What Is Drummy Render? The hollow sound that matters.

Drummy render is pool render that has lost its bond with the concrete shell behind it. When you tap it, it sounds hollow, like a drum, instead of dull and solid. The render looks fine from the surface, but there is a gap behind it, and sooner or later that patch lets go. It is one of the most common reasons an old pool surface fails.

The tap test

You can check for drummy patches yourself. It only works on an empty pool, or on the parts you can reach above the waterline such as steps and the top of the walls.

1. Use a small hard object, like the handle of a screwdriver or a coin.
2. Tap the surface lightly and listen. Solid render gives a short, dull tap.
3. Move across the area in a grid, and listen for a hollow, higher, drum-like sound.
4. Note where the hollow spots are. Patches often cluster around steps, the waterline and the floor near the main drain.

A trained ear finds drummy render a lot more reliably than a first-time tapper. We sound out the whole shell when we inspect, and again once the old surface is off.

What causes drummy render

Render sticks to the shell through a mechanical and chemical bond. Anything that weakens that bond can leave a hollow pocket behind the surface. The usual causes are:

• Poor prep at the original build or last resurface: a shell that was not cleaned or keyed properly before the render went on
• Water getting behind the surface through hairline cracks, failed tile joints or small leaks
• Movement in the shell or ground, including reactive clay in parts of western Sydney
• Pressure from groundwater pushing against the shell when the pool is empty or the valve is not working
• Age and wear: the bond weakens over time, especially under thermal cycling in hot summers

Often it is a combination. The cause matters, because if you re-render without dealing with the reason it went hollow, the same patch can come back.

What happens if you ignore it

A drummy patch is not an emergency, but it only gets worse. As water works in behind the render, the patch grows. Eventually the surface cracks, flakes off in a section, or leaves a rough, sharp-edged hole in the floor or wall. It also gives stains, algae and scale a place to take hold.

The mistake we see most often is painting over drummy render. The paint looks good for a short while, then lifts along with the render below it. That is a big part of why pool painting is only the honest answer on a sound shell. Our guide to paint versus resurfacing explains the difference.

How drummy render is fixed

The repair is simple in principle and fussy in practice:

• Sound out and mark every hollow area
• Cut the drummy render out back to solid material, with clean edges
• Prep the exposed shell so the new render will bond
• Re-render the patch, shaped and levelled to match the surrounding surface
• Let it cure properly before the finish goes over the top

This is done before any new surface is applied, which is why drummy render is dealt with during a resurface, while the pool is empty. Our drummy render repair page covers how we approach it, and pool repairs gives the wider picture.

Where drummy render tends to show up

It can appear anywhere, but some spots are more likely than others. Steps and benches take the most wear and the most movement of water. The waterline sits right where the chemistry and temperature swing the most. The floor around the main drain can be affected by groundwater pressure. Walls and corners show it where there has been shell movement or a past leak.

This is one reason we sound out the whole pool rather than just the obvious areas. A pool that looks fine from above the waterline can still have hollow patches lower down, and you want to know about them before a new surface goes on, not after. We also look at the tiles and coping at the same time, since loose tiles often travel with hollow render behind them.

Is drummy render always a reason to resurface?

Not always. A few small hollow patches on an otherwise good surface can sometimes be repaired on their own. If the hollow areas are widespread, or the surface is also stained, rough or flaking, a full resurface usually makes more sense. We would rather tell you that after an inspection than guess from a photo.