Level of Difficulty: Intermediate Do-It-Yourselfer - Moderate
Completion Time: 1 Day

It may become necessary to replace a sliding patio door if the glass is broken, the casing is damaged, or if the structure is leaking or letting in drafts. Otherwise, new patio doors are often installed at the same time as building a deck, by opening up an exterior wall.

Several steps are involved in this process, which is relatively simple but requires care and precision. Accurate measuring and adjusting are the keys to a successful installation.

A helper is also required for this job, as the doors are big and heavy and therefore difficult to handle alone.


  • Reciprocating saw
  • Pry bar
  • Caulking gun
  • Screwdriver
  • Snips
  • Stapler
  • Driver drill

  • Hammer
  • Level
  • Square
  • Tenon saw (or dovetail, flush-cutting or back saw)
  • Pencil
  • Measuring tape
  • Utility knife


  • Patio door (in its frame)
  • Flexible self-adhesive flashing
  • Drip edge
  • PVC trim (if standard door)
  • Building paper or roofing felt
  • Cedar shims, 1 ½" wide
  • 3" No. 8 screws
  • Insulation (wool or foam)
  • Interior trim (the old trim can be reused)
  • Finishing nails
  • Exterior caulk
This project describes how to replace a patio door, of which there are two types: custom or standard. Some of the steps may not apply to your installation. Refer to the manufacturer's instructions.

A custom patio door is made to fit the size and thickness of a specific wall opening. It therefore has to be made to order. Custom doors are used mainly with new builds, where the exterior siding does not generally come right up to the rough opening. These doors come with brick moulding (or brick mould) – the trim that covers the gap between the rough opening and the exterior siding (which may or may not be made of brick).

A standard patio door comes with a simple box frame. This type of door, available in-store, is used mainly for installations where a new opening is cut in an existing wall. Standard doors do not require brick moulding because the exterior siding comes right up to the wall opening.

When replacing a patio door, choose the same type as the old one. It is possible to replace a standard door with a custom door, but since the new custom door will be framed with brick moulding, you will need to cut away the exterior siding around the opening, and this can be tricky if the siding is made of brick.

Brick moulding sometimes comes with a strip at the back that forms a channel into which the vinyl siding slots. This strip is only used when installing new vinyl siding and must be removed if the siding is made of brick or if the old vinyl siding is to be reused.

In the case of a new installation, keep some plywood boards handy to cover the opening if the project is not completed the same day. Another way to avoid leaving an open hole in the wall is to remove the siding at the last minute.

Taking accurate measurements is very important in order to buy or order a door of exactly the right size. The rough opening is usually 1" higher and wider (in total) than the door frame, to leave space for inserting the shims for adjustment. If the door frame is resting directly against the jambs, the installation is not conform and you must to mention this when purchasing the new door.

1.1. Remove the door's interior trim to expose the jambs.

1.2. Measure the rough opening:

  • Measure the height on both sides of the opening. Measure from the lintel down to the subfloor, i.e. the wood or concrete surface on which the current doorsill rests (and not the flooring surface or underlayment).
  • Measure the width, across the bottom and top of the opening. Always measure from the jambs and not from the frame of the door you are replacing. For each pair of measurements, use the shortest of the two.

1.3. Measure the frame of the existing door: Measure the width and height, from the outside edge of the frame.

1.4. Measure the total thickness of the wall:

  • Vinyl siding: Measure from the interior drywall surface to the exterior lath on which the vinyl siding is nailed.
  • Brick siding: Measure from the interior drywall surface to the marks left by the old door on the brick or to just under half the thickness of the brick.
  • If the existing door has brick moulding, this will be covering the lath or side of brick so it will not be possible to measure them. In this case, measure the width of the existing door frame.

1.5. Measure the "brick to brick" opening (total width of the opening in the siding): measure from one brick to the other if the siding is brick, or from one J-channel to the other, if it is vinyl. If the old door has brick moulding, this measurement will be wider than the rough opening.



The following step suggests cutting the opening with a reciprocating saw. If you do not have a reciprocating saw, you can remove the PVC trim from the door casing to uncover the screws and then unscrew the casing from the jambs. All you have to do then is remove the existing shims and insulating material.

2.1. Remove the screen door.

2.2. Remove the sliding panel from the door casing, followed by the stationary panel, if possible.

2.3. Remove all exterior caulk in contact with the door casing.

2.4. Cut between the casing and the jambs using a reciprocating saw fitted with a blade capable of hacking through screws, shims and insulation material, including polyurethane.
Do not cut under the threshold. Ask a helper to hold the door casing steady while you cut.

