If you have an uneven sub surface, a underlay floor can potentially help level it out, but we don’t recommend this at all. An uneven surface under a floor can cause boards to sink and shift, potentially cracking or splitting them.
Can you use self-leveling underlayment?
Self-leveling underlayment is a great choice under any tiled floor, but it’s the absolute best choice when installing tile over an in-floor heating system.
How do I know if my floor is level before laying laminate?
Look for areas where the level doesn’t lay flat to identify high and low spots. Plane or sand high spots using a wood planer or hand sander. Check the spot with your level periodically, and stop sanding when the spot becomes level with the surrounding surface.