Cracks in Your Walls: When to Worry and When to Just Repair (Bangkok Homes and Condos)

Hairline cracks along the ceiling. A crack running up from the corner of a doorframe. A line down the wall where it meets a concrete column. If you live in a Bangkok house, townhouse or condo, you almost certainly have some of these.

The good news is that most of them are cosmetic. The less good news is that the cosmetic ones come back again and again if they are repaired the wrong way. And a small number of cracks are not cosmetic at all.

First, a clear line. RenoRova repairs walls: cracks, damp and the surfaces over them. We are not structural engineers. If anything below sounds like your crack, have a licensed civil engineer (วิศวกรโยธา) look at it before any repair. A cosmetic repair over active structural movement only hides the evidence.

Why Bangkok buildings crack

Two materials that move differently

Most Thai buildings have a reinforced concrete frame — columns, beams and slabs — with the walls between filled in with block or brick and then plastered. Concrete and block expand, contract and dry out at different rates. Where they meet, a fine crack is almost inevitable. That is why cracks appear so often along the top of a wall under a beam, beside a column, or around a doorframe.

Plaster shrinking as it dries

Fresh plaster shrinks as it cures. On a new building this produces fine random or map-like cracks in the first year or two. They are surface cracks in the plaster, not in the wall.

Heat

Walls that take direct sun swing in temperature every day, and the movement opens fine cracks, especially on west-facing walls and on top floors under a roof.

Soft ground

Bangkok sits on deep layers of soft clay, and parts of the city have a long history of land subsidence. Buildings on long piles stay put. Anything built on shorter piles or shallow footings can settle over the years. This is behind the most common serious crack pattern in Bangkok houses: the rear extension.

The rear extension problem

Townhouses and detached houses are often extended at the back — a kitchen, a laundry room, a covered area — years after the house was built. The main house usually stands on deep piles. Many extensions do not.

As the extension slowly settles more than the house, it pulls away along the joint. You see a crack where the extension wall meets the house wall, often wider at the top or bottom, sometimes with the extension floor dropping below the house floor and a gap opening under the doorway.

That crack will not stay closed with filler, because the extension is still moving. It needs an engineer to assess the foundations and the rate of movement, and the joint usually needs a flexible detail rather than a rigid repair.

A simple width guide

A widely used classification for crack damage, from the UK's Building Research Establishment (BRE Digest 251), sorts cracks by width. It was written for low-rise masonry buildings and is a rough guide, not a diagnosis, but it gives a useful sense of scale:

To measure, use a crack-width gauge or compare against a ruler and a few coins. A credit card is about 0.8 mm thick, which gives a quick check for "under 1 mm".

Width is not the only factor. A 2 mm crack that was 1 mm last month matters more than a 3 mm crack that has not changed in years.

The warning signs

Call a civil engineer if you see any of these:

• Diagonal cracks from the corners of doors and windows, especially if they are wider at one end. Diagonal cracking is a typical sign of uneven settlement.
• Stepped cracks following the mortar joints in a brick or block wall.
• Cracks that are getting wider, measured over weeks.
• Cracks in columns, beams or the floor slab. These are the structure, not the infill.
• Rust stains or flaking concrete around a crack in concrete. This can mean water is reaching the steel reinforcement, which rusts and expands.
• A crack that goes right through the wall, visible on both sides.
• Doors and windows that suddenly stick, or gaps that appear above doors.
• Floors that slope or a gap opening between floor and skirting.
• Horizontal cracks in load-bearing walls in older houses.

Thailand's engineers are registered with the Council of Engineers (สภาวิศวกร). Ask for a licensed civil engineer for a structural inspection, and keep their written findings.

In a condo, columns, beams, slabs and the outside walls are usually the building's common property. Report cracks in those to the juristic office in writing with photos. Cracks in the partition walls inside your unit are usually your own repair.

How to monitor a crack

If you are not sure whether a crack is moving, measure it before deciding.

1. Mark both ends with a pencil line across the tip, and write the date beside it.
2. Measure the width at its widest point with a gauge, and note it.
3. Photograph it with a ruler held against it, from the same position each time.
4. Repeat monthly for two or three months, ideally through a rainy season.

If the crack grows past your pencil marks or gets wider, it is moving. For a more precise record, a plastic crack monitor (tell-tale) fixed across the crack shows movement to a fraction of a millimetre.

Cracks and water

A crack in an outside wall lets rain in. Inside, that shows as a brown stain, bubbling or flaking paint, or mould that appears after storms — the problem behind many "ผนังร้าว น้ำซึม" searches.

The fix starts outside: the crack is opened and sealed with a flexible exterior sealant, and the wall coated with an elastic exterior paint that can bridge fine cracks. Then the inside wall is allowed to dry, treated and repaired. Painting the inside over a wet crack only traps the water. Our guide to prepping damp walls before painting covers the inside repair.

How cosmetic cracks should be repaired

Filling a crack is easy. Filling it so it does not come back takes four steps.

1. Open it up. The crack is raked out into a shallow V so the filler has something to grip, and the loose material and dust are removed.
2. Use the right filler. A static crack can take a standard filler. A crack that moves with heat and seasons, such as the joint between a column and a block wall, needs a flexible compound or acrylic sealant that can stretch.
3. Bridge it. Along frame-and-infill joints, mesh tape or fibreglass scrim over the crack spreads the movement so it does not reappear as a sharp line.
4. Skim, sand, prime and paint. The repair is skimmed flat, sanded, sealed with primer and painted to match.

Rigid filler in a moving crack is the reason most repaired cracks are visible again within months.

What RenoRova does, and what it costs

RenoRova's wall repair starts from ฿2,900 per wall, quoted in writing. Every job checks the crack for movement and takes moisture readings on any stain before repair, so the fix matches the cause. A crack or hole is usually repaired the same day. A damp wall needs to dry after treatment, usually two visits a few days apart. Repairs carry a two-year guarantee, and the report includes before and after photos and the readings.

If we see signs of structural movement, we will tell you to get an engineer's opinion first and will not cover it with filler. If you are repainting anyway, crack repair is part of every interior painting job.

Send a photo of the crack with a ruler or a coin beside it on LINE, and we will tell you whether it looks cosmetic and what the repair involves.