Volumetric thermal expansion

$\frac{\Delta V}{V} = \alpha_V \Delta T$ where $\alpha_V$ is the volumetric expansion coefficient. Valid only for small changes ($\Delta V/V \ll 1$): the formula uses the initial volume $V$ and ignores that $V$ grows during heating.

Be aware that $\alpha_L$ and $\alpha_V$ are different, and $\alpha_L \approx \alpha_V/3$ (under these conditions: 1) Isotropic, 2) Free to expand in all three directions, 3) Small change).

Assuming the layer has a fixed area $A$, it can only expand upward. Assuming also that $\Delta T$ is uniform over the layer, this relationship is valid: $\Delta V = A\,\Delta h$, so $\Delta h = \alpha_V \Delta T \cdot h_{layer}$, where $h_{layer}$ is the thickness of the layer that warms up.