Is OSB Subfloor More Vulnerable to Water Than Plywood?

OSB is made from wood strands pressed and glued together in layers, while plywood is made from thin sheets of wood veneer glued crosswise. The manufacturing process affects how each material reacts to water. OSB tends to absorb moisture through its edges and cut ends more readily than plywood.

When OSB gets wet, the strands can swell and the board may not return to its original shape even after drying. Plywood, with its layered structure, often resists swelling better and may dry with less permanent distortion. But both materials can lose strength and develop mold if they stay wet for long.

Plywood also suffers from water damage, but it often dries more evenly. The thin layers can separate, or delaminate, if the glue fails, which weakens the board. However, plywood is less likely to swell as dramatically as OSB, so repairs might be simpler in some cases.

That said, plywood is not waterproof. Prolonged exposure can cause rot, mold, and structural failure. If you have plywood subfloor, you still need to dry it quickly and check for damage after any water event.

If the OSB or plywood remains soft, crumbles, or has visible mold after drying, replacement is usually necessary. You can test the wood by pressing on it with a screwdriver; if it sinks in easily, the structure is compromised.

Replacing a subfloor involves cutting out the damaged sections and installing new panels. A water damage inspection can help you determine the extent of the damage. The pros can also handle the removal and repair of water-damaged flooring, so you don't have to do it alone.

• Soft spots that don't harden after drying
• Visible mold or mildew growth
• Warped or buckled areas
• Delamination where layers separate
• Strong musty smells that persist