Old, Soft, Crumbly Mortar
I had an appointment today at a house built in the 1750s. The owner has lived there for over 20 years and it was obvious that she has been meticulous about maintaining her historic home. She scheduled an appointment with me while in tears over some news given to her by a masonry contractor.
She had been noticing that some of the mortar joints on the exterior stone walls were eroding. In some spots near grade, the voids between the stones were about an inch deep. Being the conscientious old-house owner, she called some masonry contractors for repointing estimates. Only one called her back. He showed up a few days later to give her an estimate.
In less than 10 minutes of scraping and prying some mortar in a few spots, this contractor declared the old mortar to be soft and crumbling--not much more than sand and powder remaining between the stones. He explained that in order to repoint, he'd have to scrape out all the old mortar until he hits good, solid mortar. However, he declared that there was no solid mortar left, and any effort to try and save the old walls would be well into six figures. He then went off into describing how he and his carpenter friend's crew could rebuild the walls the "right" way --with wood framing and a manufactured stone veneer applied. He described a major advantage through lower heating costs because the new walls would be insulated.
I'm quite sure this masonry contractor has absolutely no experience with mortar made before the discovery of Portland cement. Most mortar in homes over 100 years old consists of mostly lime and sand, with local clay and hair from livestock often mixed in. This is a very soft, loose mortar, unlike recent mortar that contains cement.
After my evaluation, I explained to the homeowner that old mortar was not intended to be an epoxy or adhesive that glues the stones together. It was never really a congealed to a rock-solid state and it doesn't completely bond to surfaces like modern mortars. This mortar was used to "bed" the stones, filling in the spaces between the uneven stone surfaces. It's not surprising to me that this mortar performed its duty for two and a half centuries. Every stone was still right where it needs to be. I found that there had been a few small areas that were previously repointed, as the repair mortar had more of a gray tint and felt very hard.
As I expected, this old stone house just needs a few areas of repointing. That soft, crumbly mortar just needs to be raked out of the joints that need repointing, enough for the new mortar to "key-in" between the stones. When applied properly, the new pointing mortar should do just fine holding the 250-year-old mortar in place between the stones.
This isn't the first time I've encountered a contractor condemning an historic masonry structure. On more than a few occasions, I've been called in to give an opinion on stone foundations that have been declared "crumbling" from moisture problems. Many times the old soft mortar had been wrongly identified as failing because it wasn't congealed and bonded to the stones. In my experience, unscrupulous or uninformed "waterproofing" contractors point to the loose consistency of the old mortar as a major problem and a reason that the home needs immediate and expensive repairs.
I gave my relieved client the contact information of an experienced and knowledgeable historic masonry restoration contractor. I'll follow up by sending her a report of my findings and links for information on the proper repair and maintenance of old masonry and mortars.
I'm quite sure the cost of the repointing I suggested, done by an experienced restoration masonry contractor, will cost a fraction of what her initial contact would charge to destroy her historic home. I'll bet the first contractor's experience as a stone artisan is limited to gluing fake stones onto McMansion facades.
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.