Why Is My Wall Bulging or Bumpy? Hollow Plaster vs a Bad Finish

Free checks: tap the wall and listen

Do this first. It is the single most useful diagnostic on this page and it costs nothing.

• The tap test. Tap the wall firmly with a knuckle or the handle of a screwdriver, working across the whole wall in a grid, roughly every hand-width. A wall with plaster properly bonded to the masonry gives a hard, dull, solid knock. Plaster that has separated gives a hollow, drummy, higher sound — like tapping an empty box. Mark the hollow areas lightly in pencil as you go.
• Map the hollow area. Its shape tells you a lot. A hollow patch that follows a bathroom wall, a kitchen wall, an outside face or a line near the floor points at moisture. A hollow patch around an old repair or a chased channel points at bad workmanship in that repair.
• The straightedge test. Hold anything long and straight — a spirit level, a metal ruler, the edge of a shelf — flat against the wall in several directions and look for gaps behind it. This measures how uneven the surface actually is, as opposed to how uneven it looks.
• The torch test. At night, hold a torch almost flat against the wall so the beam grazes the surface. Every ridge, roller mark, putty lap and lump throws a shadow. This is how a bumpy finish is judged, and it is also how you should check work before you pay for it.
• Feel and smell it. Press the bulge gently. Sound plaster does not give. Plaster that flexes, crumbles, feels soft or feels cool and damp against the back of your hand is a different problem. A musty smell in the room adds to that.
• Look for the companions. Cracking along the edge of the bulge, a white powdery or crystalline bloom, paint lifting, or dark patches all point at moisture and salts rather than a finishing fault.

Hollow or drummy plaster: the bond has failed

Plaster works by keying into the surface behind it. When that key breaks, the plaster becomes a stiff sheet resting against the wall rather than joined to it. It bulges outward because it has nowhere else to go.

• What it looks like. A shallow dome or a raised band, often with a crack running around its edge. Tapping it sounds hollow. Pressing it may produce a faint crunch or a slight movement. The paint over it may look fine, which is why people miss it for years.
• Why the bond fails. Poor preparation when it was plastered — a dusty or unwetted wall, or a mix applied too thick in one go. Plaster laid over a smooth concrete surface without any key. Movement between two different materials, such as where brickwork meets a concrete column. Or moisture, which is the most common cause of all and gets its own section below.
• How to confirm it. The tap test, mapped across the wall. A hollow area with a defined boundary, rather than the whole wall sounding the same, is the confirmation.
• Is it DIY? No. The only real repair is to hack off the loose plaster back to sound material and re-plaster the area, then let it cure before finishing. Filling the bulge, screwing a board over it, or skimming putty across it are all ways of hiding a wall that is still letting go.

Do not treat this as urgent-danger unless it is overhead. A drummy patch on a wall can sit for a long time. A drummy patch on a ceiling should be looked at sooner, because falling plaster is heavy.

Moisture and salt behind the plaster: the classic Delhi case

The single most common bulging wall in a Delhi flat is a wall shared with a bathroom — the bedroom side of a shower wall, the wall behind a cistern, or the strip along the floor next to a bathroom door. Water gets into the masonry from the wet side and works outward.

• What moisture does. Water soaking through masonry softens the plaster from behind and breaks the bond between plaster and wall. The plaster then swells slightly and pushes out. This is why the bulge is usually on the dry side of a wet wall, and why it maps to the shower, the WC, a buried pipe run or an outside face.
• What salt does — and it is worse. Water moving through brick and mortar dissolves salts, carries them outward, and leaves them as crystals when it evaporates. Those crystals grow and expand behind and inside the plaster, and they physically push it off the wall. That is why you often see a white powdery or crystalline bloom at the edge of a bulging patch, and why the plaster there crumbles rather than cracks cleanly.
• How to confirm it. Look at what is on the other side of the wall. Check whether the bulge is worse after the monsoon, or worse when a particular bathroom is used a lot. Look for a white bloom, dark tide marks, soft crumbly plaster and paint lifting nearby. Our damp wall: how to find the cause guide walks through separating a leaking pipe from seepage from condensation.
• The order of work matters. Re-plastering a salt-damaged wall without stopping the water simply restarts the clock. The water source is fixed first, the wall is allowed to dry, then it is replastered. Skipping the drying stage is why the same patch fails twice.
• Is it DIY? No. This is a leak or waterproofing job followed by a plastering job. Wall seepage and waterproofing explains what drives that work.

If you also have black spotting in the same area, read black mould on walls. If the paint is lifting or blistering around the bulge, paint peeling and paint bubbling cover that side of it.

A bumpy wall that is completely sound: bad putty and uneven skimming

If the whole wall knocks solid and there is no damp anywhere, what you have is a finishing problem. The wall is fine. The surface on it is not flat. This is far commoner than drummy plaster and it is entirely a repaint issue.

• Putty applied unevenly or too thick. Wall putty is meant to go on in thin, even passes and be sanded flat between them. Rushed work leaves ridges where the blade lifted, laps where two passes met, and thick shoulders around repairs. Under a torch it looks like a landscape. See wall putty and primer for what putty is actually for.
• No sanding, or sanding only where it showed. Sanding is the slow, dusty, unglamorous part of the job, and it is the part that gets cut when a flat is being finished in a hurry. It is also the entire difference between a flat wall and a bumpy one.
• Patch repairs that were never feathered out. A filled crack or a plugged hole that stands proud of the surrounding wall reads as a lump forever, especially in a satin or gloss finish. Matt finishes hide unevenness; sheen finishes advertise it.
• Grit and debris rolled into the coat. Dust and bits picked up by a dirty roller or an unstrained tin leave hard specks that feel like sandpaper. Very common in Delhi where dust settles on everything on site.
• The wall was never level to begin with. Some old plastering is simply out of true by several millimetres over a length. Putty cannot correct that; only re-plastering or a levelling coat can, and it is often not worth it.

Is it DIY? Partly. Sanding down proud areas, filling hollows and repainting a single wall is within reach if you are patient. A whole flat is a lot of sanding. If this happened on work you have just paid for, patchy or streaky paint after painting covers the related finish complaints.

Repaint or re-plaster? When to call someone, and what to tell them

The decision is simple once you have tapped the wall. Solid everywhere means a repaint. Hollow anywhere means the hollow area has to come off and be re-plastered. No middle option works for long, because putty over drummy plaster fails with the plaster under it.

• Tell them the result of the tap test. “About half a square metre sounds hollow on the bedroom side of the bathroom wall, solid everywhere else” is a description someone can quote against without a site visit turning into a surprise.
• Tell them what is on the other side of the wall. Bathroom, kitchen, external face, a neighbour’s flat, or a pipe run. This is the first thing a competent person asks.
• Say whether there is any white bloom, staining, softness or smell. That moves the job from plastering to finding water first.
• Ask what happens before re-plastering. The answer you want includes hacking back to sound material with a margin around the hollow area, dealing with the water source, and letting the wall dry before finishing. If the answer is “we will fill it and putty over”, ask why that will hold.
• Ask about curing time. New plaster needs to cure before it is painted, and painting it too early causes its own failures — see why paint cracks and paint not sticking to the wall.
• If the bulge is on a ceiling, or you can see rusted steel or spalled concrete, get it looked at properly and do not work under it in the meantime. Cracking that runs in structural lines rather than around a patch belongs in cracks in walls.

For what drives the cost of the repaint that follows, see house painting cost in Delhi NCR, and best time to paint and waterproof for timing the work around the monsoon.