Sheathing and non-veneer boards + panel ratings

One-line orientation

Plywood and OSB are the common structural sheathing panels. Other board families vary by product, grade, and listing, so check the rated use rather than assuming every fiberboard or hardboard product is nonstructural.

Key points

• Plywood: veneer layers glued with alternating grain directions → strength and dimensional stability. Structural sheathing/subfloor.
• OSB (oriented strand board): long wood strands bonded with waterproof glue. Structural — exterior sheathing, subflooring, roofing, walls; cost-effective vs plywood with similar performance; edges swell with moisture if unsealed.
• Non-structural boards:
  • Particleboard — wood particles bonded under heat/pressure, often veneered; smaller fiber than OSB; furniture, cabinetry, countertop substrate; interior, not structural, highly water-sensitive.
  • MDF is ordinarily an interior finish or substrate product, not structural sheathing. Fiberboard is a broader family: most insulating fiberboard is nonstructural, but structural fiberboard and roof-deck products exist.
  • Overlays — HDO (resin-fiber overlay, smooth for concrete forms/cabinets) and MDO (melamine-resin overlay, smooth for painting).
• APA structural (span) rating: two numbers = roof spacing / floor spacing (max support spacing in inches). Example 40/20 = 40″ roof, 20″ floor.
• Bond / exposure (weather) classification:
  • Exterior — continuous outdoor exposure; fully waterproof glue.
  • Exposure 1 — withstands occasional moisture (e.g., construction delays), not long-term; ~95% of plywood.
  • Exposure 2 — limited moisture resistance; protected interior use.

Confusions / comparison

Panel	Built from	Structural?	Notes
Plywood	Cross-laminated veneers	Yes	Strong, stable
OSB	Oriented strands + waterproof glue	Yes	Cheaper than ply; edges swell
Particleboard	Small particles	No	Interior; water-sensitive
Fiberboard / MDF	Fine fibers + resin	Generally no; check product rating	Often insulating board or interior finish/substrate

Bond / exposure class	Moisture resistance	Use
Exterior	Continuous outdoor (waterproof glue)	Permanent exterior exposure
Exposure 1	Occasional moisture only	Protected, but tolerates build delays (~95% of ply)
Exposure 2	Limited	Protected interior

Related

→ Engineered wood products (this topic): structural members vs these sheet products · Wood framing systems (this topic): where sheathing acts as structural lateral bracing · Wood moisture and seasoning (this topic): why edge swelling and exposure class matter.