Sinking house, cracked floors! Check now: what's causing it, and when do you need to call in a professional?

Waking up to cracks in the floor, or feeling the house starting to lean slightly, frightens many homeowners. The problem of a sinking house and and cracked floors is serious and shouldn't be ignored, because besides spoiling how the house looks, it affects the safety of the occupants in the long term. Let's look in depth at the causes, plus simple ways to inspect your house yourself.

Common causes of a sinking house and cracked floors

This problem can have many causes, both natural and from poor construction. The main causes are:

• Piles that are too short or can't carry the load: especially in back extensions, which often use short piles, so they settle unevenly compared with the main house
• Changes in soil and groundwater: pumping of groundwater, natural ground subsidence, or water leaking under the foundations so the soil becomes saturated and slides
• Overloading the structure: placing heavy items in one spot for a long time, or adding extensions without calculating how the load is distributed
• Incorrect tiling or concrete pouring: if only the surface is cracked, it may be caused by the concrete shrinking as it dries, which isn't always due to structural settlement

A checklist to assess the severity yourself

Before you panic, use this simple checklist to assess the cracks you've found in your house:

• Not dangerous (hairline cracks): cracks smaller than 1 millimetre in plastered walls or surfaces, usually caused by the concrete shrinking; the surface can be repaired and refinished
• Watch closely (long but narrow cracks): horizontal or vertical cracks no wider than 2-3 millimetres; stick clear tape across the crack and write the date on it — if the tape tears later, the crack is still growing
• Very dangerous (45-degree diagonal cracks / sinking or leaning): diagonal cracks wider than 3 millimetres, cracks in beams or columns, or floors tilting so much that doors and windows won't open or close — a critical warning sign

For small surface cracks, you can do basic repairs yourself, such as chiselling out the existing crack, cleaning it, then using a concrete bonding agent mixed with grout, or a polyurethane joint filler, to stop water and moisture seeping in and damaging the reinforcing steel inside.

Safety warning: If you find that the cracks are growing quickly, there are diagonal cracks in beams and columns, or the house is visibly leaning, never try to repair it yourself! Structural and foundation work is delicate and a matter of life safety. Stop using that area and consult a structural engineer or specialist right away to plan shoring, house lifting or additional piles according to engineering principles.