Understanding Aesthetic Classifications of Cross-Laminated Timber (CLT)

Discover the latest innovations in mass timber, including product examples from HDR Pavilion, Element5, Sterling Structural, and Mercer Mass Timber, emphasizing their appearance standards, sustainability practices, and panel dimensions for diverse architectural applications.

Jeanie Fitzgerald Pitts

Sept. 1, 2026

4 min read

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Key Highlights

Architectural Appearance CLT features filled knot holes, smooth surfaces, and high-quality finishes suitable for exposed interior applications.
Industrial Appearance CLT allows for natural imperfections like wane, loose knots, and unfilled voids, ideal for rustic or concealed uses.
Manufacturers provide detailed specifications to ensure aesthetic consistency, including surface treatment, void management, and panel handling standards.
Product examples include fire-rated panels, sustainably sourced wood, and large-format panels up to 12 ft. wide and 60 ft. long, catering to diverse project needs.
Understanding these classifications helps architects specify the right CLT product to achieve desired aesthetics and performance in mass timber projects.

As the popularity of mass timber grows, architects are grappling with new terms important to specifying projects that look as intended. According to WOODWORKS (woodworks.org), aesthetic expectations for cross-laminated timber (CLT) are agreed upon between the building designer and manufacturer/fabricator, described in the product specifications, and available in two classifications—Architectural Appearance (AA) surface classification for a highly finished look and Industrial Appearance (IA) for more rustic applications.

Here are a few of the characteristics that define each classification.

Architectural Appearance Classification

In exposed surfaces, all knot holes and voids measuring over 3/4 in. (19 mm) are filled with a wood-tone filler or clear wood inserts selected for similarity with the grain and color of the adjacent wood.
The face layers exposed to view are free of loose knots and open knot holes are filled.
Knot holes do not exceed 3/4 in. (19 mm) when measured in the direction of the lamination length with the exception that a void may be longer than 3/4 in. (19 mm) if its area is not greater than 1/2 in. (323 mm).
Voids greater than 1/16 in. (1.6 mm) wide created by edge joints appearing on the face layers exposed to view are filled.
Exposed surfaces are surfaced smooth with no misses permitted.

Industrial Appearance Classification

Voids appearing on the edges of laminations need not be filled.
Loose knots and knot holes appearing on the face layers exposed to view are not filled.
Members are surfaced on face layers only and the appearance requirements apply only to these layers.
Occasional misses, low laminations or wane (limited to the lumber grade) are permitted on the surface layers and are not limited in length.

Several CLT manufacturers provide documents outlining appearance options for the products they produce. Here are a few examples of how manufacturers describe their Architectural Appearance CLT solutions.

SmartLam at New HDR Pavilion

Architectural Appearance-Grade cross-laminated timber (CLT) panels were used throughout the new HDR Pavilion built during the recent renovation of the College of Architecture at the University of Nebraska-Lincoln (UNL). These panels are one-hour fire-rated, made from Spruce Pine Fir with a Douglas Fir visual layer on the bottom, and meet a variety of qualifications in terms of void and knot size and appearance.

For more information, visit the product webpage.

Element5 Architectural Appearance CLT Panels

Element5's architectural appearance cross-laminated timber (CLT) provides a high-quality, visually refined surface finish designed for exposed interior applications like ceilings, walls, and floors, utilizing edge-glued Spruce-Pine-Fir (SPF) lumber to minimize gaps and present a clean aesthetic. Element5 CLT is manufactured in a FSC certified plant using sustainably sourced wood. At 3.1 m maximum width, Element5 produces the widest panels available in Canada, delivering enhanced efficiency to any build.

For more information, visit the product webpage.

Standardized Architectural Appearance CLT from Sterling Structural

Standardized Appearance-Grade CLT Panels Sterling Structural

Sterling Structural manufactures to a single appearance standard that can be used for both exposed and concealed applications. We utilize No. 2 lumber, sort for wane, and have rigorous panel handling requirements for our operations and logistics teams. By standardizing our appearance grade, we are able to use readily available lumber to get you your products faster and at a lower price point. Sterling uses high-grade planers to deliver a smooth final surface that honors the natural characteristics of each species.

For more information, visit the product webpage.

Customizable CLT from Mercer Mass Timber

Our CLT panels offer a balance of durability, beauty, and efficiency built for today’s performance standards. With a maximum dimension of 12 ft. wide and 60 ft. long, Mercer Mass Timber offers CLT panels in an Architectural Appearance (AA) surface classification (also known as Architectural Appearance-Grade for a highly finished look and Industrial Appearance (IA) for more rustic applications.

For more information, visit the product webpage.