Keystones hold a special place in architectural history. These wedge-shaped stones sit at the apex of an arch, traditionally serving as the final piece that locks all other stones into place and allows the arch to bear weight. For centuries, keystones were essential structural components — without them, arches would collapse.
But keystones also became something more than functional. They evolved into decorative focal points, often carved with faces, symbols, or ornate details that drew the eye upward. On historical buildings, keystones announce entrances, frame windows, and add visual weight to archways.
Today, most arches are built with modern structural materials like steel and concrete. The keystone is no longer structurally necessary. But architects still want that visual element — the sense of history, craftsmanship, and architectural sophistication that a well-designed keystone provides.
That’s where cement coated EPS keystones come in. These lightweight, durable architectural elements give you the historical appearance of traditional stone keystones without the weight, cost, or installation challenges of real stone or precast concrete.
Why Keystones Still Matter in Modern Architecture
Even though keystones aren’t structurally required anymore, they serve an important visual and symbolic function. An arch without a keystone can look incomplete or generic. The keystone provides a focal point that breaks up the monotony of uniform arch stones and gives the arch a sense of completion.
In historical architecture, keystones were often the most decorative element of an arch. Carved faces, acanthus leaves, family crests, or geometric patterns turned functional stones into artistic statements. Building owners used keystones to display wealth, taste, and attention to detail.
Modern architecture still benefits from these same visual principles. Whether you’re designing a new commercial building with classical elements, restoring a historical property, or adding architectural interest to a residential project, keystones provide that finishing detail that elevates the design from ordinary to exceptional.
The challenge is finding a way to include keystones that makes sense from a budget, timeline, and installation perspective. Traditional stone keystones are expensive to carve and extremely heavy. Precast concrete keystones are somewhat more affordable but still require significant labor to install and structural support to hold their weight.
Cement coated EPS keystones solve these problems while maintaining the visual authenticity of traditional materials.
How Cement Coated EPS Replicates Stone Keystones
Expanded Polystyrene (EPS) coated in a reinforced cement layer provides a unique combination of properties that make it ideal for architectural elements like keystones. The EPS core is lightweight — often 90% lighter than solid stone or precast concrete — while the cement coating provides durability, weather resistance, and the authentic appearance of carved stone.
The manufacturing process for EPS keystones starts with cutting the EPS foam core to the exact size and shape specified for your project. Unlike stone carving or concrete casting, EPS can be cut quickly and precisely using CNC equipment, which means custom keystone designs don’t require expensive molds or extensive hand carving.
Once the EPS core is shaped, it’s coated with a proprietary cement mixture that’s reinforced with fiberglass or other strengthening materials. This coating can be applied in different thicknesses and with various surface treatments to achieve the exact texture and finish you need — from smooth dressed stone to rough-hewn rustic finishes.
The cement coating can also be textured or carved while it’s still workable, allowing for decorative details like relief patterns, faces, or ornamental designs. For projects where you’re matching existing historical keystones, the coating can be finished to replicate the specific stone type, weathering patterns, and carving style of the original.
Color can be integrated throughout the cement coating or applied as a surface finish, ensuring that minor chips or surface wear won’t reveal a different base color underneath. This is particularly important for keystones, which are prominent visual elements that need to maintain their appearance over time.
Common Applications for EPS Keystones
EPS keystones work in any situation where you want the visual impact of traditional stone keystones without the associated weight and cost. Here are some of the most common applications:
Historical Restoration Projects
When restoring historical buildings, damaged or deteriorating stone keystones often need to be replaced. Original stone replacement is expensive and may not be readily available, especially if the building used a specific type of stone that’s no longer quarried. Precast concrete is heavy and may require structural reinforcement.
EPS keystones can be custom manufactured to match the exact dimensions, profile, and decorative details of the original keystones. They’re light enough that they won’t add stress to aging structural systems, and they can be installed without the heavy equipment and extensive labor that stone or concrete replacement would require.
New Construction with Classical Design Elements
Architects designing new buildings with classical or traditional styling often want to include keystones above windows, doorways, and archways. EPS keystones provide an affordable way to incorporate these details throughout a building without budget or timeline constraints.
Because EPS keystones are lightweight, they can be added to structures that wouldn’t support the weight of stone or heavy precast elements. This opens up design possibilities that might otherwise be impractical.
Residential Properties
Homeowners looking to add architectural character to their properties can use EPS keystones above garage doors, entry arches, or window surrounds. The cost and ease of installation make these details accessible for residential projects where stone or precast options would be prohibitively expensive.
