How To Replace an Exterior Door

By Robert Robillard on Exterior Repairs

Replacing an Exterior Door

Replacing an existing exterior door with a new pre-hung door is a fairly straightforward task. In most cases you measure the old door and try to get a new door the same size.

How To Replace an Exterior Door

When measuring and ordering a new door there are four things you need to know:

1. Door size:

Measure the width and height of your old door. Round these up to full inches to find the size of the replacement door you’ll need. If, for example, your door measures 35-3/4 in. wide and 79-1/2 in. tall, you’ll order a 36-in. by 80-in. door.

2. Jamb width:

Measure the existing door jamb. To do this, measure from the backside of the interior trim to the backside of the exterior trim You will want to match this width with your new door. This measurement ensures that the interior trim will fit flush to the wall without adding “jamb extensions.”

3. Rough opening:

This is the distance between framing stud to framing stud and rough sub-floor to the top header. Remove the interior trim to get accurate measurements for the rough opening.

4. Door Swing:

The easiest way to determine which way a door swings is to put your backside against the hinge jamb and swing your arm to follow the door open. the door is either a left swing or “left hand” swing door or a “right hand” swing door.

5. Trim Measurements:

When ordering a door you need to consider the exterior trim style and size. sometimes I will measure for and specify the width from exterior trim edge to edge and bottom exterior sill to top trim edge. I do this when the siding may make it necessary to have the trim exact. this measurement can sometimes be accomplished by the door company when they assemble the door.

The alternative to this is to order the door without trim and then apply it after the door is installed.


How To Replace an Exterior Door
Photo: Southern gentleman, Baker and Finish Carpenter George Gussler
Nothing was standard about this wall opening, floor or trim. so I measured for the door and jamb size and filled in the rest.

How To Replace an Exterior Door

Pre-hung doors need to sit on a level surface. The first step is to make sure the threshold sub floor is level or to shim it level if it’s not.

How To Replace an Exterior Door

The door I ordered had a larger jamb size to accommodate for the lath and plaster in this old house.

How To Replace an Exterior Door

Rubber Vycor flashing protects the sill and edges from the weather. Flashing extends behind the granite step.

How To Replace an Exterior Door

The door was plumbed in the opening and installed along the outer edge or the house sheathing. Shims were installed at the hinge locations and similar locations on the latch side. I also add solid shimming near the deadbolt.

The door was secured to the shims with galvanized 16 penny finish nails. We try to hide most of the nails behind the weatherstripping. Others get filled.
Air sealing with minimal expanding foam is next. We use the foam to seal all around the doors edge.

How To Replace an Exterior Door

Exterior trim was then added. We used pvc for the bottom piece under the threshold for two reasons; one it resists rot and two it resists takes a beating, unlike wood, from people kicking off their boots in the winter.

How To Replace an Exterior Door
The install was finished by adding a Larson storm screen combination door.

How To Replace an Exterior Door

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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzz As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising the bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob

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