By: Kevin Duffy
There are several factors to consider when choosing the material for a stone restoration/ replacement project. The restoration budget, the desired aesthetic look of the building, and whether the building is landmarked are three such factors that should be taken into account when choosing between natural stone or cast stone. Limestone is a natural material that is quarried and delivered to the site, while cast stone is a man made material.
Limestone is a traditional building material. Since it often comes from Indiana, it’s often referred to as Indiana Limestone. This building material has been used on countless buildings throughout the country; Rockefeller Center, the Empire State Building and the new Yankee Stadium are some of the most notable examples in New York City. According to the National Parks Service, cast stone, in one form or another, has been in use in the United States since the mid 1800’s. A notable historic cast stone structure is the Cleft Ridge span in Brooklyn’s Prospect Park.
Limestone is relatively strong, durable and ages well if properly maintained. Since limestone is a natural material, every piece exhibits variations in color and grain pattern; however, this is typically unnoticeable after installation and weathering. Limestone is usually more expensive, however the cost of limestone can vary greatly depending on market conditions. The price can even vary from contractor to contractor based on the contractor’s relationship with the stone manufacturer, the contractor’s markup on the supplier’s price, or the manpower required to handle different approaches to installation. Cast stone tends to be slightly stronger, or less brittle, than natural limestone. It has a compressive strength of 6,500 psi, whereas limestone is closer to 4,000 psi. Additionally, holes and slots for anchors can be molded into cast stone making installation faster and easier. Cast stone is typically more cost effective than natural stone, however this depends on the ordinance of the stone and the number of molds that are required. A new mold must be made each time a different shaped stone is cast, and each mold usually costs thousand dollars to create.
As previously mentioned, limestone ages well if maintained; however, there are costs associated with its maintenance. Since limestone is a porous material, water cascading down the face of a building can stain it. In order to clean limestone, chemical processes may be required to prevent damage to the stones during pressure washing. Cast stone does not stain as readily and repairs can be performed more easily. For example, if one unit needs to be replaced, it’s easier to obtain an exact match for the cast stone, whereas the limestone would need to weather for a few years before blending in. Additionally, when replacing damaged limestone, it’s often difficult to match new limestone to the weathered limestone. And unlike cast stone, limestone cannot be tinted to a desired shade.
Although there are some differences in how the materials age and how they are installed, the choice between natural and cast stone often comes down to preference. Even though cast stone is stronger, easier to clean or replace and usually less expensive, some owners would rather their buildings have the more classic look created by natural grain and color variations in the stone. In certain situations, the Landmark’s Preservation Commission may require natural stone; recently however, they have been more accepting of cast stone. The engineer or architect should check the LPC’s requirements early in the design process.