Flooring (C-15) · Lesson 2 of 7

Subfloor Panels and Underlayment: Spans, Edge Support, and Particleboard

"Subfloor Panels and Underlayment: Spans, Edge Support, and Particleboard" is lesson 2 of 7 in the Flooring (C-15) Learning Hub. It opens with "Flooring Starts at the Subfloor Rating" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Flooring Starts at the Subfloor Rating
  2. Footnotes h and i: Finish Floors That Extend the Span
  3. Unsupported Edges and Underlayment (Footnote j)
  4. Particleboard Underlayment and Composite-Wood Emissions
  5. Lumber Subfloors and Strip Flooring (R503.1.1)

The worked example in the course applies this lesson to a realistic job in 6 steps, from "Span needed" through "Edge support".

What you'll be able to do

  • Explain the main idea of "Subfloor Panels and Underlayment: Spans, Edge Support, and Particleboard" in plain language you can use on the exam.
  • Apply this step with confidence: "Unsupported Edges and Underlayment (Footnote j)".
  • Apply this step with confidence: "Particleboard Underlayment and Composite-Wood Emissions".
  • Apply this step with confidence: "Lumber Subfloors and Strip Flooring (R503.1.1)".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Subfloor Panels and Underlayment: What the Table AllowsJOISTS 24 in o.c. (SECTION, not to scale)square edge,no blocking24 in spangold = 1/4 in panel underlayment (footnote j)tan = subfloor panel, joint at the red lineEdge support: T&G edges, blocking, or1/4 in underlayment (Table R503.2.1.1(1) fn j)WHICH SUBFLOOR PANEL WORKS?PanelMax spanCarpet3/4 hdwd1.5 in LWC48/2424 inOKOKOK40/2020 inNOOK (h)OK (i)32/1616 inNOOK (h)NOh: 3/4 in wood finish floor at rightangles to the joistsi: 1.5 in lightweight (LWC) or approvedcellular concrete over the subfloorNeed 24 in: only 48/24 with carpet orresilient; others need a footnote creditCarpet: only 48/24. 3/4 hardwood: all three. 1.5 in LWC: 48/24 and 40/20.A 1/4 in wood panel underlayment also gives the square edges their edge support (footnote j)Source: 2025 CRC Table R503.2.1.1(1) footnotes h, i, j (Chapter 5, p. 13).CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Flooring Starts at the Subfloor Rating

The floor covering is only as good as the panel under it. 2025 CRC Table R503.2.1.1(1) lists the maximum subfloor span for each panel span rating (printed on the panel stamp): 24/16 (7/16 inch panel) 16 inches; 32/16 (15/32 and 1/2 inch) 16 inches; 40/20 (19/32 and 5/8 inch) 20 inches; 48/24 (23/32 and 3/4 inch) 24 inches; 60/32 (7/8 inch) 32 inches. Footnote b requires the panels to run continuous over two or more spans with the long (strength) axis perpendicular to the supports.

Step 2: Footnotes h and i: Finish Floors That Extend the Span

Two footnotes let the finish floor add span. Footnote h gives a maximum span of 24 inches where 3/4-inch wood finish flooring is installed at right angles to the joists; it is attached to the 32/16 and 40/20 subfloor entries. Footnote i, attached to the 40/20 entry only, gives 24 inches where 1.5 inches of lightweight concrete or approved cellular concrete is placed over the subfloor. A carpet or resilient floor earns no such credit, so the unmodified span applies.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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