Tech Talk: Glass Block Makes a Comeback

Glass unit masonry has been used more frequently in the last few years for several good reasons. It offers various levels of light transmission into buildings and, depending on the pattern or color in the glass block, can also provide more privacy than conventional windows. Glass block are low maintenance, durable and provide energy efficiency comparable to double pane windows. Glass block are traditionally laid with a relatively dry Type S or N mortar since the glass doesn’t wick any moisture from the mortar. The panels are reinforced at a maximum of 16 inches on center with joint reinforcing consisting of two W1.7 side rods with welded W1.7 cross rods. Glass block panels are laterally restrained using panel anchors at 16 inches on center along the top and sides of the panel and an asphalt emulsion isolation joint at the sill. There are several proprietary systems that provide similar performance to the more traditional glass block construction. These systems typically use U-shaped channels installed around the perimeter of the opening for lateral restraint and use joint spacers and specialized sealants to reinforce and bond the units to create the panel. After the sealants have cured, product-specific grouts are used to fill the joints between glass units in a manner similar to grouting ceramic tile. [caption id="attachment_18607" align="alignnone" width="1000"] Frosted glass window background[/caption] Sizes Glass block come in a variety of nominal sizes including (in inches): 4 x 8, 6 x 6, 6 x 8, 8 x 8 and 12 x 12. Two thicknesses of 3 inch (thin) and 4 inch (standard) are available with the standard units typically used in commercial construction and the thin units generally used for interior glass block panels. The modular nature of glass block allows the use of multiple size units within a panel to create visual interest. Code Limitations Glass block are always non-loadbearing. This means that headers/lintels and jambs must be designed to span the opening and carry the additional vertical and lateral loads. The maximum area of an exterior glass block panel may also be limited by the design wind pressure in some locations. For standard thickness block and a factored design wind pressure of 20 pounds per square foot, the area limit is 175 square feet by the Building Code Requirements for Masonry Structures (TMS 402). In high wind areas, the allowable area will be considerably less. It is a recommended to have a local engineer design any glass block opening larger than 50 square feet. For thin-unit glass block panels, the code limits the maximum area to 100 square feet on exterior walls. Construction Tips First, ensure that the opening is the correct size (and square) and that the jambs are plumb and the sill and head are level. To verify the correct opening size, multiply the nominal unit size by the number of units and add ¼ inch. For example, an opening 4 units wide and 6 units tall utilizing nominal 8-inch units should be 32 ¼ by 48 ¼ inches. For panels exceeding 10 feet in width or height, the opening may need to be increased an additional amount to accommodate additional thermal or differential movement. The two diagonal measurements to check for square should be within 1/8 inch for typical window-sized openings.
Words: David B. Woodham, P.E. Photos: jorgeantonio, KatarzynaBialasiewicz, JodiJacobson
Stop Juggling: The Best Way to Master Your Masonry Job Sites
January 2026

You’re in the business of building structures that will stand for generations. Yet, too often, the process of managing the job site feels built on quicksand. Every construction project manager knows the feeling: the constant, fra

Erin Koets
Technical Talks
Read More
Brick Jointers: Purpose, Patterns, & Practical Applications
January 2026

After leads have been built and bricks have been laid, it’s crucial to tool the joints before the mortar sets. The tool of choice for this task is a brick jointer, but it can be tricky to decide which one to use. This guide will walk through the purpose o

Mackenzie Claassen
Technical Talks
Read More
Laying It Straight: Line-Holding Accessories for Masons Who Demand Precision
January 2026

When it comes to laying brick, block, or stone, nothing replaces a good line. A tight, true mason’s line is the backbone of straight courses and clean elevations, and the hardware holding that line is every bit as critical. Bon Tool’s lineup of Line & Lin

Paul Bender
Technical Talks
Read More
The Importance of Eye Protection in Masonry
January 2026

There are few stereotypes more iconic than the image of a construction worker wearing a hard hat and safety glasses. Protective glasses are among the most common items worn by workers in the masonry field, second only to the hard hat. According to OSHA, w

Justus Everett
Safety Library
Read More