Prehung Exterior Door Installation overview
Prehung exterior door
Pre-hung doors simplify installation.
It’s not often that you can complete a project in a weekend that will save you money and dramatically improve the looks of your house. In this article, we’ll show you how to order a prehung exterior door that’ll fit like a glove. Then we’ll show you, step by step, how to get your old door out and the new one in.
If you can handle basic carpentry tasks, you’ll have no trouble installing a new prehung entry door in a day. Figure on another day to finish the details and start painting the door. You can complete most of the job with basic hand tools. You’ll need a hammer, pry bar, tape measure, level, utility knife, nail set and saw. If you decide to install new interior trim, you’ll also need a miter saw.
We ordered a top-quality prehung wood door with special 2-1/2-in.-wide exterior trim (casing) (Simpson Bungalow No. 7228) from a local lumberyard. We could have purchased a steel or fiberglass door for much less, but the style and crisp detailing of the wood door matched the house perfectly and we couldn’t resist. The door arrived about two weeks after we ordered it.
Video: How to Hang a Door
Measure your old door
Figure A: Measuring
Measure carefully for a perfect fit.
In most cases, simply order a new door the same size as the old one. If you alter the size or add sidelights, you’ll have to reframe the opening and alter many details. This usually doubles or triples the size of the job. Here are the four sets of measurements you’ll need to order a door (refer to Figure A, for extra details):
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.
Jamb width. Measure from the backside of the interior trim to the backside of the exterior trim (Figure A). Specify this jamb width when you order your new prehung door. This guarantees that the interior trim will fit flush to the wall without adding “jamb extensions.”
Rough opening. You’ll have to remove the interior trim to get accurate measurements of the rough opening. Measure the opening width between framing members and from the bottom of the sill to the top of the opening. Compare these measurements to the rough opening requirements of your new door to make sure it will fit.
Exterior opening (or “masonry opening” if you have a brick or stone door surround). Measure to the outsides of the exterior casing and then from the bottom of the sill to the top of the trim. Compare these measurements with those of a prehung door that has standard 2-in.-wide “brick molding” trim. If the framed door with standard trim is too small to completely fill the space or if you want a different trim style, you have three options. The best solution is to order a door with wider, flat casing to fit the opening. You can always add a piece of decorative molding overtop to approximate the style of your existing exterior trim. (We ordered 2-1/-n. flat casing and reinstalled the existing decorative molding.) Second, you can order your door with standard molding and fill the gap with additional strips of wood. The last option is to order the door without exterior molding and make your own to fit.
Tear out your old door
Photo 1: Remove the old door
Tap the hinge pins loose with a hammer and nail set. Then swing the door open and lift it off. Protect the floor with a dropcloth. The old door will be heavy.
Photo 2: Pry loose the old trim
Pry the interior trim loose from the door frame. Protect the wall with a wide putty knife. If you plan to reuse the trim, first score the intersection between the molding and jamb with a utility knife.
Photo 3: Pry off the exterior trim
Slice the caulk joint between the siding (brick) and exterior trim and pry the trim from the doorjamb with a pry bar.
Photo 4: Remove the jambs
Cut completely through the side jamb with a handsaw. Pry the jambs loose and pull them out of the opening.
Jamb removal close-up
Photos 1 – 4 show how to take out the old door and frame. If you plan to reuse the interior moldings, pull the nails through the backside with a pliers or nipper to avoid damaging the face. Cutting through one side jamb makes it easy to tear out the entire frame (Photo 4).
After the door frame is out, check the condition of the framing and subflooring in the sill area. Cut out and replace any rotted wood. If the sill on your new door is thinner than the one you removed, you may have to build up the sill area as shown in Photo 5. Set the sill height so the door just clears carpeting or rugs when it swings inward.
Photo 6 shows how to protect the sill from water intrusion. Buy the flashing tape from lumberyards. If you’re installing a door in a newly constructed wall, you can buy a special plastic sill flashing kit instead. One brand is Jamsil. Details will vary depending on the doorway situation. The idea is to channel water away from the wood. If your home is built on a concrete slab, the door frame will probably rest directly on the slab.
If your door is exposed to the weather, direct water away from the door with a metal drip cap overtop (Figure C). Brick openings like ours and doors protected by porches with roofs don’t require a drip cap. You’ll find drip caps at home centers and lumberyards. If the drip cap is damaged or missing, install a new one before you set the door frame in the opening. Cut the metal drip cap to fit and slide it under the siding and building paper (Figure C). If nails are in the way, slip a hacksaw blade under the siding and cut them.
Prepare the sill
Photo 5: Install the sill
Build up the sill area to the proper height with treated lumber. Add shims to level it. Fasten it with coated deck screws.
Photo 6: Install flashing tape
Cover the rough sill area with self-sticking flashing tape. Wrap it up the sides of the opening and over the front edge. Set the door in the opening, plumb it and check the fit.
Figure B: Sill detail
We were lucky. Our brick opening was level and plumb, but this isn’t always the case. Start by checking the sill area with a 2-ft. level. If you’re building it up as we show in Photo 5, it’s easy to level it with shims at the same time. Otherwise, level the sill area with pairs of shims spaced about 4 in. apart.