Commercial and Hospitality Buildings
Hotels, retail centers, and other commercial properties often use classical architectural details to create a sense of luxury and permanence. EPS keystones above entrances, in arcades, or throughout façades provide visual interest without the structural and budgetary implications of traditional materials.
Design Considerations for EPS Keystones
When specifying EPS keystones for your project, there are several design factors to consider:
Size and Proportion
Keystones need to be properly proportioned to the arch they’re completing. A keystone that’s too small will look insignificant, while one that’s too large can overwhelm the arch and look cartoonish. Traditional architecture followed established proportions where the keystone height was typically 1.5 to 2 times the height of the other arch stones.
For decorative keystones on flat-topped openings (where there’s no actual arch), the keystone still needs to relate proportionally to the opening width and the surrounding architectural elements.
Profile and Projection
Keystones typically project forward from the face of the arch or wall, creating shadow lines that make them more prominent. The amount of projection affects how dramatic the keystone appears — deeper projection creates stronger shadows but also means the keystone needs to be more securely attached and detailed to shed water properly.
EPS construction makes it easy to create keystones with significant projection without weight concerns, but you still need to consider how water will run off the projecting element and whether the projection will interfere with other building components like shutters or trim.
Surface Treatment and Detail
The level of detail on your keystones should match the overall architectural style and the viewing distance. Highly detailed carved faces or ornamental patterns work well on keystones that will be viewed up close, while simpler geometric shapes or subtle relief patterns are more appropriate for keystones on upper floors or areas viewed from a distance.
Cement coated EPS can accommodate various levels of detail, from simple wedge shapes to elaborate carved decorations. The coating can be worked while fresh to create texture and detail, or it can be applied over an EPS core that’s been pre-carved with more complex patterns.
Finish and Color
Keystone finish should coordinate with other exterior materials and architectural elements. For buildings with natural stone or brick, keystones can be finished to match or complement those materials. For painted buildings, keystones can receive the same finish as other trim elements or they can be left in a natural stone-like finish for contrast.
The cement coating on EPS keystones accepts paint, stains, and sealers just like traditional cement or stone, giving you flexibility in achieving the exact appearance you need.
Installation and Long-Term Performance
One of the biggest advantages of EPS keystones is how straightforward installation is compared to stone or precast alternatives. The light weight means keystones can typically be installed by one or two workers without mechanical lifting equipment. This reduces both labor costs and installation time.
EPS keystones are attached using construction adhesive and mechanical fasteners appropriate for the substrate material. For brick or stone veneer applications, keystones can be set into the veneer as it’s being laid. For retrofit applications or installation on existing buildings, keystones can be attached to the wall surface using adhesive and fasteners, with joints sealed to prevent moisture infiltration.
The cement coating protects the EPS core from weather, UV exposure, and physical damage. Properly installed and finished EPS keystones require minimal maintenance — typically just periodic cleaning and resealing every 5 to 10 years depending on exposure conditions.
Unlike stone keystones, which can crack from freeze-thaw cycles or settle over time, EPS keystones are dimensionally stable and resistant to weather-related damage. The lightweight construction also means there’s no long-term concern about keystones becoming loose or pulling away from the structure due to their own weight.
Why Choose Patterson Whittaker for EPS Keystones
Patterson Whittaker has over 30 years of experience manufacturing cement coated architectural EPS products for both commercial and residential applications. Our keystones are engineered to provide the authentic appearance of traditional stone while delivering the practical advantages of modern lightweight construction.
We can manufacture keystones to match historical examples, work from your custom designs, or help you develop keystone designs that complement your project’s architectural style. Our manufacturing process allows us to create one-off custom pieces as easily as producing multiple identical keystones, giving you flexibility whether you’re replacing a single damaged keystone on a historical building or specifying matching keystones for multiple openings on a new construction project.
Every keystone we produce is finished to your specifications and ready to install when it arrives at your job site. We provide technical support and installation guidance to ensure your keystones are properly attached and finished for long-term performance.
Get Started With EPS Keystones for Your Project
If you’re working on a project where keystones would add architectural interest and historical authenticity, cement coated EPS from Patterson Whittaker provides a practical, affordable solution that doesn’t compromise on appearance or quality.
Contact Patterson Whittaker today at 206-953-5209 (US) or 604-285-6550 (Canada) to discuss your keystone needs and learn how our cement coated EPS products can enhance your architectural design.
