Morning all, new member here after some advice. I have a 1940s house with floorboards to the ground floor that were nailed and appeared to be stuck down to the concrete slab with what appears to be bitumen or asphalt. It varies in thickness from 1mm to around 6mm. It is soft to touch with a sharp object. Can I apply Ardex straight over it, Ardex DPM then N/a or do I need to remove it ?