How to Install Transom Windows

Transom windows can be installed by themselves (as interior windows), or as decorations above doors and cased openings.  Standard carpentry tools such as nails and hammers are needed, as well as a few simple components to frame the transom.

In addition to the cased opening components, it is necessary to have adequate material for the fabrication of a jamb frame, a stop molding (for holding the sash in place), and a mullion (for installing the transom above your door).

Any homeowner who is looking to add both beauty and functionality to his or her home should consider installing transom windows. Most people won’t have any idea of where to start, so here are the basic instructions for completing transom window installation above a door.

Those that are really looking to take on this project themselves should first get a detailed set of instructions from the window manufacturers. This set of instructions is strictly to get an idea of the process. We are not responsible for any accidents, warranty voids or mishandling. We highly recommend allowing professionals, like ourselves, to replacing your windows. We offer the best warranties and coverage, aside from manufacturers, for replacement windows that are available to homeowners in the New England area.

Step 1. The first thing that is needed is a calculation of the final opening width. Based on this measurement, a transom jamb frame will need to be measured to fit that same width (about a quarter inch bigger on all side than the actual dimension of the door slab). This frame will be made using flat jamb material. The transom sash might need to be cut down slightly in order to fit inside the frame of the jamb.

Step 2. The transom sash should be centered inside the jamb frame and fastened to it. It is often easier to do this while lying on a flat surface to inhibit twisting during assembly. Temporary spacer blocks should be used to hold the transom sash in place until the stop molding can be applied. It is recommended that spacing nails or screws be fastened every eight to twelve inches.

Step 3. The stop molding needs to be cut and assembled on both sides of the sash to provide a completed look for your transom. This has the added advantage of covering any gaps that might exist between the jamb frame and the sash.  Some common trim profiles that are used for this purpose include cove mold, quarter round, and square stop. Install shorter (vertical) stop pieces first.  This will allow for the bending of longer pieces into place to obtain a tight fit.

Step 4. The transom unit (the assembled sash and frame) should now be fastened on top of the cased opening or door.  To accomplish this, nail the transom jamb to the head jamb of the door (or cased opening) beneath it.

Step 5. Once everything is assembled, it will be necessary to measure the door and transom together as one unit.  Cut the casing (the two legs and the one header) and install on one complete side of the transom.  Then case the other side of the assembled door / transom after installing the door in the rough opening.  If the transom and the door are assembled directly at the foot of the opening, it is much easier to secure the door in its final position.

Step 6. After applying the casing, complete the transom trimming process by installing the mullion between the jamb frame and the door head jamb.  Adjust the mull detail by installing spacer blocks between the jambs and using a wide mullion to cover the gap.

Step 7. Next, the assembled unit (cased on one side) will need to be placed into the rough opening by plumbing and tacking.  Then shim and nail behind the jamb on the open, uncased, side.

Step 8. After shimming, the uncased side should be measured, cut, and completed.

Step 9. Repeat Step 6 for the unfinished side of the transom.

The process for installing a transom over a cased opening or by itself is completed in the exact same way (without the doors).

These are the basic steps one would take to install a window. Of course, after reading these instructions, some homeowners may prefer toleave the task to the professionals. United Home Experts can help; the company specializes in window installation and a variety of other home improvement projects for those living in the Eastern New England area.