How To Straighten Old Walls. It takes only a few basic tools to plumb and straighten walls — mainly a good level and some nylon dry lines. Because these walls are not as strong as poured concrete walls, they need to be straightened much sooner.
Signs of weakness in the wall include horizontal cracks along the the wall should also be straightened if the blocks in the wall shatter or if a block in the wall shifts more than 1 inch laterally.
Both walls currently are plastered and papered but are nowhere near level with bumps all over and about 50% of the plaster has obviously come away the right hand wall moved with subsidence years ago and so has moved about 5inches in some places from level. Because these walls are not as strong as poured concrete walls, they need to be straightened much sooner. Once you're done, you should have one side that is completely straight, so you can remove the guide piece and then flip the wood around to straighten the other side. Here's how to do it with just a table saw.