Dundonald wrote:

John Rumm wrote:
Dundonald wrote:

The question I have is what is the best tool to do the job? I've
borrowed a circular saw but it seems a bit like a sledge hammer to
crack a nut.

You could use your jigsaw if you break the end off a blade so that at

full stroke it is just long enough to reach through the floorboard.
Then
you should be able to jigsaw along the centre line of the joist
without
actually cutting the joist (much!).

Thanks for your reply. What method or technique is best to break the
end of the blade?

If you own a dremel, then use a cut-off wheel to chop off the end of the
blade, and form the end into the proper shape WRT the last tooth.
(look at the end of the existing one)