An Inside Look at a Fast-Rising Building Material
It’s no secret mass timber construction is gaining new adherents with commercial building owners, developers and architects. The appeal of mass timber’s aesthetics, sustainability, renewability, durability, speed of construction, off-site prefabrication and code compliance has inspired a new generation of designers specifying it across nearly all property types.
The 2015 International Building Code (IBC) prescriptively lists mass timber assembly in Type IV construction. The IBC also allows construction across other types, especially Type III and Type V. There is a strong likelihood you will see more mass timber projects coming your way as architects continue to embrace this versatile, code-compliant building material.
This installment of Code Counts looks at a key structural element used in many mass timber projects – nail-laminated timber (NLT). Why NLT? What is NLT? How has it become a preferred mass timber building material? Here’s a brief look at NLT, gathered from a comprehensive, new 142-page handbook titled Nail Laminated Timber — U.S. Design & Construction.
NLT: How and Why
Mass timber products in the U.S. include glued-laminated timber (GLT), cross-laminated timber (CLT), dowel-laminated timber (DLT) and NLT. The rise of NLT as a specification favorite is largely due to domestic accessibility: NLT does not require a special manufacturing facility and is fabricated with readily available dimension lumber. By contrast, alternative mass timber elements like CLT must be imported from Canada and Europe for the most part (at least one U.S. mill is now certified by the American National Standards Institute and the American Plywood Association for CLT panel manufacturing).
Nail-laminated timber is created by placing dimension lumber (nominal 2x, 3x or 4x thickness and 4-inch to 12-inch width) on edge and fastening the individual laminations together with nails. Often specified for floors and roofs, NLT can also be used for walls, elevator shafts and stair shafts. Plywood/ oriented strand board added to one face can provide in-plane shear capacity, allowing NLT to be used as a shear wall or diaphragm.
Architects like NLT because it allows the creation of monolithic “slab” panels that support a broad range of architectural opportunities. The IBC recognizes NLT as code-compliant for buildings with varying heights, areas and occupancies that allow for Type III, Type IV or Type V construction.
Fire and Life Safety Performance
Prescriptive requirements for fire and life safety performance in IBC Section 101.3 are based on construction type, which is linked to use and occupancy classification, as well as building height and area. NLT is listed as a prescriptive heavy timber assembly in Type IV construction. However, the IBC also allows NLT in all types of construction. A look at each:
Type IV Construction
Nail-laminated timber in Type IV buildings is used mostly for roofs, floors and interior walls. However, two code exceptions permit combustible material in exterior walls:
- IBC Section 602.4.1: Where fire retardant treated wood is used
- IBC Section 602.4.2: Where CLT is used and covered with fire-retardant-treated wood sheathing, gypsum board or noncombustible material
These provisions can be applied to NLT in exterior walls. Additionally, NLT can be applied to Type IV construction to the following areas:
- Floors – IBC Section 602.4.6
- Roofs – IBC Section 602.4.7
- Interior Walls and Partitions – IBC Section 602.4.8.1
- Exterior Walls – IBC Sections 602.4.1, 602.4.8.2 and 2303.2
Type III Construction
Type III construction permits concealed spaces and can provide greater flexibility for use of exposed NLT, especially with non-rated building elements.
Type V Construction
Typical Type V is often light-frame construction. Section 602.5 of the IBC indicates that Type V construction allows structural elements, exterior walls and interior walls to be constructed of any material permitted by code, including NLT. Type V-B, where no fire resistance rating is required, provides greater flexibility for NLT use.
Type I and Type II Construction
The use of NLT is understandably limited in Type I and Type II construction. However, IBC Section 603.1 offers exceptions, notably:
- Fire-retardant-treated wood used in most roofs and non-loadbearing wall applications
- Heavy timber roof construction in a Type I-B or Type II building
In applications where heavy timber is permitted, NLT is not required to be fire-retardant treated. Nail-laminated timber may also be used as permanent formwork for a concrete slab in either type.
Alternative Means and Methods
Designers increasingly specify NLT in buildings beyond prescriptive parameters. Section 104.11 of the IBC provides a path for code approval through alternative means and methods. For example, designers may consider enhancing certain fire safety features to gain NLT use in areas not prescribed by code in Type I or II construction.
Nail-laminated timber is an old method of construction that’s undergoing a design renaissance. Code officials may find it beneficial to take a more comprehensive look at this rejuvenated building material. Nail Laminated Timber – U.S. Design & Construction Guide offers the latest code-based research to assist understanding and decision-making.
The views and opinions expressed in this article are those of the Softwood Lumber Board and do not necessarily reflect those of the International Code Council, or Hanley Wood