Vinyl flooring over concrete.

I wouldn’t recommend fixing 5.5mm plywood directly onto concrete. Plywood is intended for timber subfloors, and if it’s bonded or screwed to concrete without the correct method, it can lead to movement or moisture issues. The best solution is to bring the concrete up to the same height as the plywood by applying a suitable smoothing or self-levelling compound. This will create one continuous, level surface ready for the sheet vinyl. Before doing this, make sure the concrete is clean, sound and dry. If there’s any risk of moisture, a damp-proof membrane (DPM) should be applied first. Once the concrete is level with the plywood, the entire floor will be suitable for the vinyl, giving you a smooth, long-lasting finish. If you’re unsure about the condition of the concrete, feel free to send some photos and I can advise on the best preparation.