Things You'll Need
Hand or power saw
Many of the problems associated with doors are due to the door or frame being out of level. A door that is not level will not swing correctly, and will put excess pressure on certain hinges. Over time the door can bind with the frame, making it difficult to open. It can also pull away from the frame, even to the point where it can no longer be relied upon to keep the opening locked and secure. Leveling a door begins when the frame is installed. The majority of wooden frames come with extra-long jambs, which allows homeowners to cut the jambs as needed to accommodate an unlevel floor.
Hang the door on the frame. Pre-hung doors will come with the door installed, but other residential door frames are sold separately from the door. By hanging the door first, you'll be able to create the proper amount of clearance between the door and floor as you work.
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Place the door and frame into the opening and measure how much space there is between the door and floor. Take a note of this measurement so you can refer to it as you work.
Remove the frame from the opening and set it aside. Place a level on the floor of the opening. Hold one end of the level on the highest end of the opening, then balance the level so the bubble is centered. Measure the distance between the floor and the bottom of the level at the low end of the opening. This number represents the difference in floor height between the two ends of the opening.
Take the elevation difference you determine in Step 3 and transfer it to one of the jambs on the frame. You must make sure to transfer it to the jamb that will sit on the higher side of the floor. Keep in mind which way the door will swing to help you choose the correct jamb. Mark this measurement on the jamb using pencil.
Check to see if the jambs are long enough before you make any cuts. Compare your measurement from Step 2 to the amount you will need to cut from the jamb, which you determined in Steps 3 and 4. If the clearance between the door and floor is greater than the amount you would need to cut plus 1/4", you will be able to make the cut safely. If not, you will have to level out the frame using wooden shims during installation.
Make your cuts, if applicable. Cut the end of the jamb based on the mark you made in Step 4. You can use a hand saw or circular saw for this task. Wear safety glasses and use caution when making this cut.
Set the frame back into the opening. Use your level to check the head and jambs to check that it is level. If there is still a large gap at the base of the door, you may cut both jambs by an equal amount to narrow this gap. Leave about 1/4" clearance between the door and floor so the door can swing freely over carpet or other flooring.
Nail the frame to the wall at each hinge location, then add more nails in the other jamb at approximately the same elevations. Keep checking to ensure the frame is square, and add wooden shims under the jambs as needed to fix any remaining issues with levelness.
If your frame doesn't have jambs that are long enough to cut, use wooden shims or scrap lumber to level out the frame. Once you place the frame in the opening, slide wooden shims under each jamb until the frame is level. You can also use a beveled board or piece of wood to span the entire threshold and even it out, though it may take some skilled carpentry work to create this board.