When the cement/plaster pulls away from a wall making a hollow space behind it. Drummy render is when the cement or plaster render coating becomes loose. When tapped with a bar it produces a hollow, weak sound (i.e. it sounds like a drum, or it sounds “drummy”). There are many different causes for drummy render and these have been debated by many plasterers. These possible causes include:  heavy vibrations, water ingress (see below), not enough cement in the mix, render went on too thick, or it may be that when the plumbers and electricians chase up the walls the dust left behind formed a membrane on the wall separating the render from the substrate. It may be some or all or none of these factors, but these are the most commonly quoted possibilities.