TJI Roof Joist Design Example (TJ-4000): 18 in. through 24 in. Depths, Roof Slopes 1/4:12 to 3:12, and StructSuite

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TJI Roof Joist Design Example (TJ-4000): 18 in. through 24 in. Depths, Roof Slopes 1/4:12 to 3:12, and StructSuite

Summary: Select deep TJI roof joists using Weyerhaeuser TJ-4000 for 18 in. through 24 in. depths: one design roof load column per combination, scan for required…

Overview

Select deep TJI roof joists using Weyerhaeuser TJ-4000 for 18 in. through 24 in. depths: one design roof load column per combination, scan for required horizontal clear span, and verify bearing and stiffener rules by depth.

Problem statement

A custom home uses deep TJI roof joists to cover a great room with a shallow roof (within the TJ-4000 limits for the 18 in.–24 in. chapter). Roof slope is 2-1/2:12, which falls between 1/4 :12 minimum and 3:12 maximum for this chapter. Design uses the non-snow ASD column Non-snow 125% — (roof live + dead) (20 + 15) psf. Joists at 16 in. on center are 18 in. deep TJI 360. The horizontal clear span between the bearing wall and the interior bearing line (horizontal projection) is 28 ft 0 in. The engineer must confirm the joist is adequate per TJ-4000 and document bearing and stiffener requirements for the depth used.

How to use the 18 in.–24 in. roof table (TJ-4000): (1) Determine appropriate roof loads and load duration factor. (2) Scan down the selected Design Roof Load column until a tabulated span meets or exceeds your horizontal clear span. (3) Select the TJI joist depth, series, and on-center spacing for that row.

General notes (TJ-4000, 18 in.–24 in.): Tables assume roof slopes from 1/4 :12 minimum to 3:12 maximum, uniform loads, the more restrictive of simple or continuous span, and L/180 total load and L/240 live load deflection. 18 in. and 20 in. TJI joists: minimum bearing length 1-3/4 in. at ends and 3-1/2 in. at intermediate bearings without web stiffeners. 22 in. and 24 in. TJI joists: same bearing lengths but web stiffeners are required at all bearings. For continuous spans, short span to long span ratio should be ≥ 0.4 to prevent uplift. For 2024 IBC, snow in ASD follows ASCE 7 combinations (0.7S noted in guide). A support beam or wall at the high end is required; ridge board alone is not adequate. For conditions outside the tables, use Weyerhaeuser software.

Workflow in StructSuite

Open the Floor Joist, Ceiling Joist & Rafter module. Select Roof Joist & Rafter, then TJI® Joist (roof). Under Depth and roof slope (TJ-4000), select 18 in. through 24 in. depths — Roof slope Between 1/4 :12 and 3 :12. Under Rafter spacing (inches on center), select 16. Under Design Roof Load, select Non-snow 125% — (roof live + dead) (20 + 15) psf. Under Joist depth, select 18". Under TJI joist designation, select TJI 360. Under Horizontal clear span, enter 28 ft and 0 in. The result is SATISFACTORY or NOT SATISFACTORY against TJ-4000 for that row and column.

Design considerations (excerpt)

Deep TJI roof chapters (18 in.–24 in.) use a single deflection column per design roof load—there is no Low/High split like the 9-1/2 in.–16 in. tables. Slopes outside 1/4 :12 to 3 :12 are outside this printed chapter; use manufacturer software. Always separate TJ-4000 roof I-joists from IBC sawn rafter checks.

Pick the Design Roof Load line that matches your controlling ASD roof combination (non-snow vs snow columns). Spacing must be 16, 19.2, or 24 in. o.c. as tabulated. The guide’s L/180 total and L/240 live criteria are built into the published spans; verify project deflection limits if they are stricter than the guide.

For 18 in. and 20 in. depths, TJ-4000 states…

Related terms

  • StructSuite
  • structural engineering
  • design example
  • framing
  • tji roof joist span 18 24

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