What effect does gluing a plywood subfloor to the joists (instead of nailing) have on floor stiffness?

Gluing a plywood subfloor to the joists enhances the overall stiffness of the floor system. This method creates a strong bond between the subfloor and the joists, allowing forces and loads to be distributed more evenly across the entire surface. The adhesive effectively reduces any potential movement between the joists and the plywood, resulting in less deflection when loads are applied.

When the subfloor is nailed down, there can be slight gaps or movement due to the potential for nail withdrawal over time or under load. This gap can lead to lower stiffness and a floor that feels bouncier or less solid. In contrast, a glued-down subfloor minimizes any such movement, ensuring a solid, stable surface which contributes positively to the structural integrity of the flooring system, thus increasing the overall stiffness.

The increased stiffness benefits not only performance but also the longevity of the flooring by reducing the likelihood of squeaks and minimizing wear over time.