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BLD-CARP03-07 · Lesson 7 · 16 clock hours · lab

Installing Prehung Exterior Doors—Weathertight and Secure

Objective. Students install a prehung exterior door unit with sill-pan flashing, threshold, and weatherstripping that achieves plumb and level alignment, correct door reveal, and an airtight perimeter seal confirmed by visual inspection.

Why Exterior Door Installation Matters

An exterior door is the building envelope's most-used opening. A door installed out of plumb or level will rack in the frame, bind seasonally, and fail to seal against air and water. Gaps at the perimeter are one of the leading sources of building heat loss and water intrusion. Done right, a properly installed prehung exterior door delivers security, energy efficiency, and long service life.

Understanding the Prehung Unit

A prehung door arrives from the factory with the door already hung on hinges inside a three-sided jamb assembly (head jamb and two side jambs). The rough opening in the wall must be approximately 2 inches wider and 2 inches taller than the door slab to leave room for the jambs, shims, and clearance. Always read the manufacturer's rough-opening dimensions—do not guess.

Exterior insulated steel door units include a thermal break and often come with pre-applied compression weatherstripping around the jamb stops. Protect the unit from moisture during storage by keeping it off the ground and covered.

Sill-Pan Flashing First

Before the door goes in, the sill of the rough opening must be waterproofed. Water that gets past the threshold drains to this pan and is directed outward. Skip this step and water will rot the subfloor and framing over time.

1. Clean the sill of debris and dust.

2. Cut a piece of self-adhered membrane flashing wide enough to wrap up both sides of the rough opening by at least 6 inches and extend 2–3 inches onto the exterior sheathing.

3. Press the membrane firmly with a seam roller, working out air pockets.

4. Fold and adhere the corners—the material should lie flat with no fish-mouths or gaps.

Some manufacturers supply a rigid sill-pan product that slopes toward the exterior; follow their instructions.

Setting the Unit Plumb and Level

Set the prehung unit into the opening with a helper. Center it side to side so the reveal—the margin of jamb visible between the door edge and the jamb stop—will be equal on both sides. The door should swing freely before fastening.

Sill first. Place a 4-foot level across the sill of the door unit. Slide shims under the sill jamb until the sill reads level. Check both directions.

Hinge jamb second. Hold the 4-foot level against the hinge-side jamb. Drive shims behind the jamb at the hinge locations until it reads plumb. Check reveal against the door edge as you go.

Latch jamb third. Bring the strike-side jamb to plumb, matching the reveal on this side. Use a framing square at the head corners to confirm the head jamb is square to both side jambs.

Fasten through the jamb into the trimmer studs using the fastener type and spacing listed on the manufacturer's fastener schedule. Do not substitute longer or shorter fasteners—this affects pull-through strength and door warp resistance. Leave the shipping brackets and spreader intact until fastening is complete, then remove them.

Threshold and Sweep Adjustment

An adjustable threshold has a vinyl or rubber gasket on its top surface that contacts the door's bottom sweep when the door is closed. Raise the threshold high enough so the gasket compresses against the sweep along the full width of the door. The test: slide a piece of paper under the closed door—it should pull out with light resistance but not slide freely. If there is no resistance anywhere, raise the threshold. If the door binds and is hard to open, lower it.

Adjust the strike plate so the deadbolt bolt engages fully into the strike box without lifting or dropping the door. A bolt that only partially enters the strike is a security risk.

Air Sealing the Shim Cavity

The gap between the jamb and the framing (the shim cavity) is an air-leakage path. Fill it with low-expansion spray foam—not standard or high-expansion foam, which can bow the jambs and rack the door out of plumb.

Apply the foam in a single pass, filling approximately two-thirds of the cavity; it will expand to fill the rest. Let it cure fully before trimming. On the exterior, press a backer rod into the joint between the jamb/brick molding and the exterior sheathing or casing, then gun a continuous, tooled bead of paintable latex caulk over it. Work the caulk into full contact with both surfaces; inspect carefully for voids, especially at corners.

Safety on This Task

Inspection Criteria

Before calling the installation complete, verify every item:

All items are required to pass.

Key terms10

Prehung door unitA door slab already mounted on hinges within a factory-assembled jamb assembly, ready to drop into a rough opening.
Rough openingThe framed opening in the wall structure sized to accept the door unit plus shims; dimensions are specified by the door manufacturer.
Sill-pan flashingA waterproof membrane or rigid pan installed at the bottom of the rough opening to capture and drain any water that gets past the threshold to the exterior.
RevealThe uniform margin of visible jamb face between the edge of the door slab and the jamb stop; consistent reveal indicates the door is correctly centered and the jamb is plumb.
Adjustable thresholdA sill component with a height-adjustable gasket or ramp designed to compress against the door bottom sweep to create an airtight, weather-resistant seal.
Low-expansion spray foamA one-component polyurethane foam formulated to expand minimally so it fills the shim cavity without exerting enough pressure to bow or rack door jambs.
Backer rodA flexible foam rope inserted into a joint or gap before caulking to control caulk depth and improve the caulk's adhesion and tooling profile.
Fastener scheduleThe manufacturer's specification listing fastener type, length, and spacing required to fasten the jambs correctly; deviating from it can void the warranty and compromise performance.
ShimA thin, tapered piece of material (often cedar or composite) driven in pairs to fill gaps and bring jambs to plumb and level during installation.
Strike plateThe metal plate mortised into the door jamb that accepts the latch bolt and deadbolt; its position determines whether the bolt engages fully for security.

