TJI Roof Joist Design Example (TJ-4000): 9-1/2 in. through 16 in. Depths, Low Roof Slope Columns, and StructSuite

Example

A one-story residence has conventionally framed roof joists (I-joists) bearing on exterior bearing walls. The roof pitch is 4:12 (6:12 or less), so the TJ-4000 table uses the **Low** horizontal-deflection column for the selected design roof load. Roof dead plus roof live are represented by the non-snow ASD column **Non-snow 125% — (roof live + dead) (20 + 15) psf** per the guide. Joists are spaced 16 in. on center. The engineer will use **11-7/8 in.** depth **TJI 230** from the 9-1/2 in. through 16 in. roof chapter. The horizontal clear span between bearing walls (measured along the horizontal projection of the joist per TJ-4000) is 19 ft 0 in. Ceiling below is not driving this example; the focus is roof table selection. **How to use the 9-1/2 in.–16 in. roof table (TJ-4000):** (1) Determine appropriate roof loads and load duration factor as required by the project and the guide. (2) If roof slope is 6:12 or less, use the **Low** column for the chosen design roof load; if slope is between 6:12 and 12:12, use the **High** column. (3) Scan down the column until you find a tabulated span that meets or exceeds your horizontal clear span. (4) Select the TJI joist series and on-center spacing that corresponds to that row. **General notes (TJ-4000):** Tables assume minimum roof slope 1/4:12, uniform loads, the more restrictive of simple or continuous span, deflection criteria of L/180 for total load and L/240 for live load, and minimum bearing length of 1-3/4 in. at ends and 3-1/2 in. at intermediate bearings without web stiffeners (for these depths), unless the guide requires otherwise. For continuous spans, the ratio of short span to long span should be 0.4 or greater to prevent uplift issues. For 2024 IBC, snow loads in ASD are adjusted per ASCE 7 combinations (TJ-4000 references 0.7S where applicable). A support beam or wall at the high end is required; a ridge board alone does not provide adequate support. For flat roofs or other loading conditions not shown, use the manufacturer’s software.

How StructSuite solves this

Open the Floor Joist, Ceiling Joist & Rafter module. Select **Roof Joist & Rafter**. Under **Rafter design basis**, select **TJI® Joist (roof)**. Under **Depth and roof slope (TJ-4000)**, select **9-1/2 in. through 16 in. depths — Roof slope 6:12 or less**. 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 **11-7/8"**. Under **TJI joist designation**, select **TJI 230**. Under **Horizontal clear span**, enter **19** ft and **0** in. StructSuite compares the span to the TJ-4000 tabulated maximum horizontal clear span for that row and column and reports SATISFACTORY or NOT SATISFACTORY.

Steps

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

   Design consideration: TJ-4000 roof tables are separate from IBC common-rafter tables: horizontal clear span, factored ASD roof load columns, and Low vs High columns for 9-1/2 in.–16 in. depths. Pick **TJI® Joist (roof)** so StructSuite uses the specifier guide data embedded in the app. If you leave **RAFTER SPANS FOR COMMON LUMBER SPECIES**, you are in IBC Chapter 23 prescriptive tables instead.

   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: The chapter label fixes which printed TJ-4000 table you are emulating: narrow depths split **Low** (slopes ≤ 6:12) vs **High** (6:12 to 12:12). Spacing must match a table column (16, 19.2, or 24 in. o.c. in StructSuite). The **Design Roof Load** row must match your ASD combination from the project (non-snow vs snow columns per guide). Deflection is L/180 total and L/240 live in the guide; long-term creep notes may apply for some cells.

   In StructSuite: Under **Depth and roof slope (TJ-4000)**, select the chapter whose full label is: **9-1/2 in. through 16 in. depths — Roof slope 6:12 or less**. 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: Depth and TJI series identify the row in the span table. Heavier series or deeper I-joists increase capacity; the example fixes **11-7/8 in.** and **230** so the user sees one complete selection path. Verify bearing length (1-3/4 in. end, 3-1/2 in. intermediate for these depths per TJ-4000 general notes) on the construction documents.

   In StructSuite: Under **Joist depth**, select **11-7/8"**. Under **TJI joist designation**, select **TJI 230**.

4. Step 4: Horizontal clear span

   Design consideration: Enter span as **horizontal projection**, not along the slope. The tool compares your span to the allowable in the selected column; if NOT SATISFACTORY, increase depth or series, tighten spacing, or reduce span. Continuous spans: keep short/long ≥ 0.4 where the guide requires it to control uplift. Ridge boards do not replace bearing at the high end — show a ridge beam or wall support in the framing plan.

   In StructSuite: Under **Horizontal clear span**, enter **19** 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.