Preserving the Past, Reinventing the Roof
- eva_hatzaki
- May 30
- 9 min read
An award-winning restoration uncovers a hidden 20th-century roofing system—and a modern solution forged through collaboration.
Honored with four awards: • 2025 Lucy G. Moses Award from the New York Landmarks Conservancy • 2025 Institutional Restoration Award from the Friends of the Upper East Side • 2024 McKim, Mead & White Award in Historic Preservation from the Institute of Classical Architecture and Art (ICAA) • 2024 Cast Stone Manufacturing Award from the Cast Stone Institute
By Michael Middleton
Preservation Architect
The National Society of Colonial Dames in the State of New York (NSCDNY) is a statewide community service oriented, patriotic and genealogical organization comprised of women who can trace their ancestry to individuals who rendered service to the cause of freedom during the American Revolution.
The organization’s headquarters, constructed between 1929 and 1930, located at 215 East 71st Street in Manhattan, is a five-story brick, limestone, and cast stone building executed in the Colonial Revival style to the designs of architect Richard Henry Dana, Jr. The building serves as the NSCDNY’s administrative center, provides social spaces for its membership, and acts as a semi-public museum of America’s colonial past. Naturally, the Colonial Revival style, a movement which reinterpreted architectural designs popular in Britain’s thirteen American colonies during the 18th century, was selected for the headquarters of this organization dedicated to the preservation of sites and objects associated with America’s founding. The building is considered one of the finest examples of the style being a unique compendium of authentic 18th century fabric and historically correct detailing inspired by twenty-four colonial era houses, ranging from Massachusetts to Virginia.
In the first decades of the 1900s, there was a rise in professional architects who began to market them-selves as expert specialists in the design and restoration of “colonial” structures. It would have been for their expert knowledge in and familiarity with colonial buildings that architects, such as Dana, would have been specifically hired by wealthy clientele. As retrospective as the Colonial Revival style may seem, there had been many technological advances since the 18th century including electricity, running water, and mechanical heating, which had become standard comforts of American life by the early 20th century. Thus, such specialist architects were called upon to use their vast knowledge of early American styles to replicate the look and feel of those times but to ultimately design modern structures. This is true of the Headquarters of the NSCDNY.
While Dana designed a building which may initially appear to have been constructed in 1730, that design also masterfully integrated modern conveniences that would have been expected by a Colonial Dame in 1930. Likewise, Dana took full advantage of 20th century building technology, such as the use of concrete, steel, and terra cotta block to provide a thoroughly modern and fireproof construction for the organization.
Between 2022 and 2024, Li/Saltzman Architects (LSA) served as the Preservation Architect for the exterior restoration of the building’s two façades, roof, and rear garden perimeter walls. The overall scope of work included the cleaning and repair of masonry façades; replacement of cast stone balustrades in-kind; the refurbishment of wood doors, windows, and shutters; slate mansard roof replacement; the replacement of copper dormer standing seam roofing; the installation of new cold fluid-applied roofing; and ironwork repairs.
Surgical installation of new roof system at 215 East 71st Street, the headquarters of the National Society of Colonial Dames in the State of New York, constructed between 1929 and 1930. — Photos: Li/Saltzman Architects
A routine slate replacement uncovered a rare historic system—leading to a custom solution that blends preservation with modern performance.
Slate Roof Replacement
The restoration had several unique challenges. Most interesting was the replacement of the historic slate Mansard roofs which had been installed over an unexpected substrate. These roofs, featuring wooden dormers, are the most salient features of the building’s primary East 71st and rear façades. After more than ninety years of weathering, the grey slate shingles, originally quarried in Bangor, Pennsylvania, were deteriorated and required replacement. It was observed that while the vertical exposure of the slate shingles was consistent, the slate shingles were laid using varying horizontal exposures of 6, 7, and 8 inches in a proportion of approximately 60%, 20%, and 20% respectively. The laying of the slate with irregular exposures was likely done to make the roof appear older or that the brand-new roof in 1930 had ostensibly already undergone a series of selective replacements over time. The team sourced, and the Landmarks Preservation Commission (LPC) approved, “Vermont Black Semi-Weathering” by New England Slate as the replacement slate for the project. With an approved replacement slate and layout configuration, the team started what it thought would be a very straightforward slate roof replacement.
During the course of slate removal work, it was discovered that the underlying material, which had served as the nailing substrate for the slate shingles, was a 2” thick gypsum concrete and not a wood sheathing as anticipated. After some additional probing, it was discovered that the gypsum concrete was installed over a system of terra cotta blocks spanning between steel angles. Skyline Restoration tested the installation of both copper and stainless-steel nails into the existing gypsum concrete assembly. In all attempts, across multiple areas of the roof, the nails did not hold and could easily be removed by hand. If single nails failed to remain installed, there was no way that the existing substrate would be capable of holding the hundreds of new nails required to secure the weight of a new slate roof. It was clear that the substrate was severely compromised, and full replacement of the gypsum concrete was required. How-ever, neither Li/Saltzman Architects (LSA) nor the project’s Structural Engineer, Gilsanz Murrary Stef-icek (GMS), had ever developed a replacement system for this type of roof assembly before. From this point on, the design team worked closely with Skyline Restoration to collaboratively explore potential modern alternative approaches to recreate the failing gypsum concrete roof substrate.
