to replace or reinforce floorboards...

================ It seems a pity to use chipboard in an old cottage. New T&G flooring is really quite affordable - about 75p per metre (4 1/2" width). That makes it about 2 or 3 time more expensive than chipboard but you'll be able to save on the cost of floor covering (carpet etc.) because it can be stained / varnished / waxed.

Cic.

"Tom Dixon" wrote | In middle of complete renovation on my 150 yr old cottage, 1st | floor is boarded with not very nice old planks 15mm, butted | together, bit wormy, quite bouncy, odd bit of rot, not too bad | but not pretty. I have extended the floor with new joists over | the old stairwell.

Is the bounce in the planks or the (old) joists? If the joists are bouncy new planks might not be the answer.

If you can, I would try *gently* propping the joists from the underside with acrow props, and seeing if the floor is still bouncy. If it is, some doubling up, herringbone strutting or fitch plates in the joists might be called for.

Owain