Traditional Masonry Archives

Repairing Sandstone Façades
Sandstone is an affordable, attractive stone soft enough to carve the elaborate designs or to chisel to a rough finish. Unfortunately, the same things that make sandstone attractive give it the potential for relatively rapid deterioration.

It is no wonder sandstone became such a popular building material in the 19th century. The affordable, attractive stone was soft enough to carve the elaborate designs favored by the Victorians or to chisel to a rough finish. From brownstone townhouses to large municipal buildings and courthouses, sandstone tells us that the building we're looking at stands strong as a rock.

But does it really? Unfortunately, the same things that make sandstone attractive give it the potential for relatively rapid deterioration. The stone is coarse-grained and contains mica, silica and clay, which swell as they absorb moisture. This opens cracks for water infiltration, which puts further pressure on the stone, eventually leading to crumbling. Furthermore, sandstone was almost always installed perpendicular to its natural bedding, making it even easier for water to infiltrate and speeding deterioration. Even the finish can weaken the surface. The chiseled finish popular on old courthouses, for example, actually opens pores in the stone and creates ledges that catch water.

Thankfully, many people today are passionate about repairing and preserving these lovely buildings, and their timing couldn't be better. New developments in materials technology combined with growing interest in traditional techniques means sympathetic, lasting repairs will keep these buildings attractive for a long time.

A stone graft, also called a Dutchman.

Repair mortar
Repair mortar, sometimes called patching mortar, is significantly different from bedding or joint mortar. John Speweik, historic masonry specialist with U.S. Heritage Group, Chicago, notes that repair mortar is not for filling cracks; rather it is built up to fill in the spots where the stone is damaged or missing. True repair mortars are formulated to resemble stone as much as possible. John D'Antoni, a technical support expert for Cathedral Stone, Hanover, Md., explains: "Our labs test sandstones to learn the physical characteristics particular to each one. We have a different restoration mortar for each one." Cathedral Stone distributes the world-renowned Jahn repair mortars and grout as well as developing its own repair materials.

Laurie Wells, vice president of sales and marketing at Old World Stone, Ltd., Burlington, Ontario, explains, "Patching mortars have been used for centuries. Usually they have a cementitious component and color-match aggregate and sometimes a pigment. The ingredients have to work out to produce the correct color — that is the most difficult aspect of preparing repair mortar."

D'Antoni confirms that color matching is a challenge. The solution is a traditional one — the human eye. "We've tried digital color with a database and scanner eye," he says. But there are too many variables — shapes, sizes and ridges — so now it's all by human eye. We keep a library of colors that match sandstone samples from all our projects. That usually gets us a close starting point. We measure pigment to a hundredth of a gram to assure accuracy. We mix a new sample one day, cure it overnight and check it at the end of the next day to see if we've got a match."

Once the mortar has been matched, the next step is to prepare the surface. First, all the loose, crumbling stone must be knocked off and solid stone exposed to receive the repair mortar. At least 1/4 inch should be cut from the stone, with a shoulder-cut edge. In other words, the hole to be patched must have sides at 90-degree angles to the surface. Sloped sides result in a feathered edge, leaving the mortar around the edges too thin and leading to premature failure. Speweik recommends installing stainless steel fasteners if the repair will be deeper than two or three inches. Wells uses a lead armature with brass screws that project to 1/2 inch of the finished surface and a net of wound copper wire.

The stone should be wet to glistening for good adhesion and proper cure, according to D'Antoni. Then, he says, "Apply one coat of very wet mortar, the consistency of peanut butter, about 1/8 inch thick." The extra moisture allows the mortar to be worked well into the substrate.

The next coat should be mixed to the recommended consistency — usually 4 1/2 parts mortar to one part water. D'Antoni says that since the ingredients in Jahn repair mortars are all natural, there is no shrinkage, meaning this second layer can be as deep as desired and go several feet across.

The second layer is applied to protrude past the front of the wall and scraped down with a screed or miter rod to about 1/8 inch in front of the wall. Once it is cured sufficiently so that it does not stick to the miter rod, about one hour, it can be shaved or carved to the desired profile. The longer it cures, the harder it becomes, making it possible to carve more precise details.

Over the next 24 to 48 hours, the mortar should be misted so it cures evenly. If it dries too quickly, the color could lighten or the bond fail.

Good application of a high-quality repair mortar is essential to a long-lasting, good-looking repair. D'Antoni knows of patches 25 years old and still performing. But there are limitations. Patches should not be used near grade, where the material could wick moisture from the ground and degrade the adjacent stone. Also, repair mortar should never be used in traffic areas like steps, according to Wells. Unskilled installation or skimping on materials can lead to problems. "Acrylics, latex — these cut down breathability and actually accelerate deterioration," Speweik says. Dark colors overloaded with pigment may not cure properly and so become crumbly. Mortar absorbs water differently than stone, so in some cases the color may not match when wet.

Stone grafting
Another method of repair is a stone graft, sometimes called a Dutchman. In this case the damaged stone is cut away as described above and a solid piece of stone is carefully fitted in and held in place with stainless anchors and epoxy. This in-kind replacement is usually preferred over alternative materials for historical integrity and performance.

The first challenge is finding the proper stone for the repair, so the graft doesn't stand out unnaturally. Sometimes stone will still be available from the original quarry. Speweik once found the right stone for a large restoration by salvaging it from a newly demolished building nearby that had been built of the same material. Wells says she sometimes removes a small piece from a less visible part of the same building. It is even possible to remove the whole stone, take a graft from the back side and replace the stone. In any case, the new graft will stand out from the rest of the substrate until it has weathered to match.

Next, Wells says, "The replacement stone must be precisely dimensioned and aligned with its spot in the wall and with the anchor holes." The tolerances should be so close as to leave only a razor-thin joint between the new and old stone. The epoxy should end at least 1/2 inch from the surface to avoid being pushed out of the joint and isolating the graft. This also allows a moisture "bridge" between the two stones so they can absorb and release moisture at the same rate.

Finally, the graft is carved or tooled to match the rest of the facade. Clearly, stone grafting is a job for a skilled mason, but the end result is an exact match and the best possible historical accuracy.

Joint mortar
Often historical stone restoration also requires repointing mortar joints. There are pointing mortars designed in the lab to be compatible with specific types of masonry, including sandstone, and new mortars can be created after lab-testing the original materials, D'Antoni says. "We can test the existing mortar," he says, "or more importantly, we can test the substrate about to be pointed and develop a pointing mortar that is suitable for that situation."

Some prefer to mix their own mortars. Wells advises taking a sample of the existing mortar and breaking it to expose a clean surface for making a match. Then she mixes mortar as close to the original as possible. "Mortar is usually slaked, not hydrated, lime with local aggregate sand," she says. "The aggregate sand used in the nineteenth century was typically dirty, whereas today's sand is washed, so sometimes a pigment must be added. Properties with historical designations usually require slaked lime mortar. Slaked lime is quite different. You make it by adding dry powder quicklime to a big pail of water. It bubbles and creates heat. You put a lid on it and leave it for at least two weeks, preferably up to a year. Slaked lime is very corrosive and the process is time consuming, but it makes the mortar stick and, with little lumps of lime in it, is more historically accurate." She recommends mixing between six and eight parts sand with one part lime and one part cement.

Repair mortars and stone grafts are great for repairing portions of a sandstone wall, but if the wall has deteriorated to the point where the structural integrity is in question, the wall should be replaced, not repaired.

Sandstone may not be as permanent as originally hoped, but with proper care and careful restoration, these lovely buildings will be around for us to enjoy for many years to come.

TM