My floor slopes — should I worry, and can it be levelled?

Almost every house built before the middle of the twentieth century has floors out of level, and most of that happened in the first few decades of the building's life. The question that matters is never how much slope there is. It is whether the slope is still moving.

Safety

What you'll need

• a long spirit level, ideally 1.8m or 2m
• a tape measure
• a laser level (optional)
• masking tape and a pencil

Step by step

1. Measure the slope, do not eyeball it.Rest a long level on the floor, lift the low end until the bubble centres, and measure the gap under it. That gap over that length is your fall. Take readings in several directions across the room — floors often fall two ways at once.
2. Find the shape of the slope.A long, smooth, even fall across a whole room is settlement, and usually old. A distinct hinge, a step, or a dish concentrated in one area means something specific is happening beneath — a failed joist, a removed wall, a cellar, a filled-in well or a slab that has heaved.
3. Look at the walls above.Diagonal cracks running from the corners of openings, cracks that are wider at one end, and doors or windows that have started binding in the last year are the signals of active movement. Old cracks are filled, painted over and stable.
4. Record it and date it.Mark reference points on the floor with tape and note the measurements. Re-measure in six months. Nothing else you can do at home distinguishes historic from active movement as reliably.
5. Decide what the floor is for.A slope that furniture wobbles on and doors swing open along is annoying but harmless. A slope that stops a wardrobe, a kitchen or a shower tray sitting properly has to be engineered around. These have different answers.
6. Choose your correction strategy.Do nothing and pack furniture. Or level the covering zone only — a levelling compound or a tapered plywood build-up, which corrects what you walk on without touching the structure. Or take the deck up and pack or replace the joists, which is the only route that also corrects the ceiling below and the skirting lines.
7. Beware of the levelling compound trap.Levelling compound flows to a level plane, which means on a sloping floor it is thin at the high end and deep at the low end. Depth costs money, adds weight, has a maximum build stated by the manufacturer and raises the finished floor at the low end, so the door and the skirting have to be dealt with too.
8. Accept partial correction where it is sensible.Levelling out the worst of a slope so a kitchen or a wardrobe sits true, and leaving a gentle residual fall, is normal practice in old houses and far cheaper than chasing dead level.

Alternatives

• Scribe and pack: Leave the floor alone and scribe the item that has to sit on it — the kitchen plinth, the wardrobe, the bookcase — to the slope. Standard practice in old houses and by far the cheapest answer.
• Levelling compound over the low area: Feather-edge compound or a deep-fill product corrects the covering plane without touching the structure. Best on solid floors and where the total build-up is modest.
• Tapered timber battens and a ply deck: On a timber floor, firrings or tapered battens screwed over the joists and a plywood deck on top gives a level surface with far less weight than compound. It adds height, so doors and skirtings move.
• Structural correction: Sistering, packing or replacing joists, or in extreme cases underpinning. Only sensible if the movement is diagnosed and only after an engineer has specified it.

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