How to remove screws from old decking
When the boards having the heads are being discarded, a simple way to remove them is to cut through the screws at the joint between the board.

You can use a reciprocating saw (often called by a brandname, Sawzall) 


or a multitool


In either case, you need to use a blade intended for cutting metal. Some blades are combination blades that work in wood or metal. Place the blades between the bottom of the decking and the supporting framing and cut just the screw. The small amount of wood taken out will not matter, but lean the blade toward the wood being discarded.

If you can't access the bottom of the deck, you can remove a section of the board to be discarded close to the screw and then cut the screw horizontally.

The stub of the old screw in the framing board will be covered by the new boards.

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