Then set the door in the opening for a test fit. Hold a level against the hinge jamb and adjust the door and frame until the jamb is plumb. Check to see how the casing fits against the siding. If the siding is so far out of plumb that the door frame and casing don’t fit in, you either cut back the siding or trim the casing. It looks better if you can cut the siding, but it’s usually more practical and easier to trim the casing. Mark the casing in areas that need trimming. Then take the door out and trim the casing with a belt sander or circular saw.
Set the prehung exterior door in the opening
Photo 7: Apply caulk
Apply a bead of polyurethane caulk along the sides and top of the door opening and at the sill according to the manufacturer’s instructions. You’ll likely need two tubes.
Photo 8: Tack in the prehung exterior door
Make sure the doorsill is level. Then center the top of the door in the opening and tack it into place with galvanized casing nails. Plumb the hinge-side jamb and tack the bottom corners.
Photo 9: Shim the hinge-side jamb
Shim behind each hinge. For large spaces, start with small squares of plywood. Then finish with pairs of shims. Make sure the hinge-side jamb remains plumb.
Photo 10: Shim the latch-side jamb
Shim at the top, middle and bottom of the latch-side jamb and at the top until the gap between the door and the doorjamb is consistent. Score the shims with a utility knife and break them off.
Photo 11: Replace the hinge screws
Replace a screw in each hinge with a 3-in. screw driven into the framing. Drive additional casing nails every 16 in. along the sides and top of the exterior trim.
Figure C: Drip cap
Make sure the building paper is intact around the frame edges. If not, slide strips of No. 15 felt behind the siding and tack it to the framing with staples. When you’re sure the door will fit, caulk along the sill and behind the casing and tip the door into the opening. Photos 8 – 10 show how to shim and nail the door. The goal is to center the door in the opening and shim the sides until they’re plumb and straight. Adjust the pairs of shims until the gap between the door and the jamb is consistent on the sides and top of the door. When you’re happy with the fit, nail through the jamb into the framing at each shim location. Then replace one screw closest to the inside in each hinge with one long enough to reach the framing. This will keep the door from sagging over time (Photo 11).
Insulate, caulk and install the trim
Photo 12: Insulate around the door
Fill the space between the doorjamb and the framing with minimal expanding foam insulation (you’ll need about two cans). After the foam has expanded and skinned over, loosely stuff any remaining space with strips of fiberglass insulation.
Photo 13: Install interior trim
Cut and install new interior trim or reinstall the old trim. If there’s a gap between the new sill and the existing flooring, cover it with a beveled transition.
Photo 14: Apply caulk backer and caulk
Press foam backer rod into the siding/trim gap. Apply a neat bead of caulk between the siding and the door trim. Cut a trim board to fit under the sill and screw it to the framing
After insulating the space around the door (Photo 12), install the interior trim. Photo 13 shows how to cover a gap between the doorsill and flooring. Complete the job by caulking the exterior (Photo 14). For gaps wider than 3/16 in., insert a foam backer (available at home centers, hardware stores and lumberyards) and apply caulk over it. Most doors require an additional trim board under the sill to support its outer edge. Finally, remove the door and paint or stain and varnish the door, jamb and trim.
Editor’s Note: Buying a New Prehung Exterior Door
Most home centers stock prehung exterior doors in a limited number of styles. Common jamb widths for stock doors are 4-9/16 in. and 6-9/16 in., and they usually include 2-in. wide brick molding for exterior trim. These doors work great for newly constructed walls and for replacing doors in newer homes. But if, after measuring the jamb width and opening sizes for your existing door, you discover that you need a different size jamb or that your exterior trim is wider, then you’ll save a lot of headaches by ordering a door to your exact specifications.
There are three types of doors to choose from. Steel doors are popular because they’re inexpensive and require little maintenance. Fiberglass doors won’t warp or rot, and the more expensive models are hard to tell apart from real wood. You’ll find the widest selection of styles in wood doors, but be prepared to spend extra time maintaining the finish.
Prehung exterior doors are available at home centers and lumberyards. If you can’t find what you’re looking for in stock, you can order it. Take along your measurements and a sketch showing which way the door swings.
If you plan to install a new entry knob and deadbolt, pick them out before you order the door. Then ask the salesperson to have the door drilled to accept your hardware. It’ll cost a little extra, but it’s well worth it to avoid the nerve-racking job of drilling into a new door. Don’t forget to order hinges that will match the finish of your hardware. Get some ideas for a new door after looking at this stunning door options.
Required Tools for this Front Door Replacement Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- 2-ft. and 4-ft. levels
- Caulk gun
- Miter saw
- Pry bar
- Reciprocating saw
Required Materials for this Front Door Replacement Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- 10d finish nails
- 10d galvanized casing nails
- 16d galvanized casing nails
- 3-in. deck screws
- 4d finish nails
- 6d finish nails
- Backer rod
- Minimal expanding foam
- Polyurethane caulk
- Roll of flexible self-sticking flashing membrane
- Transition molding Transition molding (optional; Photo 13)
- Treated lumber to build up the sill (optional)