Loose, bulging or “blown” plaster: what can be saved, and how it’s repaired

Loose plaster can usually be repaired. If a section has pulled away from its lath but is still hard and in one piece, it can be fastened back with plaster washers or a plaster adhesive, and the cracks are then taped, filled and primed. Only plaster that is crumbling, water-damaged or badly broken needs to be cut out and replaced.

Many older homes in Grosse Pointe and across Metro Detroit still have their original plaster, and after enough seasons some of it lets go. That’s rarely a reason to tear walls out. Here’s how to recognize loose plaster, what causes it, and what a repair that lasts actually involves.

What does “loose” or “blown” plaster mean?

Traditional plaster went on wet, usually in two or three coats, over thin strips of wood lath nailed across the studs and joists. The first coat was pushed hard enough to squeeze through the gaps between the strips, where it curled over and hardened into “keys”. Those keys are what hold the plaster to the wall.

When enough keys snap, the plaster is held up by little more than its neighbors. Tradespeople call it loose, delaminated or “blown” plaster. A related problem is coats separating from each other: the thin finish coat lets go of the coarser base coat beneath it, and flakes or bubbles even though the base is still tight.

The distinction matters, because the fixes differ. Plaster that has let go of its lath needs fastening back to the framing. A finish coat that is flaking needs scraping back to a sound edge and refinishing.

How can you tell if plaster is loose?

Loose plaster usually announces itself in a few ways:

• A bulge or pillow in the wall, or a dip in a ceiling, that you can see when light skims across it.
• Cracks with a raised edge, where one side sits proud of the other.
• A hollow or rattly sound when you tap lightly with your knuckles, instead of the dull, solid sound of sound plaster.
• Movement when you press gently with a flat hand. Sound plaster doesn’t give at all.
• Fine dust or crumbs collecting below a crack, or picture hooks that keep pulling out.

Press gently, and never push hard on a bulging ceiling. The goal is to find the edges of the loose area, not to test how much it can take.

What makes plaster come loose?

Plaster is rigid, and houses aren’t. Framing lumber shrinks and swells as the air dries out through a Michigan heating season and turns humid again in summer, and every cycle works the keys a little. Decades of that, plus doors slamming, heavy footsteps overhead and the occasional settling of the house, eventually break enough of them for a section to let go.

Water speeds it up. A roof leak, a dripping pipe or an overflowing tub softens plaster and rots wood lath, and loose plaster often sits right below an old leak. Past work plays a part, too: hammering for a renovation, cutting in new wiring or hanging heavy shelves can all break keys around the spot.

Where there’s an obvious cause, such as a leak, fix it before the plaster. Otherwise the repair is fighting the same problem that caused the damage.

How is loose plaster reattached?

A lasting repair secures the plaster first and hides the evidence second. No crack filler will hold loose plaster on its own, because the patch moves every time the plaster does. The usual sequence:

1. Map the loose area by tapping and pressing gently, and find where the plaster is sound again.
2. Protect the room. Furniture is moved or covered, floors are protected, and dust is contained, which matters doubly in older homes.
3. Fasten it back. Screws driven through plaster washers, thin perforated discs, pull the plaster snug against the lath or into the framing. They’re spaced around and across the loose area until it no longer moves.
4. Or glue it back. Adhesive systems made for plaster are another option: small holes are drilled through the plaster but not the lath, the dust is cleaned out, a conditioner and then adhesive are injected, and the plaster is held snug with temporary screws and washers until the adhesive cures.
5. Tape and fill. Cracks are bridged with fiberglass mesh tape and filled with setting-type compound, which hardens chemically and shrinks less than premixed patching products.
6. Skim, feather and prime. The washers and patches are skimmed over, feathered wide so they disappear, sanded and primed before paint.

When does plaster need replacing instead?

Reattachment works when the plaster itself is still sound. Replacement makes sense when it isn’t:

• It crumbles or turns powdery when you press it, often from long-term moisture.
• It’s been soaked by a leak and has gone soft, stained or spongy.
• It has already broken into pieces or fallen, leaving bare lath.
• It keeps failing after earlier repairs, a sign the lath behind it is in poor shape.

Most of the time, replacement is local. The damaged section is cut back to sound edges, loose lath is fastened, and the opening is filled, either with new plaster or setting compound or with a piece of drywall shimmed out to match the old wall’s thickness, then taped and skimmed so the seam disappears.

Whole walls and ceilings rarely need to go. Original plaster is hard, dense and quiet, and a repaired plaster wall keeps the character that makes an older home worth owning. Ceilings have a few options of their own, covered in our guide to plaster ceiling repair.

Is loose plaster dangerous?

On a wall, loose plaster is usually a slow problem. It can crack further, or drop a piece if it’s bumped, but it rarely fails all at once. A loose ceiling is different. Plaster is heavy, gravity is working on it full time, and a sagging section can come down.

If a ceiling bulges, sags or sounds hollow over a wide area, keep people out from under it, move beds and furniture if you can, and have it secured soon rather than at the next repaint.

What about lead paint and old plaster dust?

Plaster repair disturbs old layers: drilling for washers, scraping loose paint, sanding patches. Homes built before 1978 may contain lead-based paint, and the EPA’s Renovation, Repair and Painting (RRP) rule requires certified firms and lead-safe work practices for paid work that disturbs painted surfaces in pre-1978 homes, more than 6 square feet per room inside or more than 20 square feet outside. The EPA’s RRP program page has the details, and our guide to lead paint in older homes covers testing.

Some older joint compounds, textured finishes and plasters also contained asbestos. Small repairs that leave the plaster in place disturb little of it, but before cutting out or tearing down old plaster, it’s worth having a sample tested.

How soon can repaired plaster be painted?

Once the compound has set and dried, the repair is primed and painted like the rest of the wall. Every patch needs primer: compound is porous, and unprimed patches show through the finish as dull spots. Any new plaster, as opposed to compound, is alkaline and needs time to cure first; check the primer label for use over fresh plaster.

Sheen decides how much of an old wall’s gentle waviness shows, and flatter finishes forgive the most. Our paint sheen guide walks through the choices, and our guide to painting plaster walls covers calcimine, wallpaper paste and the other layers older walls tend to hide.

Prefer to have it handled? Our wall and plaster repair service fastens loose plaster, repairs cracks and holes, and paints the wall afterward, so the repair disappears.

Questions

Cracks, holes or loose plaster?See how Guiffre’s repairs walls and ceilings so the paint over them looks right.→