15 Easy Steps To Install A Prehung Exterior Door

Have you ever considered installing your door personally but concluded that it's a daunting task that should be left to professional carpenters? Then, you're probably wrong, read to the end of this article to find out why.

Most people often think that installing a door is complex and requires some technical skills. This impression will often arise if you have never learned or dared to do a basic prehung door installation yourself, and it is regardless of the kind of door involved, whether it is a French door, double door, barn door, or hinged door.

Changing your exterior door can go a long way to enhance the aesthetics of your home, provide security and improve the curb appeal. Plus, choosing a prehung door makes your life a lot easier as the prehung door comes along with an already mounted frame, and you can comfortably install the whole thing at once. Replacing your old drafty door with an elegantly new efficient one is quite an easy task and fun to do if you know the appropriate measures to be taken. Besides, you'll only need some spare time and a couple of tools and materials to get started.

It will intrigue you to know that you don't even need to be a master carpenter to do an excellent job! So if you are currently running a little project in your house and intend to save some bucks, learning to install a prehung exterior door is a good head start. In this article, you'll be learning 15 simple steps to installing your first DIY prehung exterior door. Exciting, right?  So if you are ready, join me, and let's ride!

What Basic Tools Will You Need?

door install tools

Caulk gun, Nail set, Pry bar, Chisel, Hammer, Measuring Tape, Utility knife, Reciprocating saw, Spirit level, Drill.

What Materials Will You Need?

Paintable caulk, Piece of wood, Finish nails, Weather-stripping, Extra insulation, Foam insulation, Anchor screws, Latch jamb, cedar shim, drill bits, 3/16-inch twist drill bit, 1/4-inch drill bit.

Part A: Uninstall Your Old Door and Frame

1. Measure the Old Door

prehung door measurements

It is worthy of note that, when replacing an existing prehung door with a new one, you are to ensure that the new door is of equivalent size to the existing one. Any slightest alteration in size implies that you'd have to rework the opening and make many other adjustments to the length and width of the door, which tends to make the work tedious. There are four essential measurements that you need to take, and these include

 Door Dimensions

  • Make measurements of the height and width of the existing door.
  • Round up the figures to whole inches to arrive at the actual size for the new door to be acquired.
  • By way of illustration, after taking measurements, supposing your results were 81.1/2 for the height and 30-3/4 for the width, it implies that you'd order a 30-in. by 81-in. prehung door.

 Jamb length Measurement

  • The jamb width is the distance between the interior and exterior trim; ensure accurate measurements of the hinge-side Jamb.
  • You are to stipulate the jamb width obtained when ordering for the replacement prehung exterior door; this helps ensure a perfect fitting of the wall and the inner trim without further jamb extensions.

Measurements of the Rough Opening

  • Remove the interior trim before taking the rough opening measurements; this allows you to obtain accurate results.
  • Take measurements of the opening length between the left and right frames. Also, determine the vertical height from the top of the opening to the sill at the base.
  • Compare the values you obtain to that of the requirements for the rough opening of the new door and ensure that it fits; otherwise, you might need to return it for a perfect match.

Measurements of the Exterior Opening

The exterior opening is also referred to as a masonry opening when a brick wall is involved.

  • Take measurements of the exterior opening by extending the measuring tape to the ends of the external casing. Also, measure the elevation from the top of the trim to the base of the sill.
  • Match the values you obtain with a standard prehung door having brick molding trim (two inches in width).
  • Supposing the standard prehung door is smaller in size, or you desire a trim with unique style, you can apply any of these three options:
  • Firstly, you can order a prehung exterior door with a broader casing to match the rough opening. Additionally, you may incorporate a piece of fancy molding to match the existing style of the old exterior trim.
  • Secondly, you can purchase a door with regular molding, then cover up the opening with supplemental wood strips.
  • Lastly, you can purchase a door lacking exterior molding and adjust yours to match.

2. Take off the Old Door

Take off prehung door

  • Use your hammer to knock off the hinge pins to disengage the door hinges.
  • Then gradually grab the door with both hands and remove it from the alignment. You might need someone to assist you if the door is too heavy.
  • Use a tarpaulin or sailcloth on the floor while removing the door to prevent any damages or scratches on the floor.

3. Loosen the Existing Interior Trim

Detach and lose the inner trim

  • Detach the inner trim from the framing using a pry bar or chisel.
  • Ensure to safeguard the wall while detaching by using a broad putty knife.
  • If you intend to use the trim again, ensure to scribe the point of intersection between the Jamb and the molding with your utility knife.

4. Unloose the Existing Exterior Trim

disengage the exterior trim

  • Score the caulk-joining between the exterior trim and the siding (wall or brick)
  • Using your pry bar, disengage the exterior trim from the door frame.

5. Take Off the Jamb

Disengage the jambs

  • Using a handsaw, cut off the side jamb thoroughly; this helps to weaken the Jamb, making it easier to pull out the frame completely.
  • Disengage the jambs and remove them from the opening.
  • If you want to use the interior moldings again, you'll need to carefully pull up the nails from the rear end with pliers to avoid damages to the surface.

