How to Install a Patio Door

Remove the old patio door. If you make a new opening, install a temporary support. Measure the diagonals to check the opening for square (they should be exactly equal). Use a long straight board or a string line to check for high spots in the sill and low spots on the header. When measuring for a new door, use the shortest measurements. Use shims to level the sill.

Project tip: A set of hinged patio doors installs much like a slider but also has some of the challenges of hanging a standard exterior door. Because two doors meet in the middle, any sagging or out-of-square condition will be multiplied. Provide extra strength for the hinges by removing one hinge screw and driving a 3-inch screw through the hole into the wall framing.

When shimming and aligning the unit, pay special attention to the critical point where the two doors meet. The doors should fit snugly but not so tightly that they are difficult to operate. (On wood units there is often a special piece of molding called a T astragal on the vertical edge of one of the doors.)

On a wide opening like this, also check that the sides are in the same plane. If one side leans away from the house and the other does not, the opening is "cross-legged" and the door will not slide smoothly. Stretch two lines diagonally across the opening; they should just touch where they cross. If they don't, try using a small sledgehammer to move one or both sides a bit. Otherwise, you may need to remove siding or drywall on one side and adjust the framing.

Prepare the sill according to manufacturer's instructions. In this arrangement self-adhesive flashing is applied along the sill.

Unpack the door and assemble as necessary. If the sliding panel is in place, remove it by lifting up and tilting the bottom out. It's usually best to leave the fixed panel in place; it makes the door heavy, but it helps keep the frame square. Some packing blocks should be removed; others should be left in place until you're ready to install the molding. Refer to the manufacturer's instructions.

The door shown is held in place with fasteners driven through the jamb. For a neat installation use a countersink bit to drill holes so the screw heads will be flush with the surface. Locate the holes according to the manufacturer's instructions.

Working with at least one helper, lift the door onto the sill and tilt it in. Press the unit into position against the siding. Have a helper hold the door in place while you make adjustments.

Check the door frame for level, square, and plumb; continually recheck as you shim. Tap in pairs of shims every 12 inches on the sides and the header, and every 8 inches on the floor. Tack (partially drive) nails or screws to temporarily hold the door in place. Reinstall the sliding panel and check for smooth operation. If your patio door has brick molding already attached, mark the siding for cutting now. Back out the fasteners and remove the door.

Just before you finally install the door, apply three beads of exterior caulk on the sill. Apply extra-thick caulk near shims to be sure the bottom of the threshold seals against the sill. Tilt the door back in place. Check that the threshold is straight and even. Shim the sides and the header.

If the door's jambs are the right width, the front of the jambs will be flush with the wall surface. Often, however, the jambs fall short of the wall surface. If so, make sure the distance between the jamb face and the wall surface is consistent all around, so you can fill in later with trim ripped to fit. If the jamb stands slightly proud of the wall surface, you will need to plane the jamb.

Install the sliding door and test for smooth operation. The gap between the door panel and the frame should be consistent at all points. Drive screws or nails to attach the threshold. If you see the threshold flex slightly when you slide the door, install additional shims as needed.

Drive nails or screws to attach the side jambs. At the strike (or strike box), install shims and drive a long screw to secure the strike. Install the latch, test, and adjust as needed.

Drive screws to install the head jamb. Slide the door and check the gap for consistency. You may need to back out some screws, adjust the shims, and redrive the screws.

Use a handsaw to cut the shims flush with the framing or the wall surface. If the jamb does not come flush with the wall surface, cut the shims flush with the jamb edges. If the sill does not stick out as far as the brick molding, rip-cut a piece of siding or pressure-treated lumber to fit and nail or screw it in place. You can install a kick board below the sill.

Mark a 1/4-inch reveal around the exterior edge of the jamb. Hold trim against the door for marking. Apply the trim using 10d galvanized casing nails when nailing into framing and 6d if nailing into the jamb. Use 10d galvanized casing nails for attaching integral brick molding. For flanged units, drive screws.

Rip-cut the jamb extender. To ease final installation, glue the extenders to the interior casing in advance. (In this case, the old casing could be reused because the replacement door was slightly smaller than the original door.) Be sure to clamp each piece even with the casing.

With the rip-cut pieces of 1x glued to the casing, it can be trimmed to fit and reattached to the wall.

The sliding panel can be adjusted up or down on one or both sides by turning an adjusting screw. The screw may be on the bottom face of the panel or at either end.

Check the screen panel for square. If it is out of square, stand it on one corner and pull down on the opposite corner. Install the sliding screen and adjust it so it slides easily.

Apply a bead of exterior caulk all around the exterior trim. Set any nails below the surface. Fill nail or screw holes with a dot of caulk.

Project tip: Follow manufacturer's instructions for sealing the opening. In this arrangement, pieces of building paper and self-stick flashing tape are cut to fit and installed in the correct order, so water has no chance of seeping behind and damaging the house's sheathing.