Start by describing it properly
Sloping and uneven are two different complaints and they usually have different causes.
A slope is a floor that falls consistently in one direction, often across a whole room or a whole side of the house. It generally means the supports on one side have dropped relative to the others, which points at stumps, footings or the ground.
Unevenness is a floor with dips, humps and soft spots that do not follow a single direction. That points at the floor frame itself, at individual joists or bearers, or at localised damage.
Bounce is different again, and points at joists rather than supports.
Being precise about which one you have narrows the field considerably before anyone gets under the house.
Working out how bad it is
You can do a useful first pass yourself.
- The ball test. Place a marble or a small ball on the floor in several rooms. If it rolls the same direction consistently, you have a slope and you now know which way it falls.
- A long level. A 1200mm spirit level tells you far more than a short one. Take readings in several places and note where the fall is steepest.
- Doors and windows. Which ones stick, and whether they stick at the top, the bottom or one side. That indicates the direction of racking.
- Skirting gaps. Where the skirting has separated from the floor, and whether the gap changes with the seasons.
- Cracks. Location, direction and width. Diagonal cracks out from door and window corners indicate structural movement rather than shrinkage.
- Photograph and date it. If you are unsure whether it is getting worse, marking and dating a crack is the cheapest monitoring there is.
The likely causes
Failed stumps
The most common cause under an older raised timber house. Hardwood stumps rot from the base and get eaten by termites, and as they lose height or fail entirely the floor above follows them. Usually shows as a slope, often worst near a corner or wherever the ground stays wettest.
Ground movement
Reactive clay through the hinterland and the western suburbs swells and shrinks seasonally. Poorly compacted fill near the coast consolidates over time. Both move the supports without the supports themselves being defective. Typically produces slow, seasonal, sometimes reversing movement.
Water
A downpipe discharging beside a footing, ground falling toward the house, a leaking service under the floor, or a blocked subfloor vent. Water is behind a very large share of what gets diagnosed as a structural problem, and it is often the cheapest thing to fix.
Floor frame problems
Undersized or over spanned joists, joists notched too deeply for pipes and cables, rot near wet areas, and termite damage. Produces bounce and localised dips rather than a consistent slope.
Footing or slab movement
On slab and strip footing houses there are no stumps to blame. Movement is in the ground under the footing, and the remedy is underpinning designed by an engineer rather than anything done above ground.
The house was built that way
Worth including, because it is more common than people expect on older homes. Some floors have a small consistent fall that has been there since construction and has never changed. Levels alone do not tell you this. History and evidence of movement do.
What a proper diagnosis involves
- Floor levels taken through the whole house, not one room, so the shape of the movement is mapped
- Every support probed and checked for plumb
- Bearers, joists, ant caps and fixings inspected
- The subfloor ground checked for damp, standing water and ventilation
- The outside checked, particularly where stormwater goes and how the ground falls
- Cracking recorded, including direction and width
- Where movement may be ongoing, monitoring over a period rather than a single visit
Only after that does a scope make sense. A contractor who quotes a fix without doing most of that is quoting a guess, and the guess is usually the most expensive option they offer.
Doing nothing is sometimes correct
Not every out of level floor needs work. If the movement is historic, small, stable and not causing damage, the sensible response can be to monitor it and spend the money elsewhere. Older timber houses are not flat and were never expected to be.
What changes that calculation is any of the following: movement that is clearly ongoing, cracking that is getting worse, doors and windows that no longer function, an imminent sale where a building and pest report is going to raise it anyway, or a renovation that will put tiles or fixed cabinetry onto a floor that is still moving.
That last one matters. Tiling or fitting out over a floor that has not stabilised is how a modest problem becomes an expensive one.