Why watch for warning signs?
Most house foundation problems do not happen overnight; they develop over months or years. Spotting and dealing with them early can save many times the repair cost compared with waiting until the damage is severe.
Repairing minor cracks costs at the 💰 level, while underpinning a severely damaged foundation can reach the 💰💰💰💰 level (depending on house size and repair method). Acting early is therefore far cheaper.
Sign 1: What do diagonal wall cracks tell you?
Which cracks are a concern?
← Scroll to see the full table →
| Crack type | Risk | Cause |
|---|---|---|
| Small vertical/horizontal crack < 1 mm | Low | Mortar/plaster shrinkage |
| Diagonal 45° crack, 1–3 mm wide | High | Differential settlement |
| Stepped crack | Very high | Foundation movement |
| Crack from a door/window corner | High | Structural distortion |
| Crack 5–15 mm wide, or several cracks wider than 3 mm | Very high | Moderate damage, affects serviceability |
| Crack wider than 15 mm, or bulging wall, cracked columns/beams | Critical | Risk to structural stability |
The width criteria in the table are comparable to the damage classification for low-rise buildings in BRE Digest 251 (UK): up to 1 mm (very slight), up to 5 mm (slight), 5–15 mm (moderate), 15–25 mm (severe) and over 25 mm (very severe). Width is only one indicator; always consider the crack's direction, location and growth as well. Read more on crack interpretation in Which structural cracks are dangerous?
How to check
- Place a strip of tape across the crack and mark the date
- Measure the width every 1–2 weeks to see whether it is growing
- Take photos for comparison
- If the crack grows quickly = an urgent problem; call an engineer immediately
Not every crack comes from the foundation — don't confuse it with concreting defects
Before concluding that the foundation has a problem, first distinguish whether what you see is a settlement crack or a defect from the time the concrete was cast, such as a cold joint or honeycombing, caused by interrupted pours or inadequate vibration of the concrete, not by the soil beneath the foundation.
The telltale features of these defects are that they have been in the same place since the formwork was stripped and do not grow over time, and they usually sit at construction joints between pours, such as where a column head meets a beam, along beam soffits, or at corners with congested reinforcement. Settlement cracks, by contrast, tend to run diagonally across walls and keep getting wider when monitored over several weeks. The depth and density of the concrete in these areas can be assessed with non-destructive evaluation (NDE) such as Ultrasonic Pulse Velocity or a Rebound Hammer before choosing a repair method.
Sign 2: How can you tell if a house is settling unevenly?
How to check it yourself
- Place a "marble" on the floor on a smooth surface. If it clearly rolls in one direction, the floor is tilted
- Use a water level to measure the floors in each room and record the tilt. If it exceeds 1:200 (5 mm/m), have an engineer assess it
- Measure the door heights at all 4 corners of the house. If they differ by more than 2 cm over 10 m, the settlement is considered uneven
Bangkok and its surrounding provinces once suffered severe subsidence from groundwater pumping. Since pumping was controlled, the rate of subsidence has dropped sharply. InSAR satellite data for 2014–2024 show that Bangkok subsided by about 0.3 cm/year on average (Oh et al., 2026), and most inner districts by no more than about 1 cm/year (Ahmed et al., 2024), but some areas, such as around Ramkhamhaeng, are still subsiding 2–3 cm/year (Department of Mineral Resources, 2025). Settlement therefore varies greatly from area to area. If a whole house settles uniformly at every point together with the surrounding ground, it usually does not damage the structure, but if it settles unevenly (differential settlement), that is a danger sign.
Sources: Oh et al. (2026), Geosciences Journal · Ahmed et al. (2024), Science of The Total Environment · Matichon, 25 Sep 2025 (Director-General, Department of Mineral Resources)
The 3D view below shows two common cases for houses on soft soil: the ground around a pile-supported house settling until a void forms under the floor, and an extension on a shallow foundation settling unevenly until it tilts and separates from the house, together with how to measure differential settlement.
Sign 3: Why are doors and windows starting not to close properly?
When the structure begins to distort, door and window frames lose their true rectangular shape, causing:
- Doors that stick or need a hard push
- A gap between the door and a frame that used to close tightly
- Door latches that no longer line up with the strike and must be forced
- Window glass cracking with no impact (because the frame is distorted)
How to tell the difference
Doors stick in the rainy/cool season: normal, caused by timber swelling
Sticking all year + cracks in the walls: a sign of foundation problems
Sign 4: Why are floors separating from walls or skirting boards pulling away?
