Masonry Restoration at St. Rose Print E-mail
Written by Christoph Henning   

Christoph begins by chiseling loose mortar While visiting family in Pine Island in 2010, my wife, son and I had the pleasure of attending the ice cream social of the Friends of St. Rose. I had never seen this beautiful limestone church and was thrilled by the work of the Friends. At the social I learned about the upcoming tuckpointing project and, as a professional in historic preservation, I pointed out to my brother-in-law (Mr. P.) that it was not necessary to remove mortar joints that were in good condition and that the church and its building history would be better served by a selective repair of the joints.

Well, I ended up being hired for the job and I truly enjoyed it. I had never worked with volunteers and it was fun to meet, work with and have the companionship of Jerry, Jill, Kelly, John, Tommy, Richard and others. Although I have worked with all kinds of mortars for over 20 years, masonry restoration is only a side aspect of my work. I still believe that I am better qualified than the great majority of mason contractors. Modern hard monolithic masonry with its precast panels and caulk expansion joints is so very different from the traditional soft and flexible historic masonry. It also helps to have a good understanding of building chemistry and building physics as it should be the basis for all stabilization efforts. St. Rose is my largest repointing project yet and I learned a few new skills and this is always exciting.

At the outset of the project it was important to determine what kind of mortar to use.  We used a 100% lime mortar, which was the same material that the builders used originally. While we were driving around in search of suitable stone we discovered where the sand for the mortar came from. When Kelly told me about a kiln in the woods I was even more excited. This is probably where the stonemasons burned limestone lumps to produce calcium magnesium oxide. They made lime putty by grinding the burnt lumps to dust and slaking the powder in water. Lime putty and sand are the ingredients for the mortar. We had no lime putty at our disposal and so we used the modern equivalent:  hydrated lime.

To learn more about repointing and historic masonry restoration I recommend the preservation brief # 2 of the National Park Service:  http://www.nps.gov/history/hps/tps/briefs/brief02.htm.

P6270035 At the beginning we all believed that the church had been repointed on the back side (south side) at one time because it looked different than the street side (north side). I quickly realized that it was all original and that the ingenious masons had used a simpler style for the walls not visible from the street. It was a measure of efficiency. The street side has almost perfectly straight joints and a fancy convex profile to give the impression that the stones had been cut square and uniform. In order to get those straight mortar lines the masons did not care if the convex joints (also known as “tuckpointing”, “spaghetti joints”, “varicose-vein joints” or “grapevine joints”) would run between the stones or over the stones. They are decorative rather than purely functional. The north and west facades also have the fancy convex profile, but there they are not perfectly straight.  Looking from the street, the little curves and waves are not as noticeable because of the corrective abilities of the eye. The builders must have known something about optical effects and saved themselves a lot of work. To improve the beauty of the work they also made sure that the vertical joints did have the proper spacing and ever so often they filled a joint flat to give the impression that there is a big stone rather than two little ones. Preserving the original work also preserves the story of the construction.

DSCN0310 The lower wall of the foundation has a high moisture content. This is typical for historic structures. Over time air pockets in the foundation filled in and could no longer prevent the ground moisture from being sucked into the walls by capillary action. The lime mortar that we used successfully in the upper parts of the wall hardens only by air infiltration.  It cannot be used in the lower parts because of the constant flow of water from the ground.  In order to repoint the lower section of the walls we had to use a so-called “hydraulic” lime, which can harden under water. This lime hardens by building calcium carbonate with whatever air may be available and by forming calcium silicate in another phase.

The typical problem with wet foundation walls is twofold. Even if the foundation was dug out around the church to provide drainage and a new horizontal insulation were installed to keep ground moisture from traveling upwards in the walls, the results are often unsatisfactory. The soluble salts that have developed inside the wall retain moisture. To truly dry out the walls one would have to extract a significant amount of this salt. This can be done by applying a thick layer of loose cellulose fibers soaked in distilled water to the walls. The distilled water travels in the wall, absorbs the salt and transports it to the outside of the cellulose as the water evaporates. The cellulose containing the salt can then be removed. This would have to be repeated many times until the salt concentration is measurably diminished.  In my opinion, this approach is not practical for St. Rose and most other historic buildings. However, moisture is a problem and needs to be managed. From the masonry side of this management it is essential to have the walls near the ground repointed with a soft and porous material such as lime mortar that can deliver ground moisture along the shortest route to the outside for evaporation. Water and the soluble salt it transports will always travel the easiest route. If the lower part of the walls were repointed with a much more dense and hard cement-based mortar the result could be very harmful to the masonry.  Unable to penetrate the mortar, the ground moisture would be driven higher within the walls and could be forced to travel through the stones rather than the mortar. If so, the salt would likely concentrate within the stones as the water evaporates. Salt increases its volume when it absorbs water and can create enough pressure to shatter the stones into pieces. Addressing this problem with lime mortar is a better choice, because it is a more flexible material that can absorb a lot of salt and withstand more pressure within its pores. If a lot of salt is present, the mortar may disintegrate and fall out.  It may need to be repointed more frequently, but in the long run it will protect the rest of the walls and keep moisture from traveling up or to the inside of the church.

DSCN0264 At St. Rose we used some special repointing techniques that I would like to explain. For curing and color we scratched off the tooled surface of the newly installed mortar as soon as it was hard enough and would not stick to the tool. The scratched surface shows the aggregate (sand) and helps to achieve a better color match. The color of the sand will dominate overall appearance in the long run as small amounts of lime are regularly dissolved by rain water and washed away. The scratching also aids the curing as the air can penetrate the mortar much faster and on a larger surface. While water can easily penetrate the mortar (which also aids curing and the ability of the mortar to repair micro cracks within) it will also be able to evaporate from the walls quicker. For the lower walls this ability to evaporate moisture quickly is essential as already described. For the tuckpointing it was unsuitable to scratch the surface of the convex part but we scratched the mortar next to it. At a later state of tuckpointing we used high-pigmented slurry to finish the “halfround” and this helped to match the color better. Scratching off the tooled surface of the mortar is not suitable for all applications. Around the windows the main purpose of the mortar is to keep moisture away from the wood and a smooth tooled surface will work better.

Too often restoration professionals rely on standard remedies that do not consider individual circumstances and the historic context of a building and its materials.  The National Park Services preservation brief advises against some practices such as letting the mortar “feather out” because it is thought to erode quickly, but at St. Rose we see that this type of mortar application is in excellent shape.  Tuckpointing itself has a bad name and is not recommended.  However, if a masonry job lasts 133 years that speaks for itself. The calcium magnesium lime mortar has incredible properties. It may be more time consuming to install, but it is a long-lasting and historically appropriate material. Driving around in Minnesota and seeing other buildings withstand time and weather despite a missing roof or other typically dooming factors is amazing.

Some of the masonry damage at St. Rose may be caused by structural problems.  Several cracks in the window arches suggest that either the foundation settled at some time or that the roof was too heavy. Some of those cracks have been repointed and should be monitored. If new cracks appear it would be advisable to consult a civil engineer to determine the cause of the wall movement. Hopefully this is not an issue and the settling has ended already.

(Click here to see photos of the restoration effort)

Last Updated on Wednesday, 28 September 2011 13:22