What might be happening
If the floor gives way under your foot, avoid that patch until it has been checked. There is a difference between one loose board and a floor whose supporting joists have deteriorated.
A soft patch beneath one board may be confined to the board, but a broader area that dips could involve joists or their bearings.
A timber floor can look perfectly ordinary from above while the damage is hidden underneath. The first job is to work out whether the boards themselves are affected or whether the joists and their supports need attention.
What you can look for
Lift a small accessible area carefully and inspect the underside, supports, fixing points and any nearby water pipes. Look for softness and damp rather than relying on colour.
You do not need to start taking the building apart yourself. If access is difficult, ask the person inspecting it to explain what needs to be opened up and why. Photographs of hidden defects are useful, especially when you are comparing quotations.
Putting it right
Find and repair leaks or moisture ingress first. Replace unsound boards, and have weakened supporting timbers assessed before deciding on strengthening or renewal.
Be wary of a quotation that deals only with the damaged wood and says nothing about the water or building defect behind it. Once the cause has been dealt with, the remaining timber can be checked and the necessary repairs planned.
When to call someone in
Avoid walking on a floor that seems liable to give way; keep others away until it is made safe.
A carpenter or joiner may be right for a local timber repair. If the problem involves a leak, you may need a plumber or roofer first. Where supporting timbers have moved, sagged or seriously weakened, get advice from someone qualified to assess the structure.
Questions worth asking
- Is the problem in the floorboards, the joists or where the joists sit in the wall?
- Can you show me the extent of the damage before any sound timber is removed?
- What caused it, and how will that cause be put right?