Decks, Patios, Porches and Docks - Replacing a portion of a piece of deck lumber

Hi, a few deck lumber on my deck are partially rotten. My plan is to replace just the bad section. Is there a easy way to cut the bad part up to where the underneath support reside without having to lift the whole piece off the deck to do the cutting? Any ideas is highly appreciated. Thanks!

Sabre saw. Not sure how you are intending to do this repair, but the new wood must rest on the joists at each end as well as being nailed / screwed to any joist it spans.

The intent is to cut the bad section out of the old piece up to the middle of the nearest joist on both sides. The new wood is then cut to size fit in-between. So the new wood and the old wood share a joist right in the middle line on both sides. All will be nailed to joists. A sabre saw will not work in this case since I'm cutting the deck lumber right on top of and along the middleline of a joist. The problem is what/how to not cut into the joist while keeping the lumber in its current position (not to lift it off the deck). Not sure if a circular saw set to the thickness of the lumber will do the trick. Any ideas? Thanks!

A circular saw will, indeed, do the trick - for the most part.

Set your saw to just past the thickness of your lumber, and carefully cut out the rotten section. Be sure to remove any nails near the cut BEFORE cutting - circular saw blades don't like deck nails very much...:)

This procedure will NOT cut all the way through the deck board except in the center of the board. The edges will have some material that will need to be reomved with a reciprocating saw or hammer and sharp chisel.

If you are cutting out several boards next to one another, you will be able to get the middle ones fully cut out. This should make life easier.

Remember, there is no shame in removing a perfectly good deck board to make it easier to cut out the bad ones. Save the good boards, and nail them right back in place.

I would really really recommend replacing the whole bad board instead of splicing them. Would make it a lot easier to do also.

Thank you all for the advise! The main reason I try not to remove or replace the whole board is that one end of the board happen to sit beneath another structure which I'm afraid I won't be able to put it back if removed to lift the board off the deck.

cut the rotted part between the joists using a sawsall. then lift the board from that cut enough to clear the surrounding deck boards at the point of the joists .then make a clean cut with your circular saw at the joists.
ps. prop it up before cutting with your circular saw dont hold it with your hand and saw.