While underlay isn’t strictly required, it’s highly recommended when installing solid wood flooring over concrete. Solid wood is sensitive to moisture and temperature fluctuations, and concrete can emit residual dampness even if it feels dry. Using a suitable underlay—especially one with a moisture barrier—helps protect the floor from warping, cupping, or long-term damage. In many cases, it’s also advisable to apply a liquid damp-proof membrane (DPM) for extra protection before laying the underlay and flooring.