Especially in Bangkok and its vicinity, which sit on Bangkok Soft Clay, the "Floating Floor" problem is very common, because:
- The pile-supported building does not settle
- But the floor around the house (slab on grade) settles with the soil
- A gap opens between the floor and the wall
How to check?
- Look at the skirting boards in every room to see whether they have pulled away from the walls
- Press on the floor near the walls — does it feel springy or hollow?
- Measure the gap and note the date. If it keeps widening, or the floor cracks and sinks into a dip, have an engineer inspect it (the ground around a pile-supported house can keep settling at the local subsidence rate; read the remedies in How to fix settling fill)
- For raised-floor houses, check the ground floor for cracked or leaking pipes beneath it
Sign 5: How are frequent pipe bursts related to the foundation?
When the foundation moves, the piping beneath the floor is subjected to tension/twisting, causing:
- PVC pipes cracking at the joints
- Unusually high water bills (hidden leaks)
- Water seeping through bathroom and kitchen floors
- Foul odors rising from drains (pipes broken underground)
Quick checks
- Shut off the main water supply and watch the meter. If it still turns = there is a leak
- Look under the house floor. Damp patches or green mold = a leaking pipe
- Foul odor from the bathroom floor = a broken drain pipe
When should you call a geotechnical engineer?
← Scroll to see the full table →
| Severity level | Sign | Action |
|---|---|---|
| Level 1 Monitor | Small crack < 1 mm | Mark it and monitor every 3 months |
| Level 2 Investigate | Cracks 1–5 mm, growing slowly | Consult an engineer for a preliminary assessment |
| Level 3 Urgent | Cracks 5–15 mm or growing fast + tilted floor | Call a geotechnical engineer to inspect + new soil boring |
| Level 4 Critical | Cracks over 15 mm, or cracked columns/beams, tilting columns | Have an engineer assess safety immediately; stop using at-risk areas until the inspection is complete |
What does a geotechnical engineer do during an assessment?
- Site inspection: check cracks, tilt and groundwater level
- Re-investigation (repeat soil boring): check the current soil profile against the data from the time of construction
- Install Settlement Monuments to monitor ongoing settlement for 3–6 months (see how it is measured in Settlement Points and Tiltmeters)
- Structural analysis to find the real cause
- Recommend remedies such as underpinning, micropiles or grouting
If the foundation really has a problem, how can it be fixed?
1. Micropile Foundation
Small-diameter piles, 100–300 mm in diameter, driven or bored around the existing foundation to transfer the load to a stiff soil layer. Suitable for houses with difficult access. Relative cost level 💰–💰💰 per pile (often several dozen piles per house), depending on the number of piles and site conditions. The added piles should be load-tested as specified by the engineer (read Do micropiles need testing?)
2. Compaction Grouting
Stiff mortar (low-slump sand–cement grout) is injected into the ground at discrete points to displace and compact the soil around the foundation. It works well in sandy or unsaturated soils but has limited effect in saturated soft clay, and it differs from chemical or cement permeation grouting, so the engineer must choose the method to suit the soil type (see Ground improvement methods). Relative cost level 💰💰–💰💰💰 per point, depending on depth and volume of material.
3. Underpinning
The traditional method: excavating beneath the foundation in stages and adding a new, deeper foundation. Relative cost level 💰💰💰💰 (the highest of the three methods) and complex. It works well where stiff soil is shallow; on thick soft clay such as in Bangkok, excavating deeper often still does not reach stiff soil, so supplementary piles are usually used instead.
Note: The 💰 symbols indicate relative cost levels only, not SPN prices — foundation repair/strengthening is carried out by specialist contractors, and actual cost depends on the damage and site conditions; have an engineer assess first. Read about each method in depth at How to strengthen an existing building foundation (foundation retrofit)
Don't decide on repairs yourself without an engineer's assessment — patching cracks with mortar without fixing the root cause will let the problem return before long, possibly worse than before.
Summary: notice early, measure properly, then let an engineer find the cause
The 5 signs to watch for are diagonal cracks, uneven settlement, doors that won't close properly, floors separating from walls, and frequent pipe bursts
If you find these signs in your house, don't leave them. Contact SPN Soil Engineering for advice from a geotechnical engineer. The team can send an engineer to assess the site, plan a repeat soil investigation and recommend remedies that suit your budget — and if you are about to build a new house, read more on why a soil investigation is needed before building a house to prevent these problems from the start.