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

This note walks through deep TJI roof joist selection using Weyerhaeuser TJI® joist roof span tables from TJ-4000 for 18 in. through 24 in. depths. It explains the roof slope limits for this printed chapter (1/4 :12 minimum through 3 :12 maximum), design roof load columns (non-snow and snow ASD combinations), spacing (16, 19.2, and 24 in. o.c.), depth and TJI series, horizontal clear span, deflection criteria, bearing lengths and web stiffener rules for 22 in. and 24 in. products, and how StructSuite implements the same workflow when you choose Roof Joist & Rafter and TJI® Joist (roof).

Official reference (same document as linked in StructSuite)

StructSuite links to the publisher’s document library entry for the guide:

Weyerhaeuser — TJ-4000 (Specifier’s Guide for TJI joists, USA)

Use that PDF as the authority for exact table text, footnotes, and product definitions. The discussion below is educational; always confirm details against the current TJ-4000 edition you are working under.

What StructSuite does for TJI roof joists

In the Floor Joist, Ceiling Joist & Rafter module, after you select Roof Joist & Rafter and TJI® Joist (roof), StructSuite lets you:

• Choose the TJ-4000 roof chapter for 18 in. through 24 in. depths — Roof slope Between 1/4 :12 and 3 :12 (single column set per design roof load; no Low/High split like the 9-1/2 in.–16 in. roof chapter).
• Set Rafter spacing to 16, 19.2, or 24 in. on center.
• Pick a Design Roof Load line matching the guide’s ASD factored combinations (non-snow 125% roof live+dead; snow 115% snow+dead, per app labels and TJ-4000).
• Select Joist depth (18", 20", 22", or 24"), TJI joist designation available for that depth, and enter Horizontal clear span, then read SATISFACTORY / NOT SATISFACTORY against the tabulated maximum span.
• Show deflection criteria (L/180 total load and L/240 live load per TJ-4000), bearing length notes, and stiffener requirements where the guide applies them (especially for 22 in. and 24 in. depths).

Use manufacturer software or engineered design for slopes or load cases outside this printed chapter.

Roof slope limits (18 in. through 24 in. chapter)

For this chapter, TJ-4000 roof tables are based on roof slopes from 1/4 :12 minimum to 3 :12 maximum. If the project’s geometry or architectural roof falls outside that range, the printed table may not apply; use Weyerhaeuser software or other approved methods per the publisher.

How to use the table (guide logic)

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 TJI joist depth, series, and on-center spacing for that row.

There is one horizontal-deflection column per Design Roof Load line in this chapter (unlike the Low/High pairing used for 9-1/2 in.–16 in. depths).

Design roof load columns and joist spacing

Each Design Roof Load option corresponds to a column in the digitized TJ-4000 18 in.–24 in. roof tables. Changing the load line selects a different column; tabulated allowable spans change accordingly.

Joist spacing

• StructSuite roof tables use 16, 19.2, and 24 in. o.c. (no 12 in. o.c. column in this roof digitization).

If project loads or spacing do not match a listed column, use manufacturer software or engineered design.

Depth, TJI series, bearing, and web stiffeners

• Depth picks the row family in the tables. TJI series (e.g., 360, 560) sets capacity for a given depth and spacing.
• Bearing (typical TJ-4000 wording for this chapter)
  • 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 (per typical guide notes—confirm in PDF).
  • 22 in. and 24 in. TJI joists: same minimum bearing values, but web stiffeners are required at all bearings per typical guide notes—confirm in your TJ-4000 edition.

Always complete connection and support design per the manufacturer and adopted code.

Horizontal clear span and special cell flags

Span is horizontal clear span along the horizontal projection, not length along the slope. StructSuite’s Horizontal clear span entry follows TJ-4000.

When a cell triggers footnote (1) or creep-related messaging, read the full footnote and publisher guidance in TJ-4000; StructSuite surfaces short reminders when the digitized data includes those flags.

General notes (typical TJ-4000 roof chapter, 18 in. through 24 in.)

Tables are based on

• Roof slopes 1/4 :12 minimum through 3 :12 maximum (for this chapter).
• Uniform loads.
• The more restrictive of simple or continuous span.
• Deflection criteria L/180 total load and L/240 live load (per guide).

Continuous spans

• Short span to long span ratio should be ≥ 0.4 to help limit uplift (per guide).

Snow and code

• For 2024 IBC, snow in ASD follows ASCE 7 combinations (0.7S noted in guide where applicable).

Ridge / high-end support

• A support beam or wall at the high end is required; ridge board alone is not adequate structural support in the guide’s assumptions.

Outside the tables

• Use Weyerhaeuser software for conditions not shown.

Worked example (prescriptive table check)

Given

• Roof slope 2-1/2 :12 (within 1/4 :12 to 3 :12 for this chapter).
• Design Roof Load: Non-snow 125% — (roof live + dead) (20 + 15) psf.
• Spacing: 16 in. o.c.
• Depth: 18", TJI 360.
• Horizontal clear span (trial): 28 ft 0 in.

Steps in StructSuite

1. Open Floor Joist, Ceiling Joist & Rafter → Roof Joist & Rafter → TJI® Joist (roof).
2. Under Depth and roof slope (TJ-4000), select 18 in. through 24 in. depths — Roof slope Between 1/4 :12 and 3 :12.
3. Set Rafter spacing to 16.
4. Select the Design Roof Load line above.
5. Set Joist depth to 18" and TJI joist designation to TJI 360.
6. Enter Horizontal clear span = 28 ft 0 in. and read SATISFACTORY or NOT SATISFACTORY.

Interpretation

• If NOT SATISFACTORY, increase series, depth, tighten spacing, shorten span, or add supports.
• Document bearing and stiffeners per depth (especially if using 22 in. or 24 in. depths).

Interactive design example (read-only wizard): Open this example in StructSuite

TJI roof (engineered I-joist) vs. common lumber (IBC Chapter 23 rafter tables)

Use TJ-4000 for TJI roof joists; use IBC 2308 rafter tables for common lumber when that path matches the project.

References

1. Weyerhaeuser, TJ-4000 — Specifier’s Guide for TJI Joists (USA) — Weyerhaeuser document library (TJ-4000)
2. International Building Code (adopted edition) — prescriptive roof framing in Chapter 23 for dimensional lumber when applicable.
3. ASCE 7 (adopted edition) — snow and load combinations for ASD as required by the project.

StructSuite embeds the same TJ-4000 document in the TJI roof joist UI for convenience; engineering decisions remain the responsibility of the licensed professional of record.