Replace a Patio Door

Overview: Benefits, special tools and time

If you've been putting up with a drafty patio door that sticks, fogs up or leaks during a heavy rainstorm, it's time to consider a new, energy-efficient replacement. While a high-quality door isn't cheap, it'll cut down on drafts, require almost no maintenance, and glide smoothly and latch securely. Best of all, you'll save $300-plus if you install it yourself.

At first glance, replacing such a big door may seem intimidating. But if you read through this article, you'll see that it's similar to replacing a window. We'll walk you through each step, including a special section on the critical flashing details that make the new door leakproof.

Manufacturers of higher-quality units have made installation fairly simple and straightforward. If you have experience installing a window or a swinging door, you should have no trouble with a sliding patio door. Other than basic carpentry tools, you'll only need a 4-ft. level and a screw gun. But we also recommend using a reciprocating saw fitted with an 8-in. bimetal blade to hack through shims and nails (Photo 5). Unless you're a brawny DIYer, another useful tool is a strong helper for short periods to assist with the heavy, awkward panels and door frames.

In most cases, you can replace that old slider with a smoothly operating, energy-efficient door in about a day. Add a few more hours to retrim the outside and the inside and you're done.

Step 1: Measure the opening before you buy

Before you run off to buy your slider, pull off the interior trim around the old door and measure the “rough opening” for width and height. (If you want to reuse the old trim on the new door, pull the nails through the unfinished side.) Make sure you measure to the framing right next to the old doorjamb. You may have to cut away overhanging drywall to get at it (Photo 1). To check the height, make sure you're measuring to the subfloor, that is, the wood or concrete that the doorsill actually rests on and not a secondary layer of particleboard or plywood (underlayment).You can check for underlayment by pulling off a nearby heat register and looking at the floor layers bordering the opening. Then select a door that fits your rough opening. The door catalog will list its rough opening requirements. The door dealer will walk you through sizing and door options. (See “Buying a Patio Door,” below.)

Once you get your door home, you'll save a lot of time if you finish any exposed wood surfaces on the door before beginning the installation. That'll save you from tricky brushwork cutting in around exterior claddings, weatherstripping and hardware, and keep smelly finishes out of your living quarters.

In our demonstration, we're removing a 6-ft. slider surrounded by wood trim and vinyl siding. Our new door was slightly smaller so we had to deal with a small gap surrounding the new exterior trim. Most new replacement doors will have a similar gap, no matter what exterior siding you have. If your old door has exterior trim fastened to the door frame, remove the trim and follow the installation steps we show. But if the siding comes right up to the doorframe (no trim), you'll probably have to remove the siding or cut it back and restore it after installing the door in order to achieve a leak-free installation. And if you have stucco or brick, you'll probably have to add trim to fill the gap and rely partly on high-quality caulk to seal out water. We'll talk more about this later.

You never know how smoothly a patio door tearout and installation will go, so start in the morning to give yourself the best chance to finish up before nightfall. If you're not going to beat the witching hour, cover the opening from the inside with sheets of plywood cut about 6 in. larger than the opening. Run 3-in. screws through the plywood and drywall into the framing where the holes will be covered by the interior trim. If it's chilly out or threatens to rain, also staple plastic over the exterior to keep out drafts and water.

Step 2: Remove the old door

Sliding patio doors are heavy (60 to 100 lbs. per panel), so it's best to remove the old panels one at a time and then tackle the door frame. Start with prying free any exterior trim pieces and then remove first the sliding and then the stationary panel (Photos 2 - 4). Usually the sliding panel wheels rest on the bottom track and the panel is held at the top by a removable strip of wood called the “inside head stop.” Unscrew the inside head stop and remove the panel by tipping the top into the room and then lifting the bottom rollers free of the track (Photo 2). Some doors may have a channel instead of a head stop at the top, and you may need to lower the door on its rollers using the adjusting screws at the base to gain clearance (Photo 11). Then lift the panel straight up until the rollers are clear of the track, and pull the bottom into the room to free the panel.

Removing the stationary panel can be trickier. Generally there'll be an angle bracket at the top and maybe one at the bottom that you unscrew and remove (Photo 3). Then slide the panel toward the latch to clear it from the side jamb so you can lift it free. If it won't budge, use a utility knife to cut through any paint or caulk where it touches the frame and try prying the top and bottom again. If it still won't come free, don't beat yourself up. Replace the angle brackets so it won't fall out later and go to the next step of cutting the jambs free from the opening. Cut through insulation, fasteners and shims with a reciprocating saw sporting an 8-in. bimetal blade (Photo 5). Most likely, the threshold will be glued to the floor with beads of sealant, and it may take substantial prying with a flat bar to free it. Then get a buddy to help lift out the frame along with the panel. Cut up the frame in chunks for disposal.

