Taping and Filling - Repairing Cracks in Walls and Ceilings

Most cracks indoors are not a structural problem. A new building settles during its first few years, and small cracks tend to appear where different materials meet, for example where masonry walls meet a concrete ceiling. Plaster shrinks as it dries, and walls move slightly with changes in temperature and humidity. In older houses the surface layer is often brittle, and chasing out walls for new electrics can be enough to crack it. On plasterboard, cracks usually run along joints that were not properly reinforced.

By size, cracks fall roughly into three groups:

• Hairline cracks on the surface, often after plaster or paint has dried: if they are not growing, a thin coat of filler without tape is sometimes enough
• Visible cracks up to roughly 2 mm, typically in corners, on plasterboard joints and where materials meet: these need tape and filler
• Deep or moving cracks that keep coming back or slowly widen: these need wider reinforcing mesh and more coats of filler

If a crack keeps returning after repair, runs diagonally from the corner of a window or door, is getting wider or goes right through the wall, it is no longer a decorating job. Have it looked at by a structural engineer. In a block of flats, report it first to the building management or owners' association, because it may involve the load-bearing structure of the whole building.

The method is much the same on plaster and on plasterboard. The main difference is the choice of tape and filler.

Preparation

Loose plaster and flaking paint are removed from around the crack. The crack itself is opened out into a V shape with a knife or the edge of a filling knife, so the filler has room to flow in and something to grip. Dust is brushed or vacuumed out. Old, dusty or very absorbent surfaces are primed so that the filler does not dry too quickly and bonds properly.

Applying the tape

Self-adhesive fibreglass mesh tape is stuck straight onto the dry surface. Paper tape is pressed into a thin layer of fresh filler, and the filling knife is used to squeeze out air and excess filler. The tape has to lie flat, with no bubbles or creases, otherwise every bump will show once the wall is painted.

Filling and sanding

The first coat of filler covers the tape completely and is feathered out to the sides. Once dry, the surface is lightly sanded and a second, thinner and wider coat is applied to bring the repair flush with the wall. Drying time depends on the filler, the room temperature and the thickness of the coat, so follow the manufacturer's instructions. Finally the area is sanded with fine abrasive paper or mesh, usually somewhere around 120 to 180 grit, and primed before painting.

• Paper tape is the standard for plasterboard joints: it is strong and holds its shape well, but needs careful bedding into the filler
• Self-adhesive fibreglass mesh tape is easy to apply, which makes it popular for small repairs and for beginners
• Reinforcing mesh is wider and suits deeper cracks, corners and junctions between different materials
• Corner beads protect and strengthen external corners on plasterboard

For interior walls and plasterboard, the most common choice is a gypsum-based filler or jointing compound, which sands easily. A fine finishing filler is used for the final smooth coat. In internal corners where different materials meet and the joint naturally moves, for example between a ceiling and a plasterboard partition, a flexible acrylic decorator's caulk is often used instead, because hard gypsum filler would crack there sooner or later.

The simplest case is a short crack on a flat wall in an empty room that is going to be repainted anyway. Cracks on ceilings, in corners and along the joint between wall and ceiling take longer, because the work is done from a ladder in an awkward position. Old, crumbly plaster also slows things down, since it has to be cleaned back and consolidated first, and so do moving cracks that need extra coats. In a lived-in home, furniture and floors have to be covered, which adds work too. On the other hand, it helps when the taping is done together with painting or skimming the whole room, because the following steps are carried out in one go.

• Filler without tape: not enough for a crack that moves, so it usually comes back
• Badly laid tape: bubbles, creases and edges that are not pressed down create visible ridges under the paint, or the tape peels away over time
• Rushing: filler applied over a coat that has not dried tends to sink or crack
• Skipping the primer: filler and paint grip less well, and the repair ends up with a different sheen or shade after painting
• Filling too narrow: a thick strip of filler right over the tape leaves a hump, so the second coat needs to be spread wider
• Careless sanding: knife marks and sharp edges look even worse under paint, especially in side light from a window

You can handle a small crack in a wall yourself if you can reach it from a low step ladder. Larger areas, cracks running across whole ceilings or new plasterboard work are better left to a decorator or plasterer.

1. Cover the floor and any furniture near the repair.
2. Remove loose plaster and paint, and open the crack into a V shape with a knife.
3. Brush or vacuum the dust out of the crack and the surrounding area.
4. Prime around the crack and leave it to dry as stated on the tin.
5. Fill the crack and stick or bed the tape exactly along the centre of it.
6. Cover the tape with a first coat of filler and feather it out to the sides.
7. Once dry, sand lightly and apply a second, wider and thinner coat.
8. Sand flush with fine paper, dust off and prime before painting.

• No tape edges or knife marks are visible when you shine a torch across the surface from the side
• The repair blends smoothly into the surrounding wall with no hump
• The tape is not lifting anywhere and there are no bubbles under the filler
• The surface has been dusted off and primed before painting
• Flexible filler or caulk has been used in corners that move
• After painting, the repair does not differ in sheen or shade from the rest of the wall