How to remove ceiling joist cross blocks without tearing up roof?
Tools required:


Standard wood chisel or flat bar
Hammer, 16 oz. or larger
Reciro-saw (Sawzall) and a long blade, preferrably fine-toothed


Procedure:


Cut any nails driven through the rafters and into the ends of the blocks:


Drive the rafters outward with the hammer to create a small gap between them and the blocks, one at a time, then run the saw blade through the gap. 

Cut the nails driven through the roof deck:


Drive the chisel or flat bar between the sheathing and the block at several locations to create a gap.
Cut the nails by running the saw blade through the gap. Flex the blade to force it to run along the inside surface of the sheathing and not cut into it or the blocks.