Step 3: Apply leakproof flashing

Before you begin flashing, check the floor for level. In rare situations, the floor under the door will be out of level. Sweep the floor free of debris and check it with a straight board and a 4-ft. level. If it's within 1/4 in. of level over the 6-ft. opening, let it be; the door should still slide smoothly. But correct larger variations with two long, tapered shims placed directly under the sill (use treated wood on concrete). Fill any voids with polyurethane caulk to keep out drafts. Then proceed with the flashing.

In general, the directions that come with the door will be fine for assembly and installation of the new unit. Unfortunately, they assume you're installing the door in a new wall that doesn't yet have exterior siding. Replacing an old door can be a bit more complex.

Begin flashing at the bottom. Apply flexible flashing directly under the doorsill and 2 in. up the sides (Photo 6). Flexible flashing comes in 4- to 6-in.-wide rolls and has a sticky side so it adheres to the underlying surfaces. It's thick and seals around fasteners that are driven through it. It's imperative that the flashing laps well over any deck flashing or weather barrier below the door (Photo 6). You may have to overlap two rows, as we did, to get the necessary coverage. Pros will usually rest the new doorsill (caulked) directly on the flashing, but for further protection in wet locations (rain will splash against the door bottom from all angles), add a “sill pan” as well (Photo 7). We chose a Jamsill Guard. It comes in three separate parts that glue together with PVC cement. Next apply flashing over any building paper, house wrap or sheathing along the sides and fold it around the door frame opening. We couldn't work the sticky flashing behind the vinyl trim, so we tucked No. 15 roofing felt about 2 in. under it instead.

Our slider was sheltered by a wide, low soffit, so we didn't have to worry about top flashing. But if your slider is unprotected, leave the top fin intact and slip it under the weather barrier (felt, house wrap, etc.) under the siding when you slide the new door frame into the opening. Then slip in a drip cap (usually included with the door) up under the weather barrier as well (Figure A). Follow this basic principle: Keep water flowing toward the exterior surface, just like shingles do.

Step 4: Install the new door

Installing your new slider is usually the easiest part of the job. Be aware that every manufacturer has slightly different weatherstripping systems, handle and lock hardware, and ways to fit the doors into openings, so they may not exactly match the ones shown in our photos. Read and follow the door instructions for those details.

If you have a “knockdown” (not preassembled) door frame, assemble it on the deck, garage floor or other flat surface. “Dry-fit” the assembled door frame in the opening to make sure everything fits (Photo 8), then rest it flat and put two beads of silicone caulk on the underside of the sill (where the directions call for it). Slip the door frame back into the opening and push the door fins tight against the sheathing (Figure A). (Line the fin groove up flush with the outside of the sheathing if you're not using fins.) Center the door frame in relation to the siding or trim. Otherwise it'll look bad from the outside. Then plumb, shim and screw the door in the opening (Photo 9) following the manufacturer's instructions. Be sure not to bow the jambs in or out when you drive the screws.

Follow the instructions to install the stationary panel first and then the slider. Tip the stationary panel into the opening and slide it within about 1/4 in. of the side jamb, and make sure the gap is even top to bottom. If it's more than 1/4 in. out of plumb or the jamb bows, adjust the frame for plumb and straightness and adjust the shims if necessary. Install angle clips, weatherstripping and trim as needed.

Then rest the sliding panel wheels on the tracks and tip the panel into the opening. Have a helper hold it in place while you screw in the head stop. Slide the door about 1/4 in. from the frame and check the gap from top to bottom. Raise or lower the rollers at the bottom. Adjust the rollers until the gap is even and the door rolls freely (Photo 11). Finish up by installing the handle and the locking hardware.

Step 5: Finish up on the inside

Standard slider doorjamb depths are designed to fit standard 4-9/16-in.-thick walls (2x4 studs plus 1/2-in. drywall and sheathing). If your walls are thicker, as in our case, you'll have to add extension jambs (Photo 12). Cut the top piece to fit first, then shim it and nail it in place. Then install the side extension jambs. Leave an even 1/8-in. reveal (backset) between the extension jamb and the doorjamb.

Insulate around the frame with fiberglass packed tightly against the exterior sheathing and more loosely near the drywall. (Foam insulation is a bit more difficult to use because even minimal expanding types can bow in jambs and affect weatherproofing or door operation.) Add interior trim around the door, leaving another 1/8-in. backset between the trim and the extension jambs.

Step 6: Finish up on the outside

You'll frequently be installing a door that's smaller than the one you removed. This will leave a wider gap that you'll have to fill with exterior trim. Match the new trim to the existing as much as possible. Whatever trim you choose, rip the trim slightly narrow so you leave 1/8-in. gaps on both sides. Seal these gaps with a high-quality acrylic or silicone caulk.

Finish up your installation by screwing or nailing a 3/4- to 1-in.-thick piece of cedar, treated wood or composite material directly under the overhanging lip of the threshold to support it (Photo 13). Some sliding door manufacturers offer a premade aluminum support strip as an alternative.