Worked examples4

1. The rough opening width measures 38 inches. The manufacturer states the rough opening should be 2 inches wider than the door slab. What door slab width does this rough opening accommodate, and how much total shim space is available per side?
  1. Door slab width = rough opening width − 2 inches = 38 − 2 = 36 inches (a standard 3-0 door).
  2. Total shim space across both sides = 38 − 36 = 2 inches.
  3. Shim space per side = 2 ÷ 2 = 1 inch per side.
Answer: The rough opening fits a 36-inch (3-0) door slab, with approximately 1 inch of shim/jamb space available on each side.
2. You place a 4-foot level against the hinge jamb and read a 3/16-inch gap at the top between the level and the jamb. The door height is 80 inches (6 feet 8 inches). Is the jamb within the 1/8-inch tolerance over its full height, and how much must it move?
  1. Tolerance allowed = 1/8 inch = 0.125 inch over full height.
  2. Gap measured = 3/16 inch = 0.1875 inch.
  3. 0.1875 inch > 0.125 inch, so the jamb is NOT within tolerance.
  4. Amount to correct = 0.1875 − 0.125 = 0.0625 inch (1/16 inch) at a minimum, but in practice add or remove shim material to bring the gap to 1/8 inch or less across the full 80-inch height.
Answer: The jamb fails the 1/8-inch tolerance. You must adjust shims to reduce the out-of-plumb reading by at least 1/16 inch so the total deviation is no more than 1/8 inch over 80 inches.
3. The exterior caulk joint between the brick molding and the sheathing is 1/2 inch wide. Caulk manufacturers recommend a bead depth equal to half the joint width for joints up to 1/2 inch. What depth of caulk bead should you achieve, and what is the purpose of the backer rod?
  1. Joint width = 1/2 inch.
  2. Recommended caulk depth = 1/2 × (1/2 inch) = 1/4 inch.
  3. Insert backer rod to set the bottom of the joint at 1/4 inch depth from the surface before applying caulk.
  4. Backer rod prevents caulk from bonding at three points (both sides and bottom), which would cause it to tear rather than flex with building movement.
Answer: Apply a caulk bead 1/4 inch deep. The backer rod sets that depth and ensures two-point adhesion so the caulk can stretch and compress without failing.
4. The rough opening height is 82-1/2 inches. The prehung unit's overall height (sill jamb bottom to head jamb top) is 81 inches. How much vertical adjustment space is available for shimming, and is there enough room to shim level if the subfloor varies by up to 1/2 inch across the sill width?
  1. Total vertical clearance = 82-1/2 − 81 = 1-1/2 inches.
  2. This 1-1/2 inches must accommodate the sill shim pack plus any head clearance.
  3. If the subfloor varies 1/2 inch across the sill, shimming the low side up 1/2 inch still leaves 1-1/2 − 1/2 = 1 inch of remaining space, which is sufficient.
  4. Verify after leveling that the top of the unit still clears the header before fastening.
Answer: There is 1-1/2 inches of vertical clearance. A 1/2-inch floor variation is manageable—shim the low end up 1/2 inch, confirm the head clears the header, and then fasten.
In class: Apply sill-pan flashing to a prepared exterior-wall rough opening, set the prehung unit, and shim the sill level before shimming jambs plumb; install all fasteners through jambs per manufacturer fastener schedule. · Fit and fasten an adjustable threshold to achieve full compression contact with the door bottom sweep without binding the door swing; adjust door strike plate so the deadbolt bolt engages fully. · Apply low-expansion spray foam to the shim cavity, then cap with backer rod and gun a continuous bead of paintable latex caulk around the exterior perimeter; inspect for voids.
Materials: Prehung exterior insulated steel door unit; sill-pan flashing; self-adhered membrane; shims; 4-foot level; framing square; tape measure; screw gun; fasteners per manufacturer spec; adjustable threshold; low-expansion spray foam; backer rod; caulking gun; paintable latex caulk; deadbolt lockset
Check: Door is plumb to within 1/8 inch over full height; reveal is consistent; threshold sweep compresses visibly on a piece of paper pulled under it; foam and caulk perimeter shows no voids; all criteria required to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Carpentry — Door and Window Installation
1. What is the primary reason low-expansion spray foam must be used in the shim cavity rather than standard or high-expansion foam?
NCCER Carpentry — Door and Window Installation
2. You are installing a prehung exterior door. Which sequence of shimming steps is correct?
NCCER Carpentry — Door and Window Installation
3. What does the paper-pull test verify during threshold adjustment?
OSHA 10 — Personal Protective Equipment / Hazard Communication
4. Which personal protective equipment (PPE) is most important when applying low-expansion spray foam to the shim cavity?
NCCER Carpentry — Building Materials, Fasteners, and Adhesives
5. Why must you follow the manufacturer's fastener schedule when securing the door jambs rather than selecting fasteners by general practice?
NCCER Carpentry — Door and Window Installation
6. During a final inspection, you notice the door reveal is 1/8 inch on the hinge side but 3/8 inch on the latch side from top to bottom. What is the most likely cause?
NCCER Carpentry — Exterior Finishing and Weatherproofing
7. What is the function of sill-pan flashing installed before setting a prehung exterior door unit?
NCCER Carpentry — Door and Window Installation
8. A fully engaged deadbolt is critical for door security. Which adjustment corrects a deadbolt that extends but does not enter the strike box cleanly, requiring the door to be lifted slightly to latch?
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