Plan How to Install an Interior Transom Window Above an Opening

All steps below reference instructions found in this video for the Johnson Hardware 1500 Series pocket door kit.

Material Needed: Johnson 1500 Series Pocket Door Kit (purchased from a local dealer), shims.

Tools Needed: Drywall saw, nail gun. You will also need tools required by the pocket door kit.

• Frame your rough opening per our pocket door planning instructions in the Planning section.
• Proceed through all pre-drywall pocket door installation steps detailed in the Johnson video. This includes installation of the pocket door frame header and split jamb pieces per the video.
• Hang the drywall as you normally would for a pocket door. Drywall will completely cover the area where the transom unit will be later installed.
• After drywall, you should see an opening only the size of a single door as shown in the Johnson video.
• Using the width and location of the door opening as a guide, cut the drywall above the pocket door frame header to form an opening for your transom — leaving a rectangular opening spanning vertically from the top of the pocket door frame header up to the framing header. The transom unit will set directly above the door opening, atop the pocket door frame header. A small strip of drywall should remain below the new opening that is still attached to the pocket door frame header.
• Inside the new opening, one side will blocked off with a stud and the other will be on open drywall hole. If desired, add blocking material between the free-hanging drywall pieces on the “open side,” attaching the blocking to the drywall to add stiffness .
• When your transom unit arrives, slide the unit into the opening above the pocket door frame header. Be sure that the bottom of the transom unit sits flat against the top of the pocket door frame header (wood against wood).
• Be sure that the transom unit is plumb, square and level in the opening, then place shims above the transom unit — filling the gap above (about 1/2”). Secure the transom unit to the upper framing header by nailing through shims using 2 to 2-1/2” trim nails.
• Complete the remaining steps in the pocket door installation, including installation of the split jamb along the open side and the top side.
• Review our installation tip on trimming out your transom. Know that you will need a larger piece of mullion, perhaps 3”, to span the distance from the transom jamb to the split jamb of the pocket door below.

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Material Needed: Drywall and some blocking material like 2x4s.

Tools Required: Drywall saw, saw, pencil, nail gun.

For planning purposes we treat arch and half-round transoms as if they were rectangles. That gets the rough opening right.

On installation, the drywall (or other wall sheathing) must be cut to fit the curve of the transom unit.

This is easier than it sounds, but you have to pre-hang your drywall down into the rough opening. Follow the directions below.

1. During your regular drywall hanging process, hang the sheet of drywall to be placed above your rough opening so that it overlaps into the rough opening hole. Hang it low enough vertically so that it extends slightly beyond the lowest edges of the curve on the sides of the transom jamb. If in doubt, hang it the same distance down into the opening as the sash height.
2. When your transom arrives, have an assistant help you to raise the transom unit up against the hanging drywall at the position where it will sit in the opening. Be sure to account for door or U-Jamb height if that applies to your installation.
3. Use a pencil to trace the curve of the transom jamb onto the drywall.
4. Repeat this process for both sides of the opening.
5. Using a drywall saw, cut the drywall to match the curve of the transom unit. You may want to cut an opening that is a bit more generous than your original mark -- perhaps leaving a 1/4" gap to ease sliding the transom into the opening. Any reasonable gap will be covered by the casing anyway.
6. You will be left with two smaller, more manageable pieces of drywall hanging into your opening. Place several pieces of blocking material between the hanging pieces to stiffen up the drywall for installation. Secure blocks with 2" trim nails. These blocking pieces will sit behind the casing and provide good nailing points for attachment.
7. Your preparation for an arched or half-round transom is complete. Proceed with normal installation

Casing

By their nature, arch-top and half-round transoms require require curved casing at trim out. Curved casing is widely available as flexible moulding or “flex-trim”. Carter Millwork (carterflex.com) is one of several national manufacturers of flexible moulding. Think of flex-trim as an elastomeric, rubbery material that holds a curved shape but can be adjusted to fit.

For half-rounds, the diameter is the key measurement required to order the flexible casing. Diameter for our half-round sashes is exactly the same as sash width.

Flexible casing comes in several standard curvatures that fit all intervening diameters of half rounds. You provide the manufacturer with the diameter, and they will select the curvature that can be bent into shape for your application. When ordering, the width of the casing and the sash width (diameter) will be used to calculate the length of the casing piece needed. If you would like to calculate this length yourself (in inches): (Diameter/2 + Casing Width) x 3.14.

