Transoms are simple to install alone (as interior windows), above doors, or above cased openings. Basic installation requires common carpentry tools, and only a few simple components to frame and mull your transom. The following sequence illustrates an installation above an interior door. In addition to your door or cased opening parts, you will need enough jamb material to fabricate a jamb frame, stop moulding (to hold the transom sash in place), and mullion (to join the transom above your door or cased opening).

Shown below are photos of these parts.
Flat Jamb Stop Mullion
Flat Jamb (4-9/16” shown) Stop (3/4” shown) Mullion (1-1/4” shown)
Calculate your final finished opening width, and fabricate a transom jamb frame to that same width using the flat jamb material. Note: It may be necessary to cut the transom sash down slightly in order to fit inside the jamb frame. (See the note below to ensure correct sizing of jamb frame.)
Jamb Frame Parts Assemble Jamb Frame
Cut jamb frame parts Assemble jamb frame
Note on Sizing: Sizing of the transom and frame is dependent upon the opening on which it will be mounted. In this instance, we are installing this transom above a 3/0 french door (actual door slab dimension 35-3/4”). The jamb for this door has an inside dimension of exactly 36”. We will therefore want to fabricate our transom jamb frame with an inside dimension width of 36” (height may vary), so that all components will line up properly during final assembly.

See step 5 for an illustration of how transom unit should fit over your finished opening.

Place the transom sash into jamb frame, center and fasten. Hint: We have found it easier to complete this step while lying on a flat surface. This will help to prevent twisting during assembly. In the first photo, we have rested the transom sash on temporary spacing blocks (1-3/4” blocks for a 4-9/16” jamb frame) to help keep it centered while fastening to the jamb frame.
Spacer Blocks
Use temporary spacer blocks to hold transom sash in proper position
Place transom on frame Fasten transom
Place transom into frame
(resting on spacer blocks)
(Cut transom sash to fit frame interior if necessary)
Fasten transom to frame
(Use 1-1/2” to 2” fasteners)
Note: Fastening the transom sash to jamb frame is primarily for holding it in place until the stop moulding is applied. We recommend spacing nails (or screws) every 8-12” for adequate support.
Cut and install stop moulding on both sides of transom sash. This will provide a finished look for your transom, and will cover any gaps between the transom sash and jamb frame. Feel free to use other trim profiles for your stop moulding. Quarter round, cove mould, or square stop, are common profiles for this application. Hint: We suggest installing shorter (vertical) stop pieces first. This allows you to bend longer pieces into place and get a tighter fit.
Cut stop Install stop
Cut stop moulding (8 pcs. total) Install / fit stop moulding
Fasten stop
Fasten stop between sash and frame
(this joint can be either structural or cosmetic depending on how it is nailed)
Fasten the transom unit (assembled sash and frame) on top of door or cased opening. This can be accomplished by simply nailing the transom jamb to the head jamb for the door (or cased opening) below. In this photo sequence we are installing this transom over a split jamb door, so we have propped up the door frame to help keep everything flush. Note how the transom frame is exactly the same width as the door unit below.
Fasten transom over door
Fit transom unit over door
Fastem transom to head jamb
Fasten transom unit to door head jamb
Once assembled, measure the door and transom together as 1 unit. Cut casing (2 legs, 1 header), and install on one complete side. The other side of the assembled door / transom will be cased after the door is installed in the rough opening. Note how we assemble the door and transom right at the foot of the rough opening. This way we can just stand the door right up into the opening without having to move it around unnecessarily.
Case door and transom Fastem casing
Case door and transom together Fasten casing
Complete casing installation
Complete casing installation on one side of transom / door unit
After casing has been applied, complete mulling / trimming of transom unit and door by installing mullion between transom jamb frame and door head jamb. Here we are using 1-1/4” mullion to join two 3/4” thick jamb pieces fastened together. For a mull detail that looks heavier, simply install spacer blocks between the two jambs and use wider mullion to cover the gap.
Unfinished joint
Unfinished joint between transom unit and door (before mullion installation)
Install mullion
Install mullion over joint between transom unit and door
Stand the assembled unit (cased 1 side), into the rough opening. Plumb and tack into place through the casing. Now you can shim and nail behind the jamb on the open side. After shimming, measure the uncased side, cut, and complete casing. At this point, the operation is very typical of any door installation. In some localities, it is customary to initially install the door and transom together without casing. Here you would shim and install the unit by nailing through the jamb (into the framing). Casing is applied to both sides afterwards. This method tends to work better in older construction where the walls might not be perfectly plumb. Either way works equally well, we have just found the method shown in the photos to be slightly faster in most instances.
Stand unit Plumb
Stand unit into rough opening Plumb door / transom unit
Tack into place Install shims
Once properly positioned, tack unit into place through casing Shim behind jamb at uncased side
Repeat casing/mullion install for unfinished side
Repeat installation (step 7) of casing and mullion on unfinished side
Final Note: Installing a transom over a cased opening, or alone (as an interior window), is done exactly the same way (minus the doors of course). No special materials or tools are required. For an experienced carpenter, this entire sequence can be completed in about one hour (in new construction). The actual transom portion (jamb frame fabrication and installation), adds about 20 minutes to a typical door installation. The material necessary (flat jamb, stop, and mullion) to fabricate the jamb frame, cost around $15.