Events

How should desired appearance grades and characteristics be specified for CLT, NLT and glulam?

For mass timber, specification of appearance grade varies by product. Aesthetic expectations for cross-laminated timber (CLT) and nail-laminated timber (NLT) are agreed upon between the building designer and manufacturer/fabricator and described in the product specifications, whereas grades for glued-laminated timber (glulam) are standardized.

CLT Appearance Grades

Starting with the 2015 version of the International Building Code (IBC), CLT has been prescriptively allowed in construction types where heavy/mass timber is permitted as long as it is manufactured in accordance with the ANSI/APA PRG 320 Standard for Performance-Rated Cross-laminated Timber (IBC 2303.1.4). Section 7.3 of PRG 320 states that “CLT panel appearance shall be as agreed to between the buyer and the seller.” Appendix A of PRG 320 lists options for architectural and industrial appearance grades as follows:

APPENDIX A. EXAMPLES OF CLT APPEARANCE CLASSIFICATIONS (NON-MANDATORY)

This appendix contains examples of CLT appearance classifications for reference only. These requirements are based on the appearance at the time of manufacturing. The actual CLT panel appearance requirements are recommended to be agreed upon between the buyer and the seller.

  1. A1.  Architectural Appearance Classification

    An appearance classification normally suitable for applications where appearance is an important, but not overriding consideration. Specific characteristics of this classification are as follows:
    • 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.2 (323 mm2).
    • 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.
  1. A2.  Industrial Appearance ClassificationAn appearance classification normally suitable for use in concealed applications where appearance is not of primary concern. Specific characteristics of this grade are as follows:
    • 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.

Most North American manufacturers and building designers either follow the guidelines in Appendix A, or include a section in the project specifications defining “non-exposed” and “exposed” appearance grades and stating the acceptable parameters for each (e.g., checking, knot size, color variation, etc.). Following is an example of a “Part 2: Products” specification section addressing aesthetic characteristics:

C. Appearance Classifications

  1. Non-Exposed Applications: Locations where CLT panels are concealed from view
    • ​ Shake and checks allowed, shall not exceed 36 in. or 1/4 of the panel length
    • ​Heart or blue stain allowed, not limited
    • Knots well-spaced, quantity not limited
    • Minimal wane on face permitted
    • Side pressure on exposed face not required
  2. Exposed Applications: Locations where CLT panels are exposed to view in final construction
    Exposed face to utilize “J” grade of SPF lumber

    • ​Shake and checks allowed, shall not exceed 24 in. long, none through
    • ​Heart or blue stain not allowed
    • ​Knots meet NLGA select structural limitation; select tight knots
    • ​Pitch streaks not permitted
    • Wane on face not permitted
    • Side pressure on exposed faces required

Several CLT manufacturers provide documents outlining appearance grade options for the products they produce. See this link for one example.

In Europe, terms such as WSI, ISI and NSI also relate to CLT appearance grades. However, they are rarely if ever used by North American manufacturers. See this document for an explanation of these and other common appearance terms for European CLT.

NLT Appearance Grades

Since NLT does not have a recognized standard for production, it is up to the design team (usually the architect) to note requirements for appearance and quality of panels. As with CLT, this information is typically included in the project specifications.

The raw material that goes into NLT (usually 2x framing lumber) is standardized—in the IBC, the American Wood Council’s National Design Specification (NDS) for Wood Construction, and the American Lumber Standards Committee’s Voluntary Product Standard 20—so language from those sources can be a helpful starting point. The U.S. Nail-Laminated Timber Design & Construction Guide also includes discussion of appearance grade options. For example, it notes that the following surface characteristics can be specified:

  • Species
  • Lumber grade
  • Eased or sharp edges
  • Cross section size​​

See Section 2.1.2 of the NLT Guide for additional information and panel examples, which are helpful for establishing acceptable and unacceptable aesthetic characteristics. Appearance charts can be found in Appendix A.

Glulam Appearance Grades

Glulam is manufactured to various appearance classifications—including Premium, Architectural, Industrial and Framing—for structural and architectural applications. These classifications distinguish various levels of natural growth and manufacturing characteristics that are visible on the glulam surfaces. They do not characterize or affect structural performance.

Of these classifications, Premium and Architectural are used in applications where the member may be exposed and finishing may be desirable. Premium is usually specified where appearance is the primary design consideration; however, it is usually a custom order. Architectural grade is suitable for use in applications where appearance is important but not the main consideration. Several sub-categories, including Framing-L and Industrial-L are also available. These match their main appearance classifications with the exception that the outermost plies are laminated veneer lumber (LVL) rather than dimension lumber. Additional information regarding appearance classifications can be found in AITC 110-01: Standard Appearance Grades for Structural Glued Laminated Timber and in Chapter 13 of ANSI A190.1-07 American National Standard for Wood Products: Structural Glued Laminated Timber.

Glulam is also available with either a smooth or a rough textured surface. Rough textured surfaces may require up to twice as much finish as smooth surfaces.

For additional resources see the technical note: Glulam Appearance Classifications for Construction Applications from APA.

For a list of North American mass timber product manufacturers, see the list of WoodWorks’ National Partners here or email the WoodWorks Project Assistance Help Desk at .