For arch-top transoms, the amount of rise in the arch is the key measurement for flexible casing. For arch-top transoms, the vertical left and right sides of our transom (that is, the stiles) are called legs. Obviously, the legs for arch-top transoms are shorter than the overall height of the sash, with the “rise” of the arched top rail achieving full sash height. So, the sash height minus the leg height equals the “rise” of the arch.

For ordering, the manufacturer will use the rise to select the curvature profile for your application. Next, the sash width will be used to calculate the length of material required. If you would like to calculate this length yourself (in inches): (Rise x 3) + Width.

Flexible moulding is only required for the one piece of curved casing (on both sides). The remaining casing (for legs and the bottom rail) can be ordinary straight, wood casing matching the profile of the flex-trim casing you ordered. Most major casing profiles available locally will have a flex-trim version from Carter Millwork available locally as well. Go to a local lumberyard and ask to order flex-trim.

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Material Needed: U-Jamb is made entirely of 1x material that you supply — usually paint grade pine or stain grade 1x material to match the transom you have ordered. 1x6 or 1x8 material (cut to width) is most commonly used.

Tools Required: Saw, nail gun.

1. Cut all 1x stock longitudinally so width matches the width of jamb ordered for your transom unit.
2. Cut U-Jamb legs 81-3/4" tall for a normal height opening.
3. Cut head jamb (top piece) to the same width as the transom sash you ordered.
4. Assemble the U-Jamb by placing the head jamb so that it is sandwiched between the two side legs. Affix the legs to the head jamb with several 1-1/4" trim nails shot through the face of the side legs.

The width of the U-Jamb, once assembled, should match the width of your transom unit exactly.

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You can order your transom directly from us with a custom-sized jamb already installed (that is, as a transom unit), or you can build the jamb yourself. If you have easy access to the materials, building a paint-grade jamb for a rectangular transom is a fairly straight forward (although time consuming) task. Directions below.

Pro-Tip: Building a stain-grade jamb to match your stain-grade transom sash is a much more difficult proposition — since you will most certainly have to fabricate your own stop (just like we do). Also, we always recommend buying an arch-top or half-round transom with a jamb from the factory. Building curved jambs and stop requires specialized tools and techniques.

Material Needed: The jamb perimeter is made of 1x material (1x6, 1x8, etc.) this is usually paint grade pine. We use shoe moulding for the stop material, but any type of window stop -- or even quarter round -- can be used for the stop.

Tools Needed: Table or radial saw, miter saw, nail gun.

1. Cut all 1x stock longitudinally to match depth of jamb required.
2. Cut left and right (vertical) jamb pieces to match exact height of your transom sash.
3. Cut top and bottom (horizontal) jamb pieces 1-1/2” wider than the width of your transom sash.
4. Assemble the jamb box by sandwiching vertical jamb parts between horizontal pieces. Nail through top and bottom faces with 2” trim nails to secure sides in place.
5. Transom sash thickness is exactly 1-1/16” for interior sashes. Laying the jamb box face-down on the table, fabricate temporary wood-block spacers that will be placed onto the table inside the jamb box. Properly sized spacers will suspend the sash directly in the center of the jamb once the sash is slid into position inside the jamb box.
6. Slide the sash into the jamb — setting the sash directly onto the blocks. Make very minor adjustments to the transom sash width or height if the fit is too tight. This can be done by running a miter or table saw to remove a fraction of material from one or more of the sash's sides. Once placed in the jamb on the blocks, the sash should be in the dead center of the jamb.
7. Shoot 2” nails through each outside face of the jamb and into the sash to secure it in place.
8. The stop is placed inside the jamb (around the perimeter of the sash) along the inside corner formed by the sash and the jamb. Miter cut four pieces of stop (each side) to encapsulate the entire perimeter around the sash. Affix stop with 1” brad nails. Pro-Tip: Secure the shorter, vertical pieces in place before bending and snapping-in the longer, horizontal pieces. If shoe moulding is used, place the fatter side of the shoe against the jamb side.
9. Transom unit (sash with jamb) is now complete and ready for installation.

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Material Needed: Transom sash, stop material. We use shoe moulding for the stop material, but any type of window stop or even quarter round can be used for the stop.

Tools Required: Miter saw, nail gun.

1. Place a properly-sized transom sash into the jamb opening so that the sash is aligned in the center of the jamb -- plumb and level.
2. Using just a few 1-1/4" brad nails, toe-nail the transom sash in place along the sash's side edges -- being careful that nails are placed close enough to edge that they will be covered by the stop. The stop will be placed inside the jamb (around the perimeter of the sash) along the inside corner formed by the sash and the jamb.
3. Miter cut four pieces of stop (each side) to encapsulate the entire perimeter around the sash.
4. Install the stop on both sides, locking the transom sash in place. Pro-Tip: Install the shorter, vertical pieces on the ends before snapping the longer, horizontal pieces in place. Also, if shoe moulding is used, place the fatter side of the shoe against the jamb side.
5. Your transom sash is now stopped-out and secure in the opening.

