Your Place: What 'Irish' plaster is and how it can be fixed

Posted: May 06, 2011

Question: I read your December article about basement leakage. I, too, have a basement foundation of fieldstone. The house dates to 1835 or thereabouts. You mentioned that this basement construction made use of "Irish" plaster. Can this fieldstone wall be sealed with Drylok or some other masonry waterproofing treatment?

Answer: A lot of people asked what "Irish" had to do with the style of construction, so let me explain that first. The "Irish" applies to the nation of origin of the masons, who devised a way to rid themselves of job-site debris and save on construction costs.

When the bosses told the Irish workers to do something with the clay soil excavated from digging a home's foundation, the workers mixed it with lime and used it to plaster the inside basement walls.

These construction materials must have begun turning to reddish powder in a matter of years, and gathering on the edges of the inside walls - at least, that's where I was sweeping it up in the house I once owned that was built in 1904.

In the mid-1990s, I helped out with the youth group at a church that spent several months of the year preparing to work on the Pine Ridge Reservation in South Dakota.

To gain experience, we rehabbed houses with soon-to-be homeowners for Habitat for Humanity, and every house we worked on had these crumbling basement walls. That's when the Habitat people introduced some of the teenagers to pargeting.

Pargeting comes from a French word meaning "to coat a surface with plaster." The technique, easily learned, can be invaluable to anyone who lives in a stone house built between 1890 and 1920.

What we were told was that the soil-lime interior coating had been designed to be replaceable, since its purpose was to shift when the exterior stone expanded and contracted during normal freeze-and-thaw cycles.

No one bothered to pass on the information to subsequent generations of owners, and these interior walls started to flake off.

This isn't a flaw; this is the way the coating had been designed. To fix, our instructors told us:

Check the pointing on the stone on the exterior wall. If it is loose or missing, it needs to be replaced because moisture will cause havoc with the interior walls.

The interior walls should first be cleaned with a wire brush to get all the loose material off so that the new coating will adhere to it.

Then mix one part Portland cement, three-quarters part limestone, and an amount of sand no less than 2.5 times and no more than four times greater than the amount of cement (for 10 pounds of cement, that works out to 7.5 pounds of limestone and 25 to 40 pounds of sand), and add water until the mixture is the consistency of pancake batter.

Wet the surface of the wall, and spread the mixture on - not too thick - with a trowel. If it doesn't bond to the surface, there could be too much water.

Never mix more than you can use in 30 minutes.

Can you use Drylok? If there is a moisture issue originating from the outside, no amount of Drylok will solve the problem. The water will just find a way in somehow.

First, address the moisture problem - poor drainage, crack in the foundation below grade, whatever - then coat the interior walls with Drylok or a similar product if you want to.

The pargeted wall that my group worked on was never coated with anything.

On the other hand, parts of the walls of a basement I regularly visit, pargeted in the mid-1990s, do have some damp spots, meaning that while the issues of crumbling were solved, the moisture had another source.

Again, make sure the source of the basement moisture is identified and corrected first before you tackle the inside walls.


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