The side service door on most attached garages was installed in the 1970s or 1980s, and it shows. Warped jambs, a wobbly knob, and gaps where weather strip used to be turn the door into the weak point of your whole house envelope. A proper garage side door replacement runs $250 to $475 if you DIY a prehung steel unit from Lowe’s or Home Depot, and $850 to $1,400 installed by a carpenter. The decision often comes down to whether the existing frame is rotted, which most homeowners cannot tell until they pry off the trim.
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Code Requirements That Apply
The IRC R302.5.1 requires the door between an attached garage and the dwelling to be a 20-minute fire-rated door at minimum, or a solid wood door not less than 1 3/8 inches thick, or a 20-minute fire-rated steel door. A side exterior door (one that opens to the yard, not into the house) does not have to be fire-rated but must meet local energy code, typically a U-factor of 0.30 to 0.40 depending on climate zone per IECC tables.
Most jurisdictions require a permit for door replacements that change the rough opening size but not for like-for-like swaps. Check your building department. Skip the permit and you risk a flag at resale during the home inspection.
Door Material Comparison
Steel Prehung Doors ($180 to $450)
Steel is the most common pick. A Masonite or Therma-Tru 32 by 80 inch steel prehung with brick mold and bored knob hole runs $189 to $349 at Home Depot. Insulated cores give U-factors around 0.20, which exceeds code in every climate zone. Dents in steel show easily, so add a kick plate at $25 to $40 if you push a lawnmower through the door.
Fiberglass Prehung Doors ($350 to $850)
Fiberglass resists dents and never rots. Therma-Tru Smooth-Star or Pella Encompass lines run $379 to $649 with similar U-factors to steel. The smooth or wood-grain surface paints easily and lasts 20 to 30 years with one repaint cycle.
Wood Doors ($450 to $1,400)
Solid wood doors look traditional but require more upkeep. Plan on refinishing every 3 to 5 years on south or west-facing exposures. Skip wood unless the door sits under a covered porch or your HOA mandates a specific look.
How to Tell If You Need a New Frame
Pull the interior trim with a flat bar. If you see crumbly wood, dark stains, or carpenter ant frass at the bottom of the jambs, you need a full prehung unit. If the jambs are sound, you can install a slab door for $90 to $180 and keep your existing frame.
Press a screwdriver tip into the jamb at the threshold and 6 inches up. Soft wood means rot. Tap with a hammer; a dull thud where the rest of the jamb rings indicates hidden damage. About 60 percent of garage side doors over 20 years old have at least some frame rot, especially on north-facing exposures where snow piles up.
Tools and Materials for DIY Install
- Flat pry bar, claw hammer, utility knife
- Cordless drill with #2 phillips and 5/32 inch bit
- 4-foot level and 2-foot level
- 3 inch and 1.5 inch wood screws (stainless or hot-dipped galvanized)
- Cedar or composite shims (one bundle)
- 10-pack of #8 by 16d finish nails
- Low-expansion door foam (Great Stuff Pro Window and Door, blue can)
- Backer rod and silicone caulk for the exterior
- Z-flashing for above the door (aluminum or vinyl, 36 inches)
Step-by-Step DIY Replacement
Score the interior caulk seam with a utility knife. Pry off interior casing in 2 to 3 foot sections. Remove the hinge pins, then lift the old slab out. From outside, score the exterior caulk and pry the brick mold. Cut nails or screws holding the frame to the rough opening with a Sawzall fitted with a metal-cutting blade.
Lift the old frame out, vacuum debris, and inspect the sill. If the sill plate is rotted, splice in pressure-treated 2×6 before setting the new unit. Slide the new prehung into the opening from outside. Shim the hinge side first at the top, middle, and bottom hinges. Check plumb with the 4-foot level. Drive 3 inch screws through the hinge-side jamb into the king stud. Repeat on the strike side, checking the door swings free and the gap is uniform at 1/8 inch top, sides, and threshold.
Sealing and Weatherproofing
Apply Z-flashing above the head, slide under existing siding or trim. Caulk the exterior brick mold to the siding with a 50-year polyurethane sealant like OSI Quad. Fill the gap between the frame and rough opening with low-expansion door foam, not standard expanding foam, which bows the jamb and causes the door to bind.
Install the threshold sweep adjustment per the manufacturer. Most thresholds have 4 to 6 hex-head screws that raise or lower the seal. Set the sweep so a sheet of paper drags lightly when pulled out from under the closed door.
Security Hardware and Locks
Replace the strike plate with a 3 inch model and use 3 inch screws into the king stud, not the jamb alone. A standard 1 inch screw in a 3/4 inch jamb pops out with a single kick. Add a Schlage B60N or Kwikset 980 deadbolt at $35 to $65. ANSI Grade 2 minimum. Skip the cheap entry knobs sold as a package; they fail within 5 years on a service door.
If the garage door connects to the house through a man door inside, that is a separate fire-rated door governed by R302.5.1. A self-closing or self-latching device is required in most jurisdictions starting around 2021 code updates.
Cost Summary and Timing
A DIY weekend job for a prehung steel door totals $290 to $440 in materials (door, fasteners, foam, caulk, hardware). A pro carpenter charges $450 to $800 in labor on top of the door cost, with the whole job done in 4 to 6 hours. Add $200 to $500 if the rough opening needs repair from rot. The completed garage side door replacement should give 20 to 25 years of service if installed plumb, sealed properly, and protected from direct rainfall.
