How to Install Exterior Prehung Doors

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To successfully install an exterior prehung door is a matter of ensuring the unit is held plumb and square as it is secured in place. If you use this methodical approach, the new door will provide long years of use and functionality for the home.

Things You'll Need

  • Measuring tape
  • Pencil
  • Pry bar
  • Hammer
  • Nail set
  • Builder's level
  • Framing square
  • Wood shims
  • Utility knife
  • 8d finish nails
  • 2½-inch plated deck screws
  • Check the opening with a builder's level to confirm that both sides are plumb, and then measure horizontally between the two sides at the top, middle point and bottom to ensure the sides are parallel. Check the faces of the trimmers (the two wall studs on each side of the opening) with a square to ensure they are square to the adjacent wall surface.

    If the trimmers are not plumb or square, use a pry bar to adjust the trimmer by moving it in or out at the top or bottom until plumb. Insert shims as needed to fill any gap between the trimmer and the wall stud behind it, then nail through trimmer and shims to secure in place.

  • Place a builder's level on the floor of the opening. If either side of the opening is lower than the other side, slide shims under the straightedge to adjust it until the level's bubble is centered. Always use two shims, one from each side, to keep the perpendicular surface level as well. Add shims at the center point of the opening to fill any gap that remains. Secure these shims in place with 8d finish nails. These will ensure the door sill will be supported as level.

  • Lift the door unit into the rough opening. Rest the sill on the floor or the shims installed in step 2.

  • Shift the door frame in the opening to make the space between the frame and wall framing equal on the both sides. Position the frame so that the interior surface is flush with the adjacent interior wall surface and the sides are plumb.

  • On the hinge side of door frame, insert shims behind the top hinge between the jamb and the trimmer to fill the space. Position a sufficient number of shims to hold the jamb plumb and secure with 8d finish nails through the jamb and shims into the trimmer. Check that the door remains plumb.

  • Repeat step 5 at the bottom hinge location. Check that the door remains plumb and square to the adjacent wall.

  • Repeat step 5 at the middle-hinge location.

  • Check the horizontal space, called the “reveal,” between the top of the door and the top jamb. It should be uniform and 1/8- to 3/16-inch wide across the top of the door and match the reveal down the hinged side of the door. If necessary, adjust the top reveal by pushing up the jamb. Set this reveal by inserting shims above the top of the jamb, beginning at the center and driving an 8d nail through the jamb and shims into the header beam. With these shims in place, check that the top jamb is straight and level, and that the reveal is consistent. Check that the hinge-side jamb and top jamb are square to each other.

  • On the opening side of the door, secure the jamb to the trimmer, beginning at the middle, behind the lockset location. Position sufficient shims to hold the jamb at the proper reveal space between the door and the jamb. Secure with 8d finish nails through the jamb into the trimmer. Check that the door is plumb and that the jamb is square to the adjacent wall surface.

  • Add additional shim support as needed to maintain reveal space between the door and the jamb. Add shim support at the bottom, just above the sill and spaced approximately 8 inches apart vertically up the jamb length.

  • Finish securing the top jamb by inserting shims 2 to 3 inches from the corners on both sides of the door jamb. Again, maintain a consistent reveal space between the door and the jamb. Additional shims can be added as needed.

  • Finish securing the hinge-side jamb by inserting shims at the mid-point between the hinges or as needed to maintain a consistent reveal from top to bottom.

  • Install any screws in the door sill designated by the door manufacturer’s specifications. Shim as necessary to maintain support if the floor is not level.

  • Swing the door to the open position. Remove one of the hinge screws penetrating the jamb from the top hinge. Replace this original screw with a 2½-inch plated deck screw into the trimmer. This will prevent sagging of the door over time. Be sure that the new deck screw is backed by shims and penetrates the trimmer by a minimum of 1½ inches.

  • Use a small handsaw or utility knife to trim excess shims that protrude beyond the door jamb on both sides of the opening.

  • Use a nail set to set all of the finish nails used to secure the jamb and the shims.

  • The door and frame are now secure and ready for trim, hardware and final finish.

Tips & Warnings

  • If the deck screw used in step 13 does not match the finish or head size of the original hinge screws, the screw can be inserted behind the hinge plate. This step can be repeated at the other two hinges for additional strength at high-traffic doorways.
  • Prehung door assemblies are heavy and cumbersome to maneuver, so get a helper before undertaking this project. Follow all manufacturer’s specifications during handling and installation to maintain the product warranty.

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