2.5. Remove the casing.
The threshold may be fastened down with sealant, which will mean having to twist the casing to free it, or even using a pry bar.

2.6. Clean any traces of sealant from the threshold.

3.1. Stick flexible self-adhesive flashing along the threshold and 6" up the side of each jamb.
Cover the flashing or building paper already on the exterior wall, under the threshold. Make two small cuts in the corners of the flashing to make it easier to fold down. Several widths may be necessary in order to cover the whole threshold.

3.2. Cut a piece of aluminium drip edge to the width of the rough opening, run adhesive caulk along it and slide it between the siding and the building paper along the top of the opening.

3.3. Remove the sliding panel from the frame of the new patio door.
Leave the stationary panel in place; this will keep the frame rigid.

3.4. Using a countersink bit, drill pilot holes in each side of the casing: at the bottom, in the centre and at the top (or according to the manufacturer's instructions).

3.5. With a helper, test fit the patio door in the opening.

3.6. Mark the contour of the brick moulding on the siding.
If yours is a standard door with no brick moulding, go directly to the next step: Install the door. Remove the door from the opening.

3.7. Cut away the siding along the lines marked previously.
If it is vinyl siding, cut ¼" wider than the lines, to allow for insertion of the J-channel finish trim.

3.8. Apply flexible waterproof flashing to the bare wall surfaces along the sides of the opening where the siding was cut away.
Fold the surplus flashing around the jambs.

3.9. Install J-channel trim along the edges of the vinyl siding, if applicable.




The door frame is screwed to the jambs through shims. The screws are then concealed with PVC trim, which is supplied with a custom door and can be bought separately in the case of a standard door. Standard doors can also be installed without this PVC trim and the screw heads concealed with wood plugs.

4.1. Remove the strip at the back of the brick moulding, unless new vinyl siding is being installed at the same time as the door.

4.2. Run two or three beads of caulk along the threshold of the opening.

4.3. Place the bottom of the door frame on the threshold, then tilt the door into the opening until the brick moulding rests up against the house.
If the door does not have brick moulding, the outer edge of the frame must be flush with the lath under the siding (for vinyl siding) or positioned along the line left on the brick by the old door.

4.4. Insert shims from the inside between the door frame and jambs to adjust the frame and ensure that it is centred, square, level and plumb.
Insert shims in line with the holes drilled previously and position others every 12", but always at least 8" from the corners. If shims are needed under the threshold, space them 8" from one another. Constantly check that the frame is level and square. Use enough shims to support the frame but not so many as to force it, which may cause it to bow.

4.5. Measure the two diagonals of the door frame.
These measurements must be identical (not exceeding 1/8" difference). The frame must also be level, both horizontally and vertically (plumb). Up to 1/16" of deviation is acceptable. Make any necessary adjustments.

4.6. Screw the sides of the door frame to the jambs, through the shims.
Do not screw the head or the sill.

4.7. Cover the thickness of wall not covered by the door frame.

  • For a standard door: measure the distance between the edge of the frame and the interior wall surface. Make an extension jamb "archway" so that the door frame arrives level with the drywall surface. Distance less than 2 ½": nail it (using finishing nails) directly around the frame edge. Distance more than 2 ½": position the extension using shims, then nail to the jambs. This extension will be hidden by the PVC trim.
  • For a custom door: the frame width should cover the entire thickness of the wall.

4.8. Cut off the ends of the shims with a tenon or flush-cutting saw.

4.9. Insert fibreglass insulation around the edge of the frame.
Push it in with a shim or similar implement, but avoid compressing it. Fill all the space with continuous layers of insulation (not small pieces inserted separately).
Note: Polyurethane foam insulation can be used, but this technique requires experience, since too much of this product can exert pressure on the frame and deform it.

4.10. If shims were installed under the threshold, caulk the gaps between them.

4.11. Fit the sliding door into the frame and check that it glides well and is parallel to the frame sides.

4.12. Adjust the rollers under the sliding panel if necessary.





5.1. Install the PVC trim (supplied with a custom door or sold separately for a standard door).

5.2. Caulk between the siding and the frame (or brick moulding).
If there is a gap of more than ½" between the siding and the door frame, insert a strip of ½" vinyl filler trim and then caulk.

5.3. Install the interior trim.

5.4. If necessary, depending on the shape of the door threshold, fasten a support block under the threshold, cover it with aluminium cladding and seal it all the way around.