How to Square a Threshold

Open the door on the threshold, if there is one. Push it back out of the way. Measure the threshold with a framing square. Note any areas where the threshold rises or dips.

Close the door. Darken the room, get down on the floor and look under the door. Note if you can see light anywhere along the threshold except at the corners.

If the threshold is adjustable, turn its screws nearest the areas where you see light counterclockwise with a screwdriver to raise the threshold. If there are no screws in the threshold, hammer shims beneath the threshold at the points where you see light to raise the threshold. Do not break off the extra shim length protruding from beneath the threshold yet.

Measure the threshold again with a framing square. Turn the screws to raise or lower the adjustable threshold; the threshold should be level, and the corners where it meets the door frame should form 90 degree angles.

If you had to add shims beneath the threshold, measure the threshold again with a framing square. Add or remove shims beneath the threshold until the threshold is square, forming 90 degree angles where it meets the door frame. Break off excess shim material with your hand.