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Material Needed: Shims.

Tools Required: Carpenters level, nail gun.

Unlike doors where shims perform a structural role behind hinges and the strike plate, the use of shims in a transom installation is largely for convenience as an installation aid.

During installation shims can be used along vertical joints to set the transom jamb, U-Jamb or door jamb vertically plumb.

For transoms installed into a four-sided opening, shims can be used below the jamb to set the unit level and plumb to the opening,

For transoms placed above an opening with a U-Jamb below, pay particular attention that the transom unit and U-Jamb are square to one another and are set plumb and level in the overall opening. Shims can be used to hold one side plumb while a squareness issue is addressed by manipulating the other side. If you follow our directions to install the casing on one side of the U-Jamb/transom unit assembly before placing the assembly into the opening, the assembly will hold square better during installation.

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Starting Point: Rough opening (U-Jamb).

Material Needed: Casing, mullion.

Tools Required: Miter saw, nail gun.

Casing

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

Casing profiles (that is, their cut or shape) differ regionally, so we leave it up to our customers to match the casing in their home.

• Purpose of Casing: When a house is first built, casing functions to conceal the gap between the edge of the rough opening and the jamb of the door, opening or window. Since we all associate casing with this "finished look", Transoms Direct prescribes adding casing whenever it is practical tp do so — even if there is no gap to over. So if your starting point for today’s installation is a finished, drywalled opening, we recommend you add casing around the opening even if it is not technically needed. Casing just makes the final installation look more complete.
• Reveal for Casing: Common industry practice leaves 1/4" of the face of the jamb showing (on the inner side) with the remainder of the jamb covered by the casing (outer side). This area of jamb left showing is called a "reveal." This "reveal" differs regionally, so we suggest that you measure the reveal if you have a completed home and match accordingly.
• Casing for Your Installation: Place head piece of casing atop the transom unit connected to two side pieces of casing that run the combined length of the transom unit and door U-Jamb (all the way to the finished floor). Carefully measure each piece with the reveal you have chosen, then miter cut the upper left and upper right corners. Attach casing with 2” trim nails — first to the jamb, then to the drywall.

Pro-Tip: If the installation of your upper piece of casing runs into the ceiling or conflicts with crown moulding planned for above, consider replacing the casing on the top with a piece of 1x material with no miter. The side pieces of casing should kill nicely into the 1x piece, and you can wrap the crown over the top of the 1x for a very nice, integrated look.

Mullion

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

• Purpose of Mullion: Mullion is a narrow, thin piece of symmetrical trim stock that covers the joint between the head jamb of a door or U-Jamb and the transom jamb where they are mulled together. The mullion runs horizontally covering the seam and is sandwiched between the two pieces of casing on each side (so casing is always installed first).
• Mullion for Your Installation: Measure the gap between the two pieces of casing. Cut mullion to match. You should have some reveal on the top and bottom of the mullion, but not necessarily the same reveal as the casing. The left and right end of the mullion should directly abut the casing. Install mullion with 1-1/4” trim nails.
• Zero-Gap: If the head jamb and transom’s jamb are mulled directly together, this is a zero-gap. In this case, we usually recommend 1-1/4” symmetrical mullion to cover the seam. This is most common.
• Taller Mulls: If you have increased the size of the mull, consider a wider piece of mullion or 1x4 to perform the mullion’s function.

Pro-Tip for Trim-Outs in a Rough Opening

For installations that require a U-Jamb, we always recommend pre-trimming out one side of the transom assembly while it is still on the floor. In this instance, the casing and mullion are nailed to the jamb perimeter before lifting the entire assembly into the rough opening. The trim-out on one side makes the overall assembly stiffer as it is raised into the rough opening -- which can make it easier to shim, square and level in this situation.

In order to simplify things, we do not specifically call for a pre-trim out in our installation instructions for doors and 4-sided openings. However, in these cases, most professional trim carpenters do actually trim-out one side of the assembly first because it is faster. Just beware that this technique can make shimming and squaring doors more difficult. For 4-sided openings pre-trimming is generally recommended without consequence.

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Starting Point: Rough opening (U-Jamb or Door).

Material Needed: Casing, mullion.

