Rotten floorboards in an old wooden house: how to spot and fix them

What a soft patch in the floor actually means

A springy or soft floor is not a diagnosis in itself, it is a symptom. The two most common causes are a joist that has sagged or split with age, and timber that has softened because it has been damp for a long time. Both feel similar underfoot at first, which is why owners often assume the worse case before checking.

In houses from the 1920s and 1930s, the floor structure is usually a suspended timber floor: joists running across a low crawl space or void, with floorboards nailed on top. That void is exactly where problems start, because it is out of sight and rarely inspected until something gives.

A patch near a wall, under a radiator, or close to the kitchen or bathroom is more likely to be moisture-related than a soft spot in the middle of an otherwise dry room. Location is often the first clue, before you lift anything.

Where the moisture gets in under a suspended floor

The most frequent source is ground moisture rising into a poorly ventilated crawl space. Old houses were often built with small vents in the foundation wall, and over decades these get blocked by soil, plants, or a driveway extension, cutting off the airflow the floor was designed to rely on.

Plumbing leaks are the second common cause, particularly slow ones. A dripping supply pipe or a slightly leaking waste connection under a bathroom or kitchen floor can soak the joists below for months before it shows as a soft patch above.

Rainwater is a third route: guttering that overflows against the foundation, or ground that slopes towards the house instead of away from it, pushes water straight under the timber. Renovations that seal a floor with vinyl or laminate without leaving it able to breathe can also trap moisture that used to escape naturally.

How to test with a screwdriver whether the timber is sound

A flat-head screwdriver is the simplest and most reliable tool for a first check. Press the tip firmly into the board or, better, into a joist reached through a hatch or gap, at a slight angle rather than straight in.

Sound, dry timber resists the point and gives a firm, slightly resistant feel, similar to pressing into a hard piece of wood. Rotten timber lets the screwdriver sink in with little resistance, sometimes crumbling or splitting along the grain as it goes.

Test in several spots, not just the centre of the soft patch, since rot often spreads further than the visible sag. A musty smell, dark staining, or a soft, spongy texture when you press with a thumb are all worth checking alongside the screwdriver.

• Push the screwdriver in at an angle, not straight down
• Compare resistance against a spot you know is sound
• Test around the edges of the soft patch, not only its middle
• Note any musty smell, dark staining, or crumbling
• Check both the board and, if reachable, the joist beneath it

When replacing a few boards is enough

If the softness is limited to one or two boards and the screwdriver test shows the joists underneath are still solid, replacing just those boards is usually enough. This is common where a board has cracked, warped, or taken damage from a single spill or leak that has since been fixed.

Matching old floorboards can be awkward, since widths and profiles from the 1930s rarely match what is sold today, but a reasonable match is often found from reclaimed timber or by re-using boards taken from a less visible room.

This is straightforward work that our flooring service handles as a standalone job, starting from 15 € for smaller jobs, and it is the kind of repair worth doing promptly rather than living with a spring in the floor.

When the joists need attention, not just the boards

If the screwdriver sinks easily into the joist itself, if the soft area covers more than one or two boards, or if you can see daylight or damp staining through gaps once boards are lifted, the problem has moved from the floor surface into the structure.

Rotten joists need to be sistered with new timber alongside the old, or replaced outright, and the moisture source has to be found and closed off first, otherwise new timber will fail the same way in a few years.

This is not a job to patch over with a stronger board on top. Left alone, a rotten joist can spread the load to its neighbours and widen the soft area over time, and in a worst case affect what is resting on it.

What you can check yourself and when to call someone

Checking with a screwdriver, looking at the crawl space vents, and tracing an obvious leak are all things an owner can reasonably do before calling anyone. If the void under the floor is accessible through a hatch, a torch and twenty minutes will tell you a great deal.

Call for help once boards need to come up to inspect joists, once more than a small area is affected, or once you are not confident telling rot from a simply worn board. Odd jobs of this kind are billed from 39 € an hour, with a 25 € call-out in Tallinn deducted from the bill if the work goes ahead.

Where the floor turns out to need more than a board swap, whether that is several joists or a wider look at the room's moisture and ventilation, it often makes sense as part of a flat renovation job, quoted from 180 € once the extent of the work is clear.