Prehung doors can be an easy solution when replacing an existing one, since they’re already assembled. To install it, just place some shims under its hinge side jamb to keep it plumb while using your carpenter’s level to check its plumbness.
Place a 4-foot level on the floor near the doorway and use shims to center its bubble until tacked to the floor with finish nails.
Measure the Opening
Before ordering a prehung door, it is essential that you know the size of your opening. To measure this accurately, take three horizontal measurements from left, center, and right of your rough opening – any casing must be removed first! – with the smallest measurement being your opening size.
If the level reveals that one hinge side is lower than another, place wood shims beneath its bubble at hinge jamb locations until both levels are level; adjust until both levels are plumb before nailing through them into trimmer studs.
Finalize by using a carpenter’s square to check that the frame is level and square; this will improve both its aesthetics and functionality. Plus, cutting your frame later may become necessary! Now it’s time to install your prehung door!
Cut the Jambs
Pry up the side jambs and head jamb if necessary. Remove hinge pins with a screwdriver and hammer, remove door trim, cut any caulk between casing and wall using utility knife, install z-flashing at sill to protect from water damage, then install new door.
Wood shims should be placed between the side jambs to bring them up to the height of your rough opening. If you plan on installing new flooring, make sure that there is space left for this in the budget.
Shims can be an indispensable asset when installing interior doors, but it’s essential that they be added correctly. Begin on the hinge side and add shims between frame and stud until your door swings freely and is plumb before proceeding to latch side.
Add Shims
Pre-hung doors can be installed without major modifications to walls and frames; however, in order for it to fit perfectly you’ll still need to shim it for level. Begin by positioning its frame inside its rough opening and double checking that it fits properly before starting this step.
Make sure the 4-foot level is level on both hinge and latch sides, adding shims until its bubble is in the center of both areas. Next, move to latch side and shim there until plumb. Afterward, nail frame to studs at every location where there was shim placement before double-checking with levels and other tools available that it is plumb and square to trimmer studs.
Install the Door
Prehung doors differ from slab doors in that they do not require you to build their frames; rather, they come as one piece with hinges attached and can be installed directly into any existing door frame as long as both jambs are level and of sufficient size.
Place a four-foot level on the floor near the jamb, and observe its bubble. If either hinge side is lower than latch side, add shims until the level bubble centered itself before tacking them to the floor and installing your door.
Add a bead of Z-flashing along the bottom of the door sill to prevent water damage to walls and floors, then reinstall trim around it. Test how smoothly the door opens/closes before making adjustments as necessary for smooth operation.
Finishing Touches
Once a door has been installed, it’s essential that its hinge side and latch side of the frame are level and plumb. For this, a long carpenter’s level and wood shims will come in handy.
Position the level on an even surface and set it into the doorway, noting where its bubble settles. If hinge side is lower than latch side, shim under hinge side jamb until it reaches an equal height.
Repeat this process on the latch side jamb to ensure it’s level. Next, install trim and doorknob backplates – to cover nailheads use putty that matches your casing for an unprofessional appearance that you can find at most hardware stores.