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Removing Coal Tar from Stone

Updated: Oct 9, 2020

It goes without saying that flipping homes, restoring buildings and any sort of hands-on work can come with its own challenges: challenges completely unknown until the day they appear. It could be a poor foundation, structural issues unseen until the walls came down, rotted floorboards or outdated electrical – to name a few.

The O'Rourke Building is no different; one of our current challenges involves coal tar. Coal tar has a variety of uses, but to you and I, it’s most commonly used on asphalt roads and more commonly known as just "tar." Coal tar was a common material for roofing, specifically flat roofs, many decades ago. In fact, you may recall the scene from 1994 film The Shawshank Redemption when Andy and his "friends" tar a roof.

Image rights of Castle Rock Entertainment and Columbia Pictures.

Coal tar was widely used because it was water repellant. It was viewed as a seemingly impenetrable substance that would keep the inside of homes dry and protected from various weather. It was also relatively cost efficient and, for the day and age it was used, the best form of weather proofing available.

In our case, while we do not know for sure, we suspect that the O'Rourke likely had a tar roof at one point. And even if it didn't, tar was still used on the roof ­­–– applied on the tops and sides of the stones lining the perimeter. We believe this was done with good intention to protect the building from the elements.

Coal tar on the parapet of the roof of The O'Rourke.

Fôrt and Hōm would like to remove the tar from the stone. "Getting the building back to its earliest state as much as possible is our goal, and the tar was not a part of the original roof," says Construction Manager, Greg Genho, of the project.

There are several options to remove tar. A quick Google search will show a variety of chemicals available for purchase while message-boards give suggestions ranging from gasoline or baby oil to mayonnaise or WD40. Chemical removers aren't a favorable choice, as the porous nature of the stone could result in damage and discoloration.

Another approach to removing the tar is to sand it away or grind the stone down. However depending on the area needing it, the angle could make it difficult and especially tedious. Hand sanding would be a huge time consuming task and and electric sanding could be very jarring to the stone (especially one of this age) that could result in further damage.

The O'Rourke is registered on the National Registry of Historic Places making this predicament extra special because historic building restoration must follow federal and local guidelines. Therefore, a third option, which follows these historic preservation guidelines, would be to freeze the tar with dry ice or the use of carbon dioxide and then chip it off. Based on rehabilitation standards outlined by the National Park Service, U.S. Department of the Interior, this technique is the most gentle way of removing the tar from the parapet stone. And thus, the answer to our challenge.

More photos of coal tar on the perimeter stone of the roof varying in thickness and surface coverage.

You may be thinking, "Why even bother with it?" To which Greg reminds us, "We want the building to be complete from the basement to the roof, that means nothing gets missed. "Removing the tar is essential to that and is one step closer to bringing the building back to its original glory."

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