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You put shims between the three-sided frame to which the door hinges are attached and the framing of the rough wall opening. Using shims like this will center your door in the wall opening and allows you to make small adjustments to get your door plumb and level for smooth operation. Your shims and the gap they fill will be covered when you install door trim, so don’t worry too much about the appearance.

Most modern, standard doors are 1 ⅜-inches thick, so you should aim for this thickness unless you're building a door to replace an antique door or fit into a non-standard wall opening. Thickness varies when installing a pocket door or sliding barn door, so a good rule of thumb is to measure your existing door, if possible. For standard doors, you can also measure from the inside of the door to the door stop to gauge the proper thickness.

While not strictly required, it is highly recommended to shim both sides of a door. Shimming both sides allows you to make minor adjustments for wall framing that isn't perfectly plumb, ensuring the door is level and hangs evenly. This practice helps the door swing open and close smoothly, reduces the chances of the frame warping, and can be used to center the door within the wall opening. If you are working alone, you may find it easier to remove the door slab to lighten the weight while supporting the frame and inserting the shims. With two people, the process is generally faster and easier, whether the slab is left in place or not.

Sliding doors and pocket doors are both great space savers, but they work a bit differently. A sliding door moves along a track and is always visible, sliding next to a wall or another door. On the other hand, a pocket door slides into a slot in the wall, hiding away completely when it's open. This makes pocket doors a perfect choice if you're looking to maximize every inch of floor space, unlike sliding doors that can still take up some room even when they're open.