Introduction
Walk into a lumberyard and the plywood aisle looks like alphabet soup: CDX, BC, AC, RTD, T&G, “Exposure 1,” span ratings stamped in oval logos. Each code describes something real — face quality, glue type, structural rating — and buying the wrong one means either paying for performance the job doesn’t need or installing a panel that can’t do what the job requires.
The Plywood Sheet Calculator tells you how many sheets to buy; this guide tells you which sheets. Grades, thicknesses, tongue-and-groove, and span ratings — mapped to the applications where each belongs.
Plywood Grades: What the Letters Mean
Sanded-panel grades are written as face/back — AC means an A face over a C back. Faces grade A (best) through D:
- A — smooth, paintable, minimal patches. Finish work.
- B — smooth with small patches. Good for visible painted surfaces.
- C — knotholes and patches permitted. Structural, will be covered.
- D — the roughest face. Never visible; fine as a back.
| Grade | What it is | Typical use |
|---|---|---|
| CDX | C face / D back, exterior glue | Wall sheathing, roof decks, subfloors — anywhere covered |
| BC | B face / C back, exterior glue | Soffits, garage interiors, painted visible surfaces |
| AC | A face / C back, exterior glue | Visible painted panels, signage, cabinet backs |
| RTD (Rated Sheathing) | Structural, span-rated, may be unsanded | Code-driven sheathing — the panel the stamp governs |
| Underlayment | Smooth, often 1/4″–3/8″, exterior glue | Over subfloors beneath vinyl, tile, or laminate |
| Marine-grade | No voids, waterproof glue, both faces good | Boats, docks, permanently wet exposures — premium price |
| Hardwood plywood | Hardwood face (oak, birch, maple) | Cabinetry, furniture, built-ins — not structural sheathing |
The “X” in CDX denotes exterior glue — the panel tolerates construction moisture and brief weather exposure. It does not mean the panel is waterproof; no standard construction plywood is meant to stay wet permanently. Price differences between grades are covered in the cost guide.
Thickness: Which Size Goes Where
Thickness is a structural choice, driven by span and load:
| Thickness | Metric | Typical applications |
|---|---|---|
| 1/4″ | 6.35 mm | Underlayment, crafts, drawer bottoms, curved work |
| 3/8″ (11/32″) | ~9.5 mm | Light wall sheathing, soffits (check span rating) |
| 1/2″ (15/32″) | 12.7 mm | Wall sheathing, roof decks at 16″–24″ rafter spacing |
| 5/8″ (19/32″) | 15.9 mm | Roof decks at wider spacing, single-layer subfloor in light use |
| 3/4″ (23/32″) | 19.05 mm | Subfloors over 16″ o.c. joists — the residential standard |
Note that “1/2-inch” plywood is typically manufactured at 15/32″ and “3/4-inch” at 23/32″ — the nominal name is traditional, the actual dimension is slightly under. For estimating purposes the difference is irrelevant (a 4×8 is still 32 sq ft); it matters only for fine joinery and when matching existing material.
Crucially, thickness never changes the sheet count — the calculator’s 12 × 16 ft wall needs 7 sheets (192 ÷ 32 = 6.0, +10% = 6.6 → 7) whether those sheets are 1/2″ CDX or 3/4″ T&G. What changes is suitability and price.
Tongue-and-Groove (T&G)
T&G panels have interlocking long edges. The joint between two sheets lands between joists without a support underneath — the interlock transfers load across the joint, stiffening the floor and cutting the differential movement that causes squeaks. T&G is standard for subfloors (usually 3/4″, 23/32″ actual) and common for roof decks.
T&G costs a few dollars more per sheet than square-edge. For subfloors it’s money well spent; for wall sheathing, where every edge lands on a stud, square-edge is fine and cheaper. The pro tips guide covers the installation details that make T&G perform.
Span Ratings: Reading the Stamp
Structural panels carry an APA-style stamp with a span rating like 24/16 or 32/16. The first number is the maximum roof support spacing in inches; the second is the maximum floor support spacing:
- 24/16 — rated for rafters up to 24″ on center, joists up to 16″ on center. The common wall/roof sheathing rating.
- 32/16 — heavier panel; rafters up to 32″ on center, joists up to 16″ on center.
- 40/20 — thick subfloor panel; joists up to 20″ on center.
- Exposure 1 — glue bond withstands construction moisture; not for permanent exterior exposure without covering.
- Exterior — fully exterior glue and C-or-better veneers; for permanently exposed applications like soffits.
Match the rating to your framing: 16″ on-center joists need a panel whose second number is 16 or higher. When in doubt, the code compliance guide explains what to verify with your local building authority.
Choosing for the Three Big Jobs
Match the panel to the job first; the count follows the same formula regardless. The complete guide walks through the full estimating workflow once the spec is set.
- Wall sheathing: 1/2″ (15/32″) CDX or RTD sheathing, span rating matched to stud spacing (usually 16″ o.c.). Covered by siding or housewrap — no need for sanded faces.
- Subfloor: 3/4″ (23/32″) T&G, Exposure 1, span rating covering your joist spacing. Glue to joists; leave 1/8″ expansion gaps.
- Roof deck: 1/2″ or 5/8″ CDX/RTD depending on rafter spacing and local snow/wind loads; 5/8″ where 24″ o.c. rafters or heavier loads apply. Covered by underlayment and shingles promptly — Exposure 1 is not a weathering surface.
Related Guides
- Complete Guide — the full walkthrough of the calculator, formulas, and worked example.
- Cost Guide — sheet prices by thickness and grade, plus budgeting.
- Code Compliance — span ratings, nailing schedules, and what to verify locally.
- Pro Tips — layout, staggering, and fastening advice from the trade.
- Estimating Guide — step-by-step measuring for walls, floors, and roofs.
- FAQ — quick answers to the most asked plywood questions.
Frequently Asked Questions
What does CDX stand for?
C-grade face, D-grade back, exterior (X) glue. It’s the standard structural panel for sheathing that will be covered by siding, roofing, or flooring.
Can I use CDX plywood for a subfloor?
Yes — 3/4″ CDX or RTD sheathing with the right span rating is the standard residential subfloor, ideally tongue-and-groove and glued to the joists.
Is marine-grade plywood worth it?
Only for permanently wet exposures — boats, docks, exterior structures with no covering. For sheathing that gets covered, it’s a large premium for performance the job never uses.
What’s the difference between Exposure 1 and Exterior?
Exposure 1 panels withstand construction-phase moisture but must be covered; Exterior panels use fully waterproof glue and better veneers for permanent exposure. Neither is a substitute for proper flashing and covering.
Why is “1/2-inch” plywood actually 15/32″?
Nominal sizing is traditional; panels are manufactured slightly under nominal. It doesn’t affect estimating (coverage is still 32 sq ft per 4×8 sheet), only fine joinery and matching existing work.
Do I need T&G for wall sheathing?
No. Wall sheathing edges land on studs, so square-edge panels are standard. T&G pays off on floors and roofs where joints fall between supports.
Get Your Number
Choose the panel by application — CDX or rated sheathing for covered structural work, BC/AC where the face stays visible, T&G where joints fall between supports, and thickness by span and load. Then get the count from the Plywood Sheet Calculator and price it with the cost guide. Right panel, right count, right price — that’s the whole material decision.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual sheet needs vary with layout, cuts, openings, and waste. Verify quantities with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
