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How thick should plywood be over joists?

Published in Plywood Subfloor Thickness 2 mins read

The required thickness of plywood for a subfloor over joists primarily depends on the spacing of the underlying joists. To ensure a stable and durable floor, it's crucial to select the appropriate thickness.

Understanding Plywood Thickness Based on Joist Spacing

For most standard residential applications, a specific plywood thickness is recommended to prevent excessive deflection, squeaks, and structural issues. The general guideline is as follows:

  • For joists spaced 16 inches apart or less: A minimum of 15/32-inch plywood is typically recommended. This thickness provides adequate support and stability for common joist spacing, preventing the subfloor from feeling bouncy or weak.
  • For joists spaced further apart (greater than 16 inches): A thicker plywood, specifically 3/4-inch, should be used. This increased thickness is necessary to bridge the larger gaps between joists, offering the rigidity needed to support the floor and its loads without compromising performance.

Here's a summary of recommended plywood thicknesses:

Joist Spacing Recommended Plywood Thickness
16 inches or less 15/32-inch
Greater than 16 inches 3/4-inch

Why Correct Thickness Matters

Using the correct plywood thickness for your subfloor offers several significant benefits:

  • Structural Integrity: Ensures the subfloor can adequately support the weight of furniture, people, and any overlying finished flooring without sagging or bouncing.
  • Prevents Deflection: Reduces the likelihood of the floor flexing between joists, which can lead to uncomfortable movement and potential damage to finish flooring materials like tile.
  • Reduces Squeaks: A stiff subfloor minimizes movement between the plywood and joists, helping to prevent annoying squeaks and creaks.
  • Longevity of Finish Flooring: Provides a stable base that helps extend the lifespan of your top flooring layers, such as hardwood, laminate, carpet, or tile, by preventing undue stress or cracking.

Always consult local building codes as they may have specific requirements or allowances for subfloor materials and thicknesses that supersede general recommendations. Selecting the appropriate plywood thickness is a fundamental step in constructing a sturdy and reliable floor.