Quick answer
A crack under 1 mm wide, superficial and not growing is repaired by opening it into a V, cleaning it, filling it with repair filler, bedding in fibreglass mesh tape and sanding. If it is wider than 2-3 mm, goes right through the wall, runs diagonally from a window corner or grows over a few weeks, do not fill it: call an architect or surveyor.
What you need
- Stanley knife or triangular scraper
- 10 cm and 20 cm filling knives
- 120 and 180 grit sandpaper with a sanding block
- Small brush
- Vacuum cleaner
- Paint roller and brush
- Interior filler such as Aguaplast, Pladur or Bruguer (Leroy Merlin, Bricodepot)
- Flexible acrylic filler for wall and ceiling joints
- Self-adhesive fibreglass mesh tape
- Sealing primer or stabilising solution
- Emulsion paint in the wall colour
- Crack tell-tale or dated paper tape
Step by step
- 1
Measure it and check it has stopped
Note the width with callipers and the date. If it is wider than 2-3 mm, goes through the wall or runs diagonally from a window, stop here and call a technician. If it is under 1 mm and has been the same for a while, carry on.
- 2
Open the crack into a V
With the knife or scraper, open the crack to 3-4 mm wide and 3 mm deep, removing loose plaster. A closed crack will not let the filler in.
- 3
Clear the dust
Vacuum the crack and go over it with a dry brush. Dampen it very slightly with a sponge so the plaster does not draw the water out of the filler.
- 4
Apply a coat of stabiliser
Apply a primer or stabilising solution on the crack and 5 cm either side. Leave it to dry as the tin says, normally 1-2 hours.
- 5
Fill with filler
Press the filler into the crack with the small knife, working across it, until there are no gaps. For a line between wall and ceiling, use flexible acrylic filler and smooth it with a wet finger.
- 6
Bed in the mesh if it is over 1 mm
Stick the mesh tape centred over the crack and cover it with a thin coat of filler. Smooth it with the 20 cm knife, feathering 10 cm either side.
- 7
Second coat and drying
When the first coat is dry (4-8 hours, depending on thickness), apply a very thin second coat to hide the texture of the mesh. Leave it to dry for 24 hours.
- 8
Sand and remove the dust
Sand with 120 grit and finish with 180, using the block and gentle circular movements. Vacuum and wipe with a slightly damp cloth.
- 9
Prime and paint
Apply one coat of sealing primer and then two coats of paint. Date a tell-tale and check again after three months in case it reappears. See the guide on how to paint a wall.
Cosmetic cracks and structural cracks: how to tell them apart
Not all cracks are the same, and the difference matters more than the filler you use. A cosmetic crack affects the finish (paint, plaster, render) and not the wall itself. A structural crack means the building is moving, and covering the symptom without knowing the cause only delays the warning.
These are cosmetic: hairline cracks under 1 mm, straight lines where the wall meets the ceiling, cracks that follow the joint between two different materials (plasterboard and brick, for example), fine spiderweb cracks in freshly painted plaster, and those that appear a few weeks after a refurbishment, when the plaster finishes drying.
These are suspected structural cracks: anything wider than 2-3 mm, cracks that go through the wall from one side to the other (visible on both faces), diagonals at 45 degrees running from the corner of a window or door, stepped cracks that follow the brick joints, horizontal cracks at mid height, cracks that grow or change width between seasons, and any accompanied by sloping floors or doors that suddenly rub or will not close.
| Sign | Probable cause | What to do |
|---|---|---|
| Hairline under 1 mm, superficial | Plaster shrinkage or thermal expansion | You: filler and mesh tape |
| Straight line at wall and ceiling | Natural movement between materials | You or a builder: flexible filler |
| Diagonal from a window corner | Settlement or badly distributed load | Architect or surveyor |
| Over 3 mm or through the wall | Structure or foundations | Architect or surveyor, do not fill |
| Grows over weeks | Active movement | Monitor with a tell-tale and call a technician |
How to check whether a crack is still moving
Before you fill anything, check that the crack has stopped. Draw a pencil line on each side, note the date and the width with callipers, or fit a tell-tale: a strip of clear sticky tape along the crack, dated. If the tape tears within a few weeks, the crack is moving. You can also buy graduated plastic tell-tales at Leroy Merlin and in any ferretería (hardware shop).
