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

Example

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.

How StructSuite solves this

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.

Steps

1. Step 1: Member type and TJI roof design basis

   Design consideration: 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.

   In StructSuite: Under "Select member type", select the **Roof Joist & Rafter** radio. Under **Rafter design basis**, select **TJI® Joist (roof)** — the option whose subtitle is **TJI® Joist Specifier's Guide: TJ-4000 roof span tables**.

2. Step 2: Depth chapter, spacing, and design roof load

   Design consideration: 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.

   In StructSuite: Under **Depth and roof slope (TJ-4000)**, select the chapter whose full label is: **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 the row: **Non-snow 125% — (roof live + dead) (20 + 15) psf**.

3. Step 3: Joist depth and TJI series

   Design consideration: For **18 in.** and **20 in.** depths, TJ-4000 states minimum bearing 1-3/4 in. and 3-1/2 in. without web stiffeners. For **22 in.** and **24 in.**, stiffeners are required at **all** bearings—call that out on details if you switch depths. TJI series (e.g., 360 vs 560) changes the allowable span for the same depth and spacing.

   In StructSuite: Under **Joist depth**, select **18"**. Under **TJI joist designation**, select **TJI 360**.

4. Step 4: Horizontal clear span

   Design consideration: Span entry is **horizontal clear span** along the plan projection, not sloped length. If NOT SATISFACTORY, try a higher series, deeper joist, tighter spacing, or add a support. Continuous framing: maintain short/long ≥ 0.4 per guide. Show structural support at the ridge/high end; do not rely on a ridge board alone.

   In StructSuite: Under **Horizontal clear span**, enter **28** in the ft box and **0** in the in. box. The result shows SATISFACTORY or NOT SATISFACTORY per Weyerhaeuser TJ-4000 maximum horizontal clear spans (deflection L/180 total load and L/240 live load per guide).

Live design (pre-filled)

The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.