophic. I have seen it many times. My stories about failures when building paper was inadvertently used for extra protection of masonry for the application of stucco are many, but this is my favorite war story.

Cement plaster popping off a wall.

A GRIM FAIRY TALE

At 3 a.m., the guards at the Federal Reserve Bank heard a load crash on the roof. Assuming the building was possibly under attack, the local SWAT team was scrambled to protect the Federal Reserve. It turned out that the masonry penthouse on the roof of the Federal Reserve had cement stucco with a building paper applied. The lath/paper was attached with concrete nails, and looked like a good and reasonable installation. Over the next few years, water seeped in and got behind the building paper, through the unavoidable holes and tears in the building paper (see Reason 1). The water froze, expanded and then pushed the plaster off the masonry wall.

I was with the local plaster bureau at the time and got the call to investigate. I had a presentation to do that day for the city’s largest architectural firm and wore my most impressive black suit. I went through the metal detectors and was told to wait. The guard then went on his break. The new guard spoke with me and then radioed upstairs to remind them I was still waiting. A group of distinguished looking gentlemen came to apologize and led me to the elevator. They seemed a little nervous and very apologetic, which I thought was odd. One of the gentlemen said, as he led me into the elevator, “I was certain we passed our inspection cleanly last month.”

The elevator stopped on a mid-level floor and the doors opened, I was looking out at two heavily armed guards and rows of steel cages filled with tremendous amounts of cash stacked almost to the ceiling. Another gentlemen said sternly, “You will have to check your weapon here. We have to follow the rules, no exceptions.”

“I think there may be a mistake,” I said.

The one gentleman seemed slightly annoyed and said, “No mistake, even though you are with the Bureau, you have to check your weapon.”

“Yes, I am with the plaster bureau” I responded.

“You are not with the Federal Bureau of Investigations?”

I went from hero to zero and was unceremoniously shuffled up to the maintenance room on the roof.

Popped plaster was removed to discover building paper was used over the CMU.

I JUST GOTTA HAVE IT

So, if you can’t count on a building paper with lath as a secondary moisture barrier, what should you do? Typically, nothing; cement plaster, properly mixed and applied will provide superior moisture protection; it has in Europe for centuries.

If extra moisture protection is deemed absolutely necessary, there are options. The best is to use a highly water-resistant coating over the masonry, which will allow the cement plaster to bond to it. Most EIFS manufacturers make these products-most are trowel applied to a nominal thickness of about 1/8 inch. The grout joints are also typically still visible through the coating, which allows for lath attachment at the grout joints. The important factor is to be certain the coating allows the cement plaster to bond to it.

Some consultants feel this is unexplored territory and the plaster industry is not sure what to do. Plaster bureaus have always recommended against building paper over masonry or concrete substrates. The plaster industry does not purport to be building envelope experts or scientists with a full understanding of all hygrothermal properties, but we know stucco and are experienced in many applications. If the vast experience of the plaster bureaus and common sense of what was described does not change the consultants mind, how about appealing to their romantic side; can we just leave the intimate bonds of this perfect marriage alone? W&C