A carpenter is replacing an existing exterior door. After removing the old door and cleaning the rough opening, they discover the rough opening is slightly out of square and 10 mm (3/8 inch) wider than the new door frame on one side. What is the best immediate course of action before installing the new door?

Question: A carpenter is replacing an existing exterior door. After removing the old door and cleaning the rough opening, they discover the rough opening is slightly out of square and 10 mm (3/8 inch) wider than the new door frame on one side. What is the best immediate course of action before installing the new door?

Answer options:

• Install the door as is and rely on shims to fill the gap.
• Trim down the new door frame by 10 mm to fit the opening. ✅ Add a filler strip or adjust the framing to ensure the rough opening is plumb, level, and square.
• Pack the gap with expanding foam and then install the door.

Correct answer: Add a filler strip or adjust the framing to ensure the rough opening is plumb, level, and square.

Explanation: Before installing a new door, the rough opening must be plumb, level, and square. Adding a filler strip or adjusting the framing ensures the door can be properly shimmed and fastened without compromising its operation or seal. Relying solely on shims for a large gap or cutting the door frame are poor practices, and foam is for insulation, not structural shimming.