Part B: Install the New door and Frame

6. Get the Sill in Place

Install the Sill

  • After removing the old door frame, check the state of the entire sill subflooring.
  • Cut off and replace all rotted wood.
  • Supposing the sill on the replacement door is lower than the one you pull out, you might need to build up the sill region.
  • If you intend to build up the sill area, first use the carpenters' level to check that it is horizontal; you can also level the sill with shims simultaneously, as shown in the image below.
  • Raise the sill region to the appropriate height using treated lumber and level it with pairs of shims.
  • While leveling the sill region with shims, ensure that the shim pairs are spaced apart, about 4 inches.
  • Set up the sill height such that the door can swing inwards without damaging the carpet or rug on the floor.
  • Then set the door for test fitting by gently swinging it open. Finally, use a spirit level to modify the Frame and the door to achieve a perfectly vertical jamb.
  • Examine the casing and ensure that it fits with the siding. Supposing the siding doesn't align and the casing and door frame doesn't tally, you will either have to prune the casing or trim down the siding. It's okay to cut the siding; however, it is typically more convenient and comfortable to prune the casing.
  • To carry out the pruning, first mark out the casing areas that need to be pruned.
  • Next, cut down the casing using a circular saw or belt sander.
  • Then, secure it by using coated deck screws; once the jamb levels.

7. Apply Flashing Tape

Install Flashing Tape

  • You'll need to apply flashing tape to the sill region as this helps to prevent water encroachment on the sill.
  • Supposing you are installing your prehung door on a freshly-built wall, it is advisable to use an exceptional plastic sill flashing kit instead.
  • Cover the sill region with the flashing tape and wrap it all over the sides and edges.
  • Test fit the door again, and check for plumb to verify that it fits with the installed flashing tape.

8. Apply Caulk

Apply Caulk

  • In applying the beads of polyurethane caulk, first, ensure that the building paper aligns with the edges of the Frame. Otherwise, place strides of No. 15 perforated felt on the sides and affix it to the frame with staples.
  • Once you ascertain that the door is well fitted, you can begin applying beads of caulk at the back of the casing as well as along the sill.
  • Apply beads of caulk on the top and sides of the door frame as well as the sill area; ensure to follow the manufacturer's guide.
  • You'd probably need to get two tubes of the caulk.

9. Fix In the Exterior Door

Tack in the Prehung Exterior Door

  • First, you need to ensure that the sill is level.
  • Lift the door towards the hinge area such that the door aligns with the hinges. You might need a helping hand if the door is too heavy for you.
  • Tack in the hinges at the top, middle, and bottom of the door.
  • Use your hammer to tap the hinges to fit in.
  • Check the Jamb for plumb.

10. Apply Shims to the Jamb on the Hinge Side

Shim the Hinge-Side Jamb

  • Apply shims behind every hinge.
  • When dealing with wide spaces, you should begin with little plywood squares and then apply composite shims to finish.
  • Ensure that the Jamb is plumb.

11. Apply Shims to the Door Latch Side

Shim the Latch-Side Jamb


  • The purpose of shimming is to centralize the door opening and straighten the sides to make them plumb.
  • Apply appropriate amounts of shims above the latch-side Jamb (about 6 inches); also apply shims at the middle and the rear-end using the same spacing. Additionally, use adjustment shims at the door jamb area to achieve a consistent space between the slab and the Jamb.
  • Now, slit the projecting parts of the shims and cut them off using your utility knife.
  • Once you get a perfect fit, apply nails to the Jamb at every shim location.
  • Apply screws on each hinge using 3-inch screws.
  • Drive nails on the top and at the sides of the external trim at 16 in. apart; you can use galvanized casing nails for this purpose.

12. Insulate the Space between Your Door Frame and the Jamb

Insulate Around the Door

  • Fill up the gap in the frame area with little foam insulation. Two cans will do the entire filling.
  • Allow the foam to expand and skin over, then block any residual space with shreds of fiberglass to serve as extra insulation.

13. Install the Interior Trim

Install Interior Trim

  • You can either reinstall the existing trim if it is still reusable, or you can install a brand new one.
  • If you are installing a new one, first cut it to the desired size and then install it.
  • Fill up any remaining gap in the sill area with a beveled transition.

14. Seal with Caulk Backer

Apply Caulk Backer

Caulking helps create a tight blockade to prevent snow and rain from percolating into tiny gaps and joints on the prehung exterior door, which often result in wood rot and scorched paint.

  • Apply the foam backer to the trim gap.
  • Ensure neat caulking between the trim and the siding.
  • For gaps exceeding 3/16 inches, use a foam backer before applying caulk over it.

15. Finish with Painting

New Prehung Exterior Door

  • In most instances, you might not like the new prehung exterior door's color, so you'll need to paint it your desired color.
  • Detach the door from the hinge, then paint and varnish it to suit you.

By now, you'd agree with me that exterior door installation isn't “a tough nut to crack” after all, and all your hunches about the installation process being complicated were misguided.

Ensure that you carefully follow the installation instructions provided in each step to complete the entire installation process faster. Nonetheless, if you have questions regarding any of the steps, feel free to drop them in the comment box below.

How useful was this post?

Click on a star to rate it!

Average rating 5 / 5. Vote count: 2

No votes so far! Be the first to rate this post.