Painting over a crack does not fix its cause. Before you pick up a brush, you need to know whether the crack is only in the finish, whether the wall is still moving and whether there is water behind it. This guide explains how to tell the cases apart, when to call an engineer, how to prepare the wall with filler and mesh, and what elastic paints can and cannot do.
Updated 3 November 2026. General information, not a substitute for assessment by a technician.
Key points
- A fine, stable crack in the plaster or skim: repair and paint. A crack that grows, goes through the wall or comes with sticking doors: get an engineer to assess it before you cover it.
- Covering an active crack without knowing the cause only hides the symptom. If it grows, its classification can change (BRE).
- The data sheets we read tell you to open and clean wider cracks and to reinforce with mesh where different materials meet.
- Elastic fibre fillers suit cracks with slight movement. They do not cure a structural problem.
- If the crack lets water in, fix the leak first. See damp in walls.
Types of crack
LNEC, Portugal’s national civil engineering laboratory, in a study of infill masonry walls (J. L. Miranda Dias, 2005), describes cracking in the coatings, the blocks and the mortar joints. It names as possible causes excessive vertical loads, very strong cement-rich mortars with high shrinkage, or weak mortars with poor durability. There can be one cause or several together. The study warns that cracking can let rainwater in.
In practice you can think of three families:
| Family | What it usually is | How it behaves |
|---|---|---|
| Shrinkage | Plaster or filler loses water as it dries and contracts | Fine, close to the surface, tends to settle |
| Movement or load | Different materials move differently, for example brick and concrete | Follows the joint line, may reopen |
| Settlement or structure | The ground or the structure gives way | Wide, grows, may appear on several floors |
This table is our own grouping, not an official classification. The only width criterion we could read directly comes from BRE, the UK building research body (page dated 15 April 2014), which uses Digest 251: categories 0 to 2, up to 5 mm, are aesthetic problems that mainly need redecoration. Categories 3 and 4 affect weather-tightness and the working of doors and windows. Category 5 is a stability matter and may need structural work. BRE adds that if a crack widens over time, its classification can change.
When to call an engineer
We found no official Portuguese criterion in millimetres. Our own rule of thumb, based on BRE, for calling a civil engineer:
- the crack widens from week to week;
- it is over 5 mm, or goes through the wall and shows on both sides;
- it appears on several floors or runs diagonally from window and door corners;
- doors and windows start to stick;
- the floor has become uneven or there are signs of water.
To keep track, measure the width with a ruler, photograph it with the ruler beside it and repeat every few weeks. That is our advice: BRE gives no method or interval. In a block of flats, a structural crack in a common part is a matter for the condominium: see damp and common parts.
Preparing the wall, step by step
Data sheets differ but agree on the main points. Always read the sheet of the product you use.
- Remove the cause. The Barbotek INT sheet (Barbot, 10 May 2021) says to eliminate all leaks, including rising damp, and requires a dry surface with moisture below 5%. See how to identify the damp.
- Remove what is loose. Hollow, blistered or poorly bonded plaster should come off down to firm material.
- Open the crack. Aguaplast Fibra (maker Beissier, sheet published by Robbialac, February 2024) says to widen the crack into a wedge. Barbot says to open and clean cracks over 2 mm and remove the dust.
- Primer where needed. On surfaces that shed dust or sand, Barbotek INT calls for a fixing primer or bonding bridge. On very absorbent surfaces, a regulating primer.
- Fill.
- Crack up to 2 mm: fine repair filler, maximum layer 2 mm (Barbotek INT).
- Crack over 2 mm or structural: bonding primer plus acrylic mastic, or repair render (Barbotek INT).
- Crack with movement: elastic fibreglass filler in layers up to 3 mm, extending 3 to 4 cm each side. Over 3 mm, fill first with a filling compound (Barbot, ref. 8800008, November 2015).
- Mesh at joints. Where different materials meet, such as concrete and brick, Barbotek INT says to reinforce with mesh, for example fibreglass.
- Dry and sand. Barbotek INT gives 4 hours before sanding and 24 hours before painting, for a 1 mm layer. Barbot gives 24 hours per mm for the elastic filler. Sand wearing a mask, gloves and goggles, as the sheet asks. Aguaplast Fibra says it is hard to sand and should be smoothed with a damp spatula before it dries.
- Regulating primer and paint, as Barbotek INT asks. See how much paint you need and which paint for each room.
If the wall has salts or mould, deal with them first: salt, mould.
Elastic paints: what they do
An elastic paint forms a film that follows small movements. The Barbot Membrana Elástica sheet (10 December 2024) says it resists shrinkage cracks, is for exterior renders, and that cracks over 2 mm must be opened and treated first, with elastic filler for moving cracks. Revisal Elástico Liso (Isaval, version of 5 March 2025) is a facade coating, with elongation at break of 98% per its sheet.
Two conclusions. First, the sheets we read for these paints are for facades. We read none for an interior elastic paint. Second, Barbot’s own sheet tells you to treat larger cracks before painting: elastic paint complements the repair, it does not replace it. For “anti-damp” paints, see what they promise and what they do.
When it is not just a paint job
If the crack meets any item on the engineer list, do not paint: get it assessed. If there is a water stain next to a ceiling crack, fix the leak first (leak from upstairs, painting ceilings). If paint is peeling, see why paint fails.
What we could not confirm
- Portuguese criteria in millimetres for calling an engineer: no official source found. The Ordem dos Engenheiros was not consulted on this.
- Classification of crack types from an official Portuguese source: the table is ours. The LNEC study deals with masonry under compression and is not a homeowner guide.
- BRE Digest 251 itself was not read. We read only a BRE page that summarises it, with no table of millimetres per category.
- Interior elastic paints: no sheets read.
- Prices and duration of the repair: not confirmed.
- Data sheets may have newer versions.
Sources (read on 3 November 2026)
- LNEC, J. L. Miranda Dias, Avaliação das deformações da zona em redor de juntas de argamassa de paredes de alvenaria sujeitas a fendilhação devido a acções de compressão vertical, 2nd Mortars Congress, 2005: abstract, introduction and section 2.1.
- BRE, Assessing cracks in houses, 15 April 2014: categories 0 to 5, monitoring, when to consult a professional. Summary read through an automated tool.
- Barbot, Barbotek INT data sheet, revision 10 May 2021, read in full.
- Barbot, Massa Elástica com Fibra de Vidro, ref. 8800008, November 2015, read in full.
- Barbot, 1316 Membrana Elástica, 10 December 2024, read (properties and preparation).
- Robbialac, Aguaplast Fibra data sheet (maker Beissier), February 2024, read in full.
- Isaval, Revisal Elástico Liso, version 8, 5 March 2025, read (description, technical data, preparation).