What thickness of plywood should you use for a roof?

Jese Leos
Jese Leos
Published on 19-Dec-2025
What thickness of plywood should you use for a roof?

If you’re building or re-roofing, picking the right plywood thickness matters. Too thin and the roof feels bouncy, shingles or metal can crack, and fasteners pull out easier. Too thick and you waste money and add unnecessary weight. Below I’ll explain the common choices, why they matter, what types of plywood to use, convert thicknesses into mm, give examples, and finish with a quick checklist.

Quick answer (if you’re in a hurry)

  • Rafter/joist spacing 16" on center (o.c.) → use ½ inch (12.7 mm) plywood.
  • Rafter/joist spacing 24" o.c. → use ⅝ inch (15.9 mm) plywood, or at least ½ inch of a high-rated sheathing like APA-rated 7/16" OSB if code allows.
  • Heavy loads, long spans, or if you want extra stiffness → use ¾ inch (19.05 mm) plywood.

Always check local building code and roof covering manufacturer instructions.

Why thickness matters?

Plywood on the roof (roof sheathing) is the layer your roofing material — shingles, metal, tiles — sits on. Thicker panels:

  • Bend less under foot traffic and snow loads
  • Hold nails/screws better (less pull-through)
  • Make the roof feel solid and reduce cracking of roof finish

Thinner panels save money but can flex, causing early wear on shingles or noisy metal panels.

Common thickness options and when to use them

Below are the usual choices builders use in most houses:

1) ½ inch (0.5") — about 12.7 mm

  • Good for: roof rafters at 16" o.c. and typical residential roofs with shingles or most standing-seam metal panels.
  • Why: gives a solid surface and is a common minimum for many local codes when rafters are 16" o.c.

Conversion (careful math):
1 inch = 25.4 mm.
So 0.5 × 25.4 = 12.7 mm.

2) ⅝ inch (0.625") — about 15.875 mm (≈15.9 mm)

  • Good for: rafters at 24" o.c., long spans, heavier roof coverings, or when you want less deflection.
  • Why: thicker panel reduces bounce and spreads loads better at wider spacing.

Conversion:
0.625 × 25.4 = 15.875 mm → round to 15.9 mm.

3) ¾ inch (0.75") — about 19.05 mm

  • Good for: very wide spans, roofs that will carry heavy loads (solar panels, heavy tile), or when you want a very stiff deck.
  • Why: lowest deflection and best fastener holding.

Conversion:
0.75 × 25.4 = 19.05 mm.

4) 7/16 inch (0.4375") OSB — about 11.11 mm

  • This is commonly used as an economical sheathing product (OSB, not plywood) for roofs with 24" o.c. rafters only if local code or manufacturer allows it.
  • Conversion: 0.4375 × 25.4 = 11.1125 mm → ≈11.1 mm.

Bottom line: ½" plywood is the safe standard for 16" o.c.; ⅝" for 24" o.c.; use ¾" when you need extra stiffness.

Types of plywood for roof sheathing (and which one to pick)

1. CDX plywood (exterior-grade sheathing)

  • Most common for roofs.
  • “C” and “D” are face/back grades; “X” means exterior glue.
  • Pros: affordable, widely available, designed for roof use.
  • Pick this for most normal residential roofs.

2. APA-rated sheathing / Structural plywood

  • Graded by industry standards for strength and stiffness.
  • Often labeled with span rating and use (e.g., “sheathing”).
  • Pros: predictable performance — great if you want to be certain about deflection.

3. OSB (Oriented Strand Board)

  • Often cheaper than plywood. Common sizes: 7/16", 15/32", 23/32".
  • Performs well if kept dry; commonly specified in modern builds.
  • If choosing OSB, follow the exact span/thickness guidance and keep it dry during construction.

4. Pressure-treated plywood

  • Use only where wood touches masonry or in areas prone to moisture contact.
  • Don’t use pressure-treated unless required — it’s more expensive and heavier.

5. Marine plywood / hardwood plywood

  • Overkill for normal roofs — expensive and unnecessary unless special conditions apply.

Recommendation: For most roofs, CDX plywood or APA-rated sheathing at the thickness recommended for your rafter spacing is the right choice.

Fastening & installation tips (won’t use fancy terms)

  • Use the nail/screw size recommended by code and the plywood maker. Usually roofing nails or ring-shank nails for better holding.
  • Stagger sheet joints and leave a small gap (usually 1/8") between sheets for expansion.
  • For 24" o.c. rafters, consider nailing pattern closer or use a thicker panel.
  • Install underlayment (felt or synthetic) and follow roofing manufacturer instructions — plywood alone isn’t the weather barrier.

Examples (real-like, simple math)

Example 1 — Small house, rafters 16" o.c.

  • Rafter spacing = 16 inches
  • Recommended plywood thickness: ½ inch (0.5")
  • Conversion: 0.5 × 25.4 = 12.7 mm → use 12–13 mm plywood (commonly sold as ½").

Example 2 — Garage roof, rafters 24" o.c.

  • Rafter spacing = 24 inches
  • Recommended plywood thickness: ⅝ inch (0.625")
  • Conversion: 0.625 × 25.4 = 15.875 mm → use 15–16 mm plywood (commonly sold as 5/8").

Example 3 — Roof that will carry solar panels

  • Solar panels add point loads and require stiffer deck. Even if rafters are 16" o.c., consider ¾ inch (0.75") plywood for extra stiffness.
  • Conversion: 0.75 × 25.4 = 19.05 mm → use ~19 mm plywood.

Common mistakes to avoid

  • Using ½" plywood on 24" o.c. rafters (it may feel bouncy and can shorten shingle life).
  • Ignoring roof covering manufacturer specs — metal panels or tiles can require thicker sheathing.
  • Skipping underlayment or poor nailing — fastening matters as much as panel thickness.
  • Leaving no gap at panel joints — wood swells with moisture.

Quick checklist before you buy

  • What is your rafter/joist spacing? (16" o.c. or 24" o.c.?)
  • What roofing material will you install? (asphalt shingles, metal, tile?)
  • Do you need to carry extra loads? (solar, HVAC, heavy tile?)
  • Local building code and inspector: check first.
  • Pick CDX or APA-rated sheathing at the thickness that matches spacing and loads.

Final plain-language recommendation

For a normal house with standard roof materials:

  • Use ½ inch (12.7 mm) plywood if rafters are 16" o.c.
  • Use ⅝ inch (15.9 mm) plywood if rafters are 24" o.c.
  • Use ¾ inch (19.05 mm) if you expect heavy loads or want extra stiffness.

Always check your local building code and the roofing manufacturer’s instructions. If in doubt, talk to your roofer or building inspector — a small extra cost on plywood can save you headaches and repairs later.