Check the width once a month for three months, and take photos with a ruler alongside. If it does not change, it is stable and filling it is safe. If it changes, the filler will open up again, and the important thing is to understand why: leaks at the foundations, a failing floor slab, overloading or tree roots. That is not something we solve with filler. A technician has to look at it.
Common mistakes when filling cracks
- Filling without opening the crack. Filler smeared only over the surface falls off. You have to open the crack into a V with a scraper or a Stanley knife so it can grip.
- Filling a crack that is still moving. It reopens within weeks. On walls that expand and contract, use flexible filler, not a rigid one.
- Using plaster or cement where flexibility is needed. On lines between wall and ceiling, or between different materials, a plaster filler cracks again.
- Forgetting the mesh. Without fibreglass mesh tape, the crack comes back on anything wider than 1 mm.
- Painting before it dries. The filler shrinks, leaves a dip and the crack still shows.
- Ignoring damp. A crack with a damp stain nearby is a different problem. See how to get rid of damp on a wall.
When to call a professional, and who to call
For cosmetic cracks, we do it ourselves. Repairing a crack or a flaking patch costs €50-120 and a builder is €30-40 an hour, with a fixed price over WhatsApp from a photo with a ruler alongside. It is on the small building works at home page and in the builder's rates. If what you have are holes from old wall plugs, first read how to fill holes in a wall.
When a crack looks structural, we do not fill it. We ask you to call an architect or a surveyor (aparejador) who will measure it, write a report and decide whether propping, injecting or reinforcing is needed. That is licensed work with technical responsibility, and it is not part of what we do. Once the technician gives the go-ahead, we come back and restore the finish.
If you live in a building with a comunidad de vecinos (residents' association) and the crack is on the facade, party wall, stairwell, patio or garage ceiling, it is a common element. Tell the administrator, who can order a technical inspection of the building, and do it before repairing, because the insurer usually asks for nothing to be touched until it has been seen.
Cracks and climate: heat, salt air and coastal buildings
In Valencia, Alicante, Torrevieja or Benidorm the walls go through cycles of summer heat and winter damp. Terraces and south-facing facades expand and contract, which is why cracks are common where a terrace meets the building and on outside walls. In addition, salt air on the seafront corrodes the reinforcing steel in concrete balconies and overhangs, and a crack with rust stains, exposed steel or pieces falling away is a serious matter.
In holiday-home developments with a pool, watch for cracks near the pool shell and the terraces, in case of leaks. In flats from the sixties and seventies in the area, it is not unusual to find very old plaster that cracks when you drill into it or when a partition is changed, with nothing structural going on. In any case, if you are in doubt, a photo on WhatsApp lets us tell you whether we see it as cosmetic or whether we recommend a technician.
Frequently asked questions
When is a crack in a wall dangerous?
When it is wider than 2-3 mm, goes through the wall, runs diagonally from the corner of a window or door, grows over a few weeks or appears alongside doors that rub and sloping floors. If you see exposed steel or pieces falling away, call an architect or surveyor as soon as possible.
Can you fix a crack that keeps moving?
You can disguise it, but it will reopen. First you have to find out why it moves, and that is a technician's job. If the movement is only expansion between materials, flexible filler with mesh gives a good result and lasts for years.
What filler should I use for cracks in interior walls?
For fine cracks, a powder or ready-mixed filler such as Aguaplast or Pladur. For lines between wall and ceiling or between materials, a flexible acrylic filler. For cracks wider than 1 mm, add self-adhesive fibreglass mesh. All of it is at Leroy Merlin or Bricodepot.
How much does it cost to have you come and fix a crack?
Repairing a crack or a flaking patch costs €50-120 and a builder by the hour is €30-40. With a photo on WhatsApp we give you a fixed price, and if we see a crack that looks structural we tell you and recommend a technician before anything is touched.
How long should I wait before painting after repairing a crack?
Between 24 and 48 hours, depending on the thickness of the filler and the humidity in the home. In winter and in bathrooms with no window it can take longer. If it crumbles when you sand it or feels cold and damp, wait another day: painting over damp filler leaves marks.