By: Kelly Grogan
March 24, 2021
Stone is an age-old building material that has been used in construction since the first Roman road was built in 312 B.C. In New York City, stone, brick, and other forms of masonry became the main building materials after devastating fires ravaged houses and stores that were mainly constructed of timber in the 19th century. The appearance of the city was greatly transformed as the building code laws reacted to these tragedies and discouraged the use of timber. Still today, ornamental stone enhances the fascinating appearance of our architecture and adds to the picturesque New York City skyline. With so many historic stone facades that are aging and deteriorating in a densely populated city, it is not only necessary to perform proper maintenance for historic preservation efforts, but also for public safety.
So how can you do your part to ensure your masonry façade is properly maintained? One of the first steps is to hire a professional engineer or architect who specializes in exterior work to perform a survey of your façade and evaluate the condition. If the building consists of original stone from over 50 years ago, the inspection is likely going to reveal stone units with visual cracks or spalls that warrant repairs or replacement depending on the severity. In addition to natural stone, other typical forms of masonry units consist of terra cotta, concrete, cast stone, and concrete masonry unit (CMU) blocking. Repairs to damaged masonry units can include repointing stone joints, routing out cracks or removing an area of damaged stone and installing a repair mortar, or performing a more in-depth patch repair with reinforcement. If a stone unit cannot be salvaged by performing one of these repairs, the next option is to replace it with an entirely new masonry unit.
There is not always a clear line of when replacement is necessary, and there are a lot of things to consider when deciding which repair option to pursue. One consideration that can make this line a little clearer is determining if a stone failure is isolated or systemic because this determination can greatly guide the restoration efforts. A common example would be to think of a stone water table that extends the width of a building at the street-facing façade. The water table is a wide band consisting of smaller stone units side-by-side with sealant or mortar joints between each unit. If the fascia of only one stone unit is spalled amongst the whole band of units, you are looking at an isolated failure and the remedy to this spall would likely be a stone patch repair. However, if you see cracks and/or spalls in multiple stone units, you are probably looking at a systemic failure and replacement will most likely be necessary.
A typical example of a possible symptom of a systemic failure could be if the steel framing that supports a stone water table has significant rust accumulation. At the surface, the expansion of the rusting steel will eventually show itself as cracks or spalls in the stone units, dislodgement of the masonry unit, and/or discoloration from the underlying rust accumulation. If a crack extends through several stone units or across the full width of the stone band, the cause of the failure will definitely not be addressed by performing crack and/or patch repairs. In this instance, to properly address the source of the masonry failure, it would be necessary to remove the existing stones, prepare and waterproof the steel, and then re-install the existing stones that were salvageable and replace the existing stones that were deteriorated to a point beyond repair.
Another consideration that can suggest whether to repair or replace a masonry unit is the role of the unit as a component of the wall assembly. A 4-inch-deep decorative stone medallion is likely not supporting the weight of the façade above and, therefore, is not essential to the integrity of the wall assembly. However, a stone lintel above a window opening is a structural component and has to support the façade above. If there is a vertical crack at mid width of a stone lintel, it is likely due to the stress of the weight above and should be replaced to ensure proper load path.
If the line between repairing or replacing is still blurry after performing a thorough visual and hands-on inspection, there are also financial and timing implications that may factor into an owner’s decision to repair or to replace a stone. A downside of replacing stone is that is can be very costly (we all know that budgets are limited) and it can significantly delay a project schedule. The molds for intricate ornamental masonry units can be priced at a couple thousand dollars before even adding on the cost of material and labor for obtaining and installing the replacement stone. The process involves removing the existing stone and delivering it to the fabricator, iterations of shop drawings, and time for the new masonry unit to cure and eventually be delivered to site. Depending on the type of masonry unit, the schedule of the fabricator, and the proximity of the fabricator to the site, owners can expect a lead time of 6 to 12 weeks from the time the shop drawings are approved to the time a replacement masonry unit arrives on site. At the same time though, replacing an existing damaged stone with a whole new masonry unit ensures that all of the failed material has been removed and any underlying conditions, such as steel with rust accumulation or a failure in the backup, can be appropriately addressed. Owners should consider that a new masonry unit will definitely last longer than patch repair mortar.
Since budget and schedule are often at the forefront of an owner’s priorities, pursuing a repair option is usually a very appealing choice. The cost per square foot of patch repair in comparison to the cost per square foot of replacement is significantly lower. Additionally, patch repair mortar is typically readily available and quicker to obtain on-site, outside of the time it takes to select a color to match the existing stone. Stone repairs can last up to several years ONLY IF the preparation work and repairs are properly performed by a contractor. It is crucial for longevity of a repair to have a design professional oversee the repair process to ensure it is performed according to the intended design and manufacturer’s specifications. Depending on how intricate a decorative masonry unit is, it may also be very difficult to match the existing color, texture, and shape utilizing a patch repair mortar in lieu of casting a mold for a new unit.
In the above article, we discussed some of the typical concerns to consider when determining if stone should be repaired or replaced on your façade. However, the most important note to take away is that public safety should always be held paramount in making a decision between repairing or replacing an existing stone. If there is any doubt that a stone repair will not have a useful life or there is a potentially unsafe condition, owners should rely on the recommendation of a trusted design professional and always err on the side of caution.