How to Install Prehung French Doors

Installing pre-hung French doors is a project that the DIY homeowner can do however it is a fairly complicated project and requires a second person to help lift the pre-hung French door into position. In addition, it is always important to read the manufacturers installation instructions prior to installing and they should take precedence over the general instructions listed below. To begin the installation of a prehung French door, unpack it from the box and remove the surrounding packing material and any wood or cardboard skid boards.

Make sure however to leave any pre-hanging clips that hold the doors closed within the door frame in place. Removing them will make the installation of the door more difficult, if not impossible. See how to frame in a rough door opening here.

Before actually installing prehung French doors make sure the doorframe is fully prepared to accept the new door unit.

If the door unit is to be installed on an outside wall make sure there is house wrap folded around the door frame and that it is stapled on the inside of the door frame. In addition, it is also wise to attach a piece of ice and water shield around the bottom of the door frame.

Next, nail a couple of small board stops (approximately 2”x4” in size) to the door frame opening on the inside of the doorframe, or from the opposite side in which you will actually insert the door into the doorframe. Apply two board stops near the top of the door frame opening and two near the bottom. These small board stops will help to ensure the door does not accidentally fall through the door opening, and will also act as a sort of fence to align the prehung French doors flush with the inside wall.

Finally apply a thick bead of caulking along the floor, or sill, of the door threshold before placing the prehung French door unit in place.

Once you have prepared the door opening for receipt of the door, center the bottom of the door unit in the door frame opening and lift or slide the door unit into place.

After the bottom of the door unit has been positioned in place, tilt the top of the door inwards so that it sits squarely in the door frame opening.

Once the prehung French door unit is roughly in place, check to make sure it’s face is plumb with the inside and outside walls and that there is a small gap between the sides and top of the door and the door opening. Then temporarily secure one hinge jamb side to the door frame using a couple of long screws (3” to 3.5” in length). Place one screw near the top of the door and the other near the bottom (about 6” from the top and bottom of the door unit).

Next check the gap along the top of the door unit and make sure that it is even throughout the entire width of the door unit. If it is not, shim the base of the door so that the gap is even along the entire length of the door unit. Also make sure that the spacing between the latch jamb is not too wide so that the doors cannot be latched. If it is too wide, then shim directly underneath the sill to narrow the spacing. If it is too narrow, then shim underneath the hinge jambs accordingly.

After confirming with a level that the gap across the length of the door is even and that the latch spacing is sufficient to properly latch the doors make sure that the unit is still plumb.

Next shim the other hinge jamb as necessary, applying shims approximately six inches from the bottom of the sill and six inches from the top of the door unit. Also apply shims directly behind the hinged areas of the door unit. Do not install shims above the door head jamb. Shims should only be used at the base or sides of the door unit.

Use a carpenters level or other straight edge to check to make sure you do not bow the door jamb as you install the shims. Also recheck to make sure that the door is flush with the wall surfaces.

Use a square and check all four corners of the door unit to make sure they are square. If they are not square apply shims as necessary to square up the corners. Again do not apply shims above the door head jamb.

On the inside of the door unit make one final check that the door is still plumb and that the gap is still even along the top and sides of the door and door jamb. Then use long 3-3.5” screws to fasten the hinge jambs to the door frame opening at the shim points. Then remove the door stops.

Next remove any pre-hanging clips from the doors and test the doors to make sure they swing freely and operate properly.

After confirming that the doors operate properly stuff the gaps between the door unit and the rough door opening with fiberglass insulation or foam insulation.

Next install the working hardware to the doors and ensure that they operate correctly.

Install door trim to both sides of the prehung French door if it is an interior French door.

If it is a patio French door then install door trim on the inside. On the outside of the door unit caulk around the door unit before re-applying house siding or exterior trim. Once the exterior door trim is installed, run another bead of caulk around the door trim.