In order to jack up a house from underneath, you have no way of knowing how far the house moves unless you have a reference line. Using a grade stick nailed to a nearby floor joist, place your laser line on the ground (out of the way) so it hits the grade stick. Mark the stick with a sharpie. Then get working on the bottle jack and you can watch the house move relative to the sharpie line and laser line. Once you've reached your predetermined height adjustment, stop and add additional support so the bottle jack isn't alone in supporting the weight of the house.
The plastic is a vapor barrier (aka vapor retarder) to protect and separate the structural grout from the floor joist, rim joist, or girder.
Finally, if you're going to jack up an old house, please hire a licensed contractor. Abode Building Company LLC is a licensed Florida General Contractor.
