Exterior Door Installation: The Step-by-Step Guide

A door that sticks every August, whistles in January, or grows a mysterious floor stain by March almost always failed at installation, not manufacture. Exterior door installation is really a waterproofing job with a door in the middle of it — the hanging part takes an hour, but the sill pan, flashing, and shimming determine whether the unit lasts thirty years or rots its own subfloor in five. This guide walks the full process for a prehung unit, the type virtually all replacements should use (see our prehung exterior door guide for why), with the checkpoints that separate a weathertight install from a callback.

Tools, Materials, and Time

• Tools: 4-foot and torpedo levels, drill/driver, pry bar, reciprocating saw or hacksaw blade (for old fasteners), utility knife, caulk gun, hammer, tape measure
• Materials: prehung door unit, sill pan kit or flexible flashing tape ($20 to $40), cedar or composite shims, 3-inch exterior screws, low-expansion window/door foam, exterior sealant (polyurethane or high-quality hybrid), backer rod
• Time: 4 to 8 hours for a like-for-like swap; a full day if the opening needs repair

Plan for weather — the opening will be a hole in your house for a few hours — and recruit a second set of hands for lifting; a fiberglass unit with glass runs 80 to 130 pounds.

Step 1: Measure and Order Right

Pull the interior casing and measure the actual rough opening: studs to studs, header to subfloor. A prehung unit wants a rough opening about 2 inches wider and 2 to 2-1/2 inches taller than the slab — 38 x 82-1/2 inches for a standard 36×80 exterior door. Confirm jamb depth (4-9/16 inches for 2×4 walls, 6-9/16 for 2×6) and the hand: stand outside; hinges on the right is a right-hand inswing. Wrong-hand doors are the industry’s most common return that cannot be returned.

Step 2: Remove the Old Door and Frame

Pop the hinge pins and remove the slab, then cut the old frame free — a reciprocating saw run between jamb and stud slices nails and screws fast, and the jamb legs pry out in pieces. Take the old sill out carefully and inspect what is under it. Soft, dark, or crumbly subfloor gets cut back and replaced now; setting a new door over rot just gift-wraps the problem. Dry-fit the new unit in the opening to confirm fit before any sealant comes out.

Step 3: Build the Sill Pan — the Step That Matters Most

Every opening eventually lets some water past the door, and the sill pan is what routes that water back outside instead of into the floor framing. Use a rigid pan kit (Jamsill Guard, SureSill, $25 to $40) or form one from flexible flashing tape run across the sill and 6 inches up each jamb leg, back-dammed at the interior edge, with corners lapped shingle-style. Slope matters: the subfloor should pitch slightly toward the exterior; a few strategic shims or a bead of sloped sealant under the pan achieves it. Then run two heavy beads of sealant across the pan where the door sill will land — but leave the pan’s front drainage edge open. Never caulk the front of a sill pan shut.

Step 4: Set, Plumb, and Shim the Unit

Stand the unit into the opening from outside, pressing the brickmold or nailing flange into a bead of sealant. Now the sequence pros follow:

1. Hinge side first. Shim behind each hinge location, get the hinge jamb dead plumb in both directions with a 4-foot level, and tack it with 3-inch screws through the shims.
2. Check the head gap. Close the door and read the reveal — the gap between slab and frame — across the top and down the latch side. It should be an even 1/8 inch, the thickness of a nickel, all the way around.
3. Shim the latch side at the strike height plus top and bottom, adjusting until the reveal is uniform and the door closes without rubbing.
4. Replace two screws in each hinge with 3-inch screws driven through shims into the stud. This carries the door’s weight to framing and stops the sag that plagues screen-tightened installs.

Test relentlessly during this step: the door should swing freely, stay put at any position (a door that self-swings means the frame is out of plumb), and latch without lifting or slamming.

Step 5: Insulate, Flash, and Seal

Fill the shim gap between frame and framing with low-expansion door-and-window foam — standard gap-filler foam bows jambs inward and binds the door. Outside, tape the jamb flanges or brickmold to the weather barrier shingle-style: sides first, head flashing last so it laps over the side pieces, and install a metal or vinyl drip cap above the brickmold if the siding does not already provide one. Caulk the sides and top of the exterior casing joint with polyurethane sealant over backer rod where gaps exceed 1/4 inch. Leave the bottom edge uncaulked for drainage. Inside, seal the jamb-to-drywall joint, reinstall or replace casing, and adjust the sill cap (most modern sills have screws that raise the threshold cap) until a dollar bill dragged under the closed door shows light, even resistance.

Step 6: Hardware and Final Adjustments

Install the lockset and deadbolt, and swap the strike plate screws for 3-inch versions into the stud — the cheapest security upgrade in the building. Check weatherstripping contact at all points with the dollar-bill test; kerf-mounted weatherstrip can be nudged in or out of its slot to tune contact. Prime and paint or stain all six sides of a wood or fiberglass door promptly, including top and bottom edges; unsealed edges are how steel doors rust from the inside and wood doors drink moisture and warp.

The Five Mistakes Behind Most Failed Installs

• No sill pan. The number-one cause of rotted subfloor under doors. Caulk under the sill is not a pan; it fails in a few seasons and then traps the water it was meant to block.
• Shimming to the wall instead of plumb. If the wall itself leans, a door shimmed flush to it leans too, and it will swing open or shut on its own forever. Trust the level, not the framing.
• Short screws. Factory hinge screws only reach the jamb. Without 3-inch screws into the studs, the hinge side sags within a year and the latch starts missing the strike.
• High-expansion foam. Standard gap foam generates enough pressure to bow a jamb 1/8 inch inward — precisely enough to bind the door. Only use cans labeled for windows and doors.
• Backwards flashing laps. Every layer must shed onto the layer below it, outside-in: pan first, then side tapes over the pan legs, then head flashing over the sides. Reverse any lap and you have built a funnel into the wall.

Weather, Timing, and Old-House Curveballs

Swap doors in dry weather above about 40°F when possible — sealants and flashing tapes bond poorly on cold, damp surfaces, and foam cures badly below freezing. In older homes, budget mental margin for what the tear-out reveals: out-of-square openings (fixable with shims up to about 3/8 inch of racking, beyond that you reframe), no header over the door in very old balloon framing, or previous “repairs” of stacked caulk hiding rot. If the opening needs more than an hour of correction, stop and cover it — a properly tarped opening overnight beats a badly rushed install by daylight. Finally, order the door before demolishing anything. It sounds obvious, and every lumberyard has stories about the customer standing in a doorless house waiting three weeks on a special order.

DIY or Hire It Out?

Contractors charge $250 to $700 labor for a straightforward prehung swap, $1,000 to $2,500-plus when the opening needs resizing, structural repair, or new siding integration. The DIY line is honest self-assessment on two skills: reading a level and sequencing flashing tape. Get the flashing shingle logic and the even-reveal shimming right and a first-timer can produce a professional result in a day. Rush the sill pan and the most expensive door on the shelf will still rot the framing around it — slow down exactly there, and the rest of the install is just carpentry.