Sounds like you're referring to adjustable steel piers used to hold up steel I beams installed in the basement of new homes. Typically, these have a plate on the top and bottom and a screw that you adjust in or out until the proper height is set. The base is intended to be surrounded by concrete to avoid shifting the piers after installation. You could weld the plates on and bolt them to the concrete to keep it from being shifted. As for having the strength to lift a sagging concrete roof you describe, they are not meant to be jacks and may require you to do the jacking with a bottle jack first. The columns are commonly available at building supply centers and steel companies. Here's a page that will show you what I'm referring to: Click Here to see adjustable columns Go to the bottom of the page.