What you probably want is a tenon saw and a good supply of swear words.
A multitool will help as well, although don't get cheap blades and you probably want to do most of the cutting with the saw.
If you don't like the sound of that, don't cut the boards on top of the joist - instead, cut along the side of the joist (many times faster and less of a faff) and fix a second piece of timber along the joist to fix the replacement boards to. Not sure how in line with good building practise that is or whether it meets regs, but it's what I'm going to have to do in some cases. A few chunky coach bolts through the joist done up nice and tight should do the trick I reckon.
Cutting the boards is a right pain in the arse.
What you probably want is a tenon saw and a good supply of swear words.
A multitool will help as well, although don't get cheap blades and you probably want to do most of the cutting with the saw.
If you don't like the sound of that, don't cut the boards on top of the joist - instead, cut along the side of the joist (many times faster and less of a faff) and fix a second piece of timber along the joist to fix the replacement boards to. Not sure how in line with good building practise that is or whether it meets regs, but it's what I'm going to have to do in some cases. A few chunky coach bolts through the joist done up nice and tight should do the trick I reckon.