Black Locust is appearing in more specifications every year — driven by Ipe’s CITES Appendix II listing, tightening embodied carbon requirements, and clients who increasingly ask hard questions about where their materials come from. But because it’s a specialty species, many spec writers are drafting Black Locust sections for the first time. This guide covers the practical details: where it belongs in your spec, what language to use, and the handful of requirements that prevent RFIs and change orders down the line.
Where Black Locust Fits in Your Specification
For building projects, Black Locust decking is most commonly specified under MasterFormat Section 06 15 00 (Wood Decking). Siding applications fall under Division 07 exterior cladding sections, and site applications — boardwalks, permeable pavers, site furnishings — typically live in Division 32. If your office uses a master spec, the existing tropical hardwood decking section is a reasonable starting point: the fastening, gapping, and substructure requirements are broadly similar, and the species-specific data below replaces the Ipe or Cumaru references.
Basis-of-Design Language
Specify the species by botanical name: Black Locust (Robinia pseudoacacia). Because Black Locust is not a commodity lumber product, a basis-of-design approach with a named supplier is the most reliable path to getting what you drew. If your spec allows substitutions, tie them to performance criteria rather than species lists:
- USDA decay resistance classification: Very Resistant (highest tier), untreated
- Janka hardness: minimum 1,700 lbf
- No chemical preservative treatment of any kind
- Documented chain of custody; FSC-certified material where the project requires it
This performance-based language matters. It prevents the substitution of pressure-treated lumber or uncertified tropical species that meet a hardness number but fail the sustainability and durability intent of the specification.
Key Properties for the Spec
- Janka Hardness: 1,700 lbf
- USDA Decay Resistance Class: Very Resistant (above-ground applications rated 30+ years; see real-world lifespan data)
- Modulus of Rupture: 22,000 psi
- Density: ~48 lbs/ft³
- Treatment: None — durability is natural, from flavonoid compounds present throughout the wood
Grades, Sizes, and Length Realities
Standard decking boards are available in 4″ and 6″ widths and lengths up to 12 feet. This is the single most important thing to know when detailing: Black Locust does not grow like a plantation softwood, and long clear boards are limited. For runs beyond 12 feet, specify finger-jointed boards — they perform equivalently and are the standard solution for boardwalks and long-deck commercial work. Detailing your board layout around available lengths at the design stage avoids the most common Black Locust RFI.
Grading follows conventional hardwood practice: higher grades are cleaner and more uniform; lower grades carry more knots, mineral streaking, and color variation. Many designers deliberately specify character grades for the natural aesthetic. Either way, name the grade in the spec and require a physical sample submittal — don’t leave it to the supplier’s default.
Fasteners, Gapping, and Substructure
- Fasteners: Stainless steel only. Standard and even coated fasteners can react with the wood’s tannins, causing staining and accelerated corrosion. Write stainless into the spec explicitly.
- Pre-drilling: Required at all fastener locations. Note it in the installation article.
- Gapping: 1/8″ to 3/16″ between boards for expansion.
- Acclimation: Boards must acclimate to site conditions before installation.
- Ventilation and drainage: As with any hardwood deck, specify airflow under the deck surface and positive drainage.
Submittals to Require
- Physical samples of the specified grade
- Technical data sheets with species properties
- Chain of custody documentation (FSC certificates where the project requires certified material)
- Fastener product data confirming stainless steel
- BIM/CAD files for detailing, where needed
Common Specification Mistakes
The mistakes we see most often: copying an Ipe section and leaving 16- and 20-foot board lengths in the schedule; permitting generic “coated” fasteners; specifying film-forming finishes (they peel — penetrating oil or natural weathering are the correct maintenance paths, see the buyer’s guide for details); and omitting the acclimation requirement. All four are easy to avoid at the drafting stage and expensive to fix in the field.
Specification Support
Robi Decking’s technical team works with architects and landscape architects on specifications regularly — we can review your draft section, provide technical data sheets, BIM files, and LEED documentation, and help with take-offs. For the broader case for the species, see Why Black Locust Belongs in Your Next Green Building Specification.
Want to see Black Locust in action across a wide range of climates and project types? Browse our project gallery.
