Hollow-Sounding Plaster in Ghana — The Tap Test Guide 2026
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Hollow-sounding plaster has already failed. The bond between plaster and blockwork is gone and there is an air gap behind it — what is holding it up is its own edges. Painting it, filling it or skimming over it buys a few months. The only question worth asking is how far the debonding extends, and a tap test answers it in twenty minutes.
How to run the test
- Use your knuckle or a hammer handle. Not the hammer head — you want to listen, not to break anything.
- Work a grid of about 300 mm. Systematic beats random. Start in a corner and work across in rows.
- Listen for two notes. Sound plaster gives a sharp, tight tap. Debonded plaster gives a deeper, drummy, slightly resonant note. Once you have heard both you cannot mistake them.
- Mark as you go. Chalk or pencil on every hollow spot. You are drawing a map of the failure.
- Keep going past the visible damage. This is the part people skip, and it is the whole point of the exercise.
Reading the map
What you have marked usually tells you the cause.
| Pattern | Likely cause |
|---|---|
| Band along the bottom, fairly level top edge | Rising damp. Needs a damp-proof course, not replastering. |
| Patches on one elevation, worse after rain | Penetrating damp through a porous external wall. |
| Vertical strip, often near a bathroom or kitchen | Plumbing leak in a chase. |
| Running down from ceiling level | Roof, parapet or gutter leak above. |
| Scattered, no pattern, no damp | Poor application — dusty blockwork, no wetting down, over-rich mix. |
| Around window and door reveals only | Movement, or water tracking in at the frame. |
That last row is worth noting. Not every hollow patch is damp. On new work it is often simply that the blockwork was dry and dusty when the plaster went on, and nothing was done about it.
How much to take off
Remove everything that sounds hollow, plus 300 mm beyond the last hollow spot in every direction.
That margin is not conservatism. Where damp is involved, salts have travelled further through the blockwork than the debonding shows. If you cut back only to the edge of the drummy area you leave salt-loaded block right at the joint, and the new plaster fails along that line within a season.
On a wall where the whole lower band is hollow, take the full band off rather than patching. A patchwork of old and new plaster on a damp wall will show as a patchwork within a year regardless of how well it is finished.
Before you replaster
Three things, in order:
Deal with the water. If the tap test pointed at a leak, fix the leak. If it pointed at rising damp, you need a damp-proof course — we are explicit about this in why plaster keeps falling off damp walls, and no surface product changes it.
Let it dry. Weeks, not days. This is why the November to March window matters for this work in Ghana.
Prepare the exposed blockwork. Old block and mortar that has been wet and then dried is usually chalky. A sand fixing agent penetrates 5-10 mm and binds that weak surface, giving the new plaster something solid to key into instead of dust.
A note on timing the test
Run the tap test at the end of the rainy season rather than in February. Damp-related debonding is at its most extensive when the wall is wet, and testing a bone-dry Harmattan wall can understate how far the failure has spread.
Send a photo of the marked-up wall and we will tell you what you are looking at. Call 0555 548 001 or 0241 737 026, or book a free assessment in Greater Accra.
Related Resources
- Why Plaster Keeps Falling Off Damp Walls
- Rising Damp vs Penetrating Damp in Ghana
- White Powder on Concrete Walls (Efflorescence)
- Wall Leakage Repair in Ghana
- Sand Fixing Agent in Ghana — Complete Guide
Fest Ghana Limited is the trading name of FGL Waterproofing Solution Limited Company, a company registered in Ghana — Registration No. CS218171026. Company information.