Introduction #
A transom window is a fixed window set above a door opening, usually to bring extra light into the space. Installing one is part of the same trim and finish carpentry workflow as the door below it, on new construction or a remodel.
A transom is not a stand-alone unit like an interior window frame, built and set as its own separate unit. It sits directly above the door, and the two are trimmed as one unit: the casing has to run continuous between them, holding one unbroken reveal the whole way. The frame and jamb detail for the transom follow the same approved style as the door, per the plans for the job. Confirm that against the plans before you fasten anything, not after the casing goes on.
Before You Begin #
Before you install the transom window, confirm that the following prerequisites are complete.
Prerequisites #
- Ensure the front door is installed and hardware is in place.
- Verify that drywall and rough framing are complete in the installation area.
- Confirm that any electrical or plumbing rough-ins are complete above the door.
- Check the rough opening above the door against the plans before you build or set the transom frame.
What You Need To Know #
When you install the transom window, understanding these core techniques will save you time and prevent callbacks.
- The transom is not a stand-alone frame. It sits directly above the door and has to align with it. The casing has to run continuous with the door casing, holding the same 3/16″ reveal the whole way up.
- Use shims to adjust the window frame as needed to achieve a perfect fit.
- Consider using spray foam insulation around the window frame for a tight seal.
Tools #
- Level - Essential for ensuring the window is perfectly aligned.
- Shims - Used to adjust the window frame for a precise fit.
- 15-Gauge Finish Nailer - For fastening the transom frame to the framing, 2-3/4″ nails.
- 18-Gauge Brad Nailer - For the casing, 1-3/4″ brads.
- Air Compressor - Needed to run the pneumatic nailers.
Materials #
- Transom Window Unit - Sized to fit the opening above the door.
- Spray Foam Insulation - Low-expansion, door-and-window-rated foam for sealing the gap between the frame and the rough opening.
- Trim Material - MDF or Prime Pine for casing and finishing.
- Wood Glue - For gluing every mitered casing joint.
Installation Steps #
Step 1. Prepare the Opening #
Ensure the opening above the door is clear and framed to the correct dimensions for the transom window. Check it against the door below before you go any further.
- Check the rough opening size against the window unit dimensions.
- Remove any obstructions or debris from the opening.
- Confirm the opening lines up with the door below, per the plans, before you build the transom frame.
Step 2. Position, Plumb, and Fasten the Transom Frame #
Set the transom frame into the opening directly above the door, per the plans. Shim, check, and fasten each point as you go, instead of shimming the whole frame first and coming back to nail it. Shimming and nailing one point is a single step, not two.
- Check the frame with a level as you go. It needs to read plumb and level, matching the approved style and the plans for the opening.
- Shim behind the jambs as needed to hold plumb and level. Fasten through the frame and shims into the framing at that point with a 15-gauge nailer (2-3/4″ nails), the same as jamb work on the door below. Move to the next point and repeat.
- Work around the frame checking plumb, level, and alignment with the door below as you go.
Step 3. Insulate Around the Frame #
Apply a low-expansion, door-and-window-rated spray foam around the transom frame to seal the gap between the frame and the rough opening.
- Ensure even application of foam to avoid bulging or distortion.
- Allow foam to cure before proceeding with trim installation.
Step 4. Case the Transom to Match the Door #
Casing on a transom is not a separate job from the door casing below it. Mark the reveal at 3/16″ on the transom frame, the same reveal you are holding on the door jamb. That keeps the line running continuous, with no step where the two meet. Measure, cut, and dry-fit each piece before you nail anything, the same way you would casing a window.
- Set a combination square to 3/16″ and mark the reveal line on all sides of the transom frame, matching the door casing’s reveal below it. On stain-grade material keep the marks light and sand them off before the casing goes on, since pencil marks show through stain; on painted material this does not matter.
- Cut 45-degree miters on the corners. Glue every miter, on painted and stained casing alike, and press it tight before you nail: glue does the holding at a miter, not the nail.
- Nail the casing with an 18-gauge, 1-3/4″ brad nailer. Never cross-nail a miter to close it. A liberal bead of glue and a tight, held joint is what holds it, especially on MDF, where a nail through end grain barely holds at all.
- Check the reveal against the door casing below as you go. If the two don’t line up, cheat the reveal slightly rather than break the line. A continuous look across the whole opening matters more than holding an exact 3/16″ on every inch.
Step 5. Tighten, Clean, and Check #
With the transom cased to match the door below, close out the install. At this stage you are checking joints and fasteners, not finishing the wood: the finish coat is the painter’s job, not ours.
- Work fresh glue into any miter that is not fully closed, press it tight, and wipe off the excess with your finger while it is still wet.
- Set every brad below the surface with a nail set. Anything left proud will show through the finish.
- Check the reveal one more time, top to bottom, door and transom together: it should read as one continuous line with no step where the two meet.
- Sweep up the scraps and put the debris in the dumpster.
Filling the fastener holes is the painter’s step, not ours. What he needs from us: tight glued miters, a consistent reveal, casing flat to the wall, and fasteners set below the surface.
Pro Tips #
Pro Tip: Hold the same 3/16″ reveal through the whole opening, door and transom together. Let it drift even a little at the point where the two meet and it shows the moment the casing goes on.
Common Mistakes #
- Mistake: Misalignment with the door below can cause aesthetic and functional issues. Always use a level and adjust with shims.
- Mistake: Inadequate insulation leading to drafts. Ensure spray foam is applied evenly around the frame.
- Mistake: Casing the transom as a separate box instead of tying it to the door casing below. Hold the same reveal and check alignment against the door before you nail anything.
QC / Punch Checklist #
- Reveal is a consistent 3/16″ on the transom and reads continuous with the door casing below.
- All miters are glued tight with no visible gaps. On painted casing, glue the miter, then putty and sand it smooth. On stained casing, skip the filler and rely on a tight, glued joint instead.
- Every brad is set below the surface, nothing proud to the touch.
- Gap around the frame is fully sealed with spray foam, no bulging or gaps left.
- Swept clean, no scraps left on the floor.
Related Guides #
The transom sits directly above the door, and the casing runs continuous between them. From there, the rest of the trim carpentry on the opening is the same layout and nailing you already know from casing a door or a window.
- How to Install a Pre-Hung Door - The door this guide assumes is already installed and hung before you build the transom.
- How to Case a Window - The picture-frame casing technique the transom uses: reveal, miters, and nailing pattern.
- How to Install an Interior Window Frame - A stand-alone window frame in its own rough opening, the version of this job that does not tie into a door.
- How to Install a Mullion Strip - If your transom is split into more than one light, this covers the strip between them.
