Do’s and Don’ts for protecting your historic masonry

News January 23, 2014 0 Comments

A stone wall in need of some maintenance.

A stone wall in need of some maintenance.

by Peter Saylor

Recognized as a National Historic District, Chestnut Hill has been described by David Contosta and Carol Franklin in their newest book, “Metropolitan Paradise,” as an assemblage of “delectable details” formed by a sympathetic bond responding to original land forms, connecting fabric of forest vegetation, natural materials, and an astonishing variety of housing types with a remarkable range of architectural expression.

Blurring the boundaries between nature and the built environment, Chestnut Hill has found richness in what the authors coin “the Wissahickon Style.” A key element of this style came from local quarries and stone masons utilizing Wissahickon schist substantially in masonry construction, giving the community a unique texture and color that blended the buildings and garden walls into the surrounding bedrock.

Naturally, after over 100 years, much of this masonry heritage has reached a stage that demands simple maintenance or sometimes significant restoration pointing.

Understanding the “do’s and don’ts” as a homeowner about to make masonry repairs to a piece of the Historic District begins with selecting the right mason and having the fundamental knowledge about how to achieve the best results. The following guidelines for mortar color, mixes and joint styles, compliments of the schist specialist firm of Saxon Restoration LLC, serves as an excellent suggestion for the best chances for success.

Let’s start by saying that the most important part of any restoration job is the preparation. The joints should be removed to a sound foundation, at least as deep as the width of the joint and thoroughly cleaned.

Mortar color ranges all over the spectrum. Historically, the color comes from the sand. Brown, white and yellow sand all primarily determine the color of the joints. The amount of lime and cement also contribute. White or gray cement also makes a big difference. Lime proof dyes and pre-blended mortar mixes exist today, which makes matching color easier. But it still takes a trained mason and a good eye to really get close. Frequently you will see bright gray patches of mortar in an old wall. They stand out like a sore thumb. This occurs typically from the lack of experience of the mason, who may only know one way to do it. Samples are always a good option, while understanding perfection will most likely never be achieved in a mortar match due to biological growth, weathering and staining, and the effect UV rays and carbon have on the fading and staining of the joints. But you can get very close!

Historic stone and mortar mixes, which were used in this region, are comprised of sand cement and lime. Portland cement only began being incorporated in the mixes around the turn of the 20th century. Prior to that most mortar mixes were lime based mixes. Today, hydraulic lime mortar mixes are being re-introduced into restoration projects more and more.

The important part of a mortar mix is that it is “softer” than the stone. Masonry gets wet, it’s permeable. As the moisture vapor leaves the stone, as it dries, the ideal scenario is that the vapor leaves via the mortar joints. Moisture will always find the path of least resistance. That’s why the joints should be softer. They are intended to be sacrificial over time, thus prolonging the life of the stone.

When using Portland cement, a typical historic mix is 1:1:6. –1 part cement, 1 part lime, 6 parts sand. Some mortar mixes are as soft as 1:2:9. Hydraulic lime mixes consist of only sand and lime. Either is an acceptable historic mix. After extensive use with both types, the lime mix seems to function better with the wall system for preserving the life of the wall.

That being said, it is usually best to use the type of mortar mix the wall was constructed with. If necessary, although it is usually not, a mortar mix analysis can be performed in a lab from a mortar sample.

A mistake people make repairing their walls is that the mortar is too hard. This causes the stone to deteriorate over time as the mortar joints remain hard. This is evidenced by the cupping of the stone and the loss of the stone faces. Also, if the joints are not prepared correctly, they have no depth or bite and cannot last more than a couple of northeast winters.

There are three joint styles commonly seen in our Historic District. Each has variations on size and texture. Ribbon Joint: This joint appears as a ribbon, with its top and bottom cut into the stone edge, leaving a smooth surface in the middle. Weather Joint: This style is similar to the ribbon except that only its bottom is cut into the stone edge. The idea behind this joint is to shed water by creating a drip edge at the cut line. This is intended to keep water off of the stone face (to some extent). Rake Joint: This joint has no smooth face. The face is removed and the edges are pushed into the stone edge creating a superior seal. While rake joints look nice, they are recessed and the stone does take most of the water intrusion.

Developing a discerning eye and an appreciation for the importance of knowing what to look for will help insure that Chestnut Hill’s Wissahickon Style endures and that masonry’s contribution to the correct texture and color of the Historic District is in good hands.

Peter Saylor, FAIA, is a partner of SaylorGregg Architects and a member of the Chestnut Hill Historical Society’s Historic District Advisory Committee.

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