Book Tile Roof Systems
Figuring out what exactly the existing roof system was and how it was intended to work historically was our first task. After initial research, it was determined that what had been uncovered was a “book tile” terra cotta roofing system. As the name suggests, these relatively large structural clay tiles have the general appearance of a closed book. Each tile has one concave and one convex end so that adjacent tiles can be interlocked in addition to being held together with mortar. Groupings, or panels, of book tiles would then be installed spanning between a system of tee irons or steel angles, themselves supported by larger steel purlins and then by an overall steel roof structure below. Book tile roofing systems were popular during the early 20th century primary for sloped roof applications as a way to provide a fire-proof roof structure for buildings in urban centers. As terra cotta block is brittle and hollow, these systems needed to be finished with a one- to two-inch-thick lightweight concrete or mortar topping. The lightweight topping would serve both as a protective layer over the terra cotta block and as a nailing surface for slate, tile, or metal roofing.
Typical wood dormer before and after restoration, at the headquarters of the National Society of Colonial Dames in the State of New York in NYC. Photos: Li/Saltzman Architects
Several modern materials were tested—but none could match the original system’s strength, fire rating, and waterproofing. The search for a solution continued.
Replacement In-kind?
The next challenge was to find a replacement material that could serve as nailing surface for the new slate roof, protect the intended fire-rating of the terra cotta book tile system, and maintain the historic planes of the roof surface, dormer sidewall cladding, and copper flashing locations. Naturally, the simplest solution would be to replace the compromised gypsum concrete in kind or with a modern equivalent material. This was where the team started. It was found that United States Gypsum (USG) prduced a modern gypsum concrete material. LSA contacted the technical representative for USG who explained that they could offer us their Securock Gypsum Concrete Patch. However, after further discussion, USG could not warranty their product as a full substrate replacement. It’s intended use is for localized patching of damaged areas of gypsum concrete toppings and USG had never used it in the capacity that we were requiring for the Headquarters. With USG out, the team’s next idea was perhaps not a true in-kind replacement but with a modern grout or cementitious coating. Skyline invited the technical representative from the Sika Corporation (Sika) to visit the building to assist in exploring op-tions for the use of one their products to rebuild the compromised gypsum concrete. After some brain-storming, the technical representative informed the team that no current product manufactured by Sika would be suitable as a replacement material as their products would be too hard for nails to pene-trate. Furthermore, even if they could penetrate, Sika would then not be able to provide a warranty if their product was used as a nailing surface for a new slate roof due to the possibility of water infiltra-tion from the hundreds of small penetrations.
Densdeck?
As it was clear that replacement in-kind was not option, the team then needed to find an alternative that could function in a similar, but modern, way to the historic gypsum concrete. One suggestion was to consider the use of roof board products manufactured by Georgia-Pacific as a fire-rated nailing surface for the new slate roof assembly. After reviewing several of these, the team thought that Georgia-Pacific’s DensDeck might be a suitable candidate. This roof coverboard has a high fire-resistance rating, resists up-lift from wind, and is impact resistant. These traits coupled with the fact that we could install multiple boards on top of each other to rebuild the historic plane of the roof two inches above the book tile made us believe that we had finally found a solution for the roof. Unfortunately, the technical representative gave us another reality check. It was found that this approach would not be feasible as DensDeck is largely used for flat roof applications and that the pull value for the DensDeck would not be as strong as other materials such as plywood to support the weight of a new slate roof.
Slate roof replacement in progress (top). Typical dormer copper standing seam roof replacement (above). — Photos: Li/Saltzman Architects
The Winner
After about a month of due diligence to no avail, the team realized that it might not be a single product but an entire system which might be required to replace the roof. After some internal discussions, Skyline came back and informed the team that not only had they encountered this type of roof system before, but they had devised a replacement system for it at a nearby landmark, the Sloan Mansion, on East 68th Street. Now having a model, the team was finally able to design a new roofing system which more or less mirrored what had been successfully installed there. The replacement system would be comprised of three elements; galvanized steel hat channels, stainless steel TEK self-drilling screws, and exterior grade fire-retardant treated plywood.
The metal hat channels served several purposes. First, their shape allowed us to push up the overall new roof assembly so that we could re-establish the historic roof plane approximately 2” above the surface of the terra cotta book tiles. The system of hat channels also worked as the base on which the new plywood could be screwed and fastened. The elevated sections of the hat channel profiles protected the brittle terra cotta block below and eliminated the need to make penetrations into it to secure the plywood. Of course, the hat channels and everything above still needed to be secured to the terra cotta and steel roof structure. For this, Skyline had to very surgically attach the flanges of the hat channels to the interior side flanges of the steel tee angels via the TEK self-drilling screws. This required two groups of workers; one at the exterior to align the hat channels at the approximate locations of the tee angles and a second crew in the attic of the building to ensure that the screws were lining up with the steel and not shattering the terra cotta book tiles. Once the system of hat channels was placed over the terra and steel structure of the Mansard and dormer roofs, the installation of everything above was rather straightforward. The new fire-retardant treated plywood served to maintain a fire-rating for the roof, diminished with the removal of the original gypsum concrete, in addition to being the base for the installation of the underlayment and the final nailing surface for our new slate shingles.
South elevation roof before and after restoration, at the headquarters of the National Society of Colonial Dames in the State of New York in NYC. — Photos: Li/Saltzman Architects
Award Winning Collaboration
The willingness to collectively work together to create the best solutions for projects is one of main reasons that LSA enjoys working with Skyline Restoration. In this case, through the coordinated efforts of the Architect, Engineers, and Contractor, we were able to successfully develop a solution for an unusual roofing condition which was able to respond to aesthetic, technical, and structural requirements. The exterior restoration of the Headquarters of the NSCDNY was completed in June of 2024. For the meticulous work, including recognition of the distinctive roof replacement, the project has been honored with four awards; a 2024 McKim, Mead & White Award in Historic Preservation from the Institute of Classical Architecture and Art (ICAA); a 2024 Cast Stone Manufacturing Award from the Cast Stone Institute; the Friends of the Upper East Side’s 2025 Institutional Restoration Award; and a 2025 Lucy G. Moses Award from the New York Landmarks Conservancy. 🀰
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