I had six old oak floorboards to lay in my 500 year old house. My
problem was that the floor was not flat but curved down 10 cm towards
one corner

I tried

- Fixing one end of the floorboard in position and putting a heavy
weight on the other end. Result - no noticeable deflection in five
days

- As above but laying a wet towel on top of the floorboard where the
bend needed to be. Result - no noticeable deflection in five days

- At which point I gave up pussyfooting around and enthusiastically
used a wallpaper steamer where the bend needed to be. That was the
magic ingredient which made the floorboard sufficiently flexible that
I could stand on it to get screw and penny washer fixings in position.
Result - one week later I loosened a floorboard and found that the
board had deflected by a couple of centimetres.

I dont trust it to stay deflected so I fixed it back down with screw
and penny washer. In a month or two I will remove the penny washers
and countersink the screws

Anna
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~ ~ Anna Kettle, Suffolk, England
|""""| ~ Lime plaster repair and conservation
/ ^^ \ // Freehand modelling in lime: overmantels, pargeting etc
|____| www.kettlenet.co.uk