Tools Required: Miter saw, nail gun.

Casing

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

Casing profiles (that is, their cut or shape) differ regionally, so we leave it up to our customers to match the casing in their home.

• Purpose of Casing: When a house is first built, casing functions to conceal the gap between the edge of the rough opening and the jamb of the door, opening or window. Adding casing creates a "finished look."
• Reveal for Casing: Common industry practice leaves 1/4" of the face of the jamb showing (on the inner side) with the remainder of the jamb covered by the casing (outer side). This area of jamb left showing is called a "reveal." This "reveal" differs regionally, so we suggest that you measure the reveal if you have a completed home and match accordingly.
• Casing for Your Installation: Plan head piece of casing atop the transom unit. Connect two side pieces of casing that run the combined length of the transom unit and door or U-Jamb (all the way to the finished floor). Carefully measure each piece with the reveal you have chosen, then miter cut the upper left and upper right corners. Attach casing with 2” trim nails — first to the jamb, then to the drywall.

Pro-Tip: If the installation of your upper piece of casing runs into the ceiling or conflicts with crown moulding planned for above, consider replacing the casing on the top with a piece of 1x material with no miter. The side pieces of casing should kill nicely into the 1x piece, and you can wrap the crown over the top of the 1x for a very nice, integrated look.

Mullion

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

• Purpose of Mullion: Mullion is a narrow, thin piece of symmetrical trim stock that covers the joint between the head jamb of a door or U-Jamb and the transom jamb where they are mulled together. The mullion runs horizontally covering the seam and is sandwiched between the two pieces of casing on each side (so casing is always installed first).
• Mullion for Your Installation: Measure the gap between the two pieces of casing. Cut mullion to match. You should have some reveal on the top and bottom of the mullion, but not necessarily the same reveal as the casing. The left and right end of the mullion should directly abut the casing. Install mullion with 1-1/4” trim nails.
• Zero-Gap: If the head jamb and transom’s jamb are mulled directly together, this is a zero-gap. In this case, we usually recommend 1-1/4” symmetrical mullion to cover the seam. This is most common.
• Taller Mulls: If you have increased the size of the mull, consider a wider piece of mullion or 1x4 to perform the mullion’s function.

Pro-Tip for Trim-Outs in a Rough Opening

For installations that require a U-Jamb, we always recommend pre-trimming out one side of the transom assembly while it is still on the floor. In this instance, the casing and mullion are nailed to the jamb perimeter before lifting the entire assembly into the rough opening. The trim-out on one side makes the overall assembly stiffer as it is raised into the rough opening -- which can make it easier to shim, square and level in this situation.

In order to simplify things, we do not specifically call for a pre-trim out in our installation instructions for doors and 4-sided openings. However, in these cases, most professional trim carpenters do actually trim-out one side of the assembly first because it is faster. Just beware that this technique can make shimming and squaring doors more difficult. For 4-sided openings pre-trimming is generally recommended without consequence.

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Starting Point: Rough opening (In Wall).

Material Needed: Casing, mullion.

Tools Required: Miter saw, nail gun.

Casing

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

Casing profiles (that is, their cut or shape) differ regionally, so we leave it up to our customers to match the casing in their home.

• Purpose of Casing: When a house is first built, casing functions to conceal the gap between the edge of the rough opening and the jamb of the door, opening or window. Adding casing creates a "finished look."
• Reveal for Casing: Common industry practice leaves 1/4" of the face of the jamb showing (on the inner side) with the remainder of the jamb covered by the casing (outer side). This area of jamb left showing is called a "reveal." This "reveal" differs regionally, so we suggest that you measure the reveal if you have a completed home and match accordingly.
• Casing for your installation: For all 4 sides, carefully measure each piece with the reveal you have chosen, then miter cut the corners. Attach casing with 2” trim nails — first to the jamb, then to the drywall. You may need to place small shims behind the casing to compensate for the slight bow that the drywall corner bead creates in the wall.

Pro-Tip: If the installation of your upper piece of casing runs into the ceiling or conflicts with crown moulding planned for above, consider replacing the casing on the top with a piece of 1x material with no miter. The side pieces of casing should kill nicely into the 1x piece, and you can wrap the crown over the top of the 1x for a very nice, integrated look.

Pro-Tip for Trim-Outs in a Rough Opening

If you prefer, the casing can be nailed to the jamb perimeter on one side of the transom before lifting the entire assembly into the rough opening. Most professional trim carpenters do actually trim-out one side of the assembly first because it is faster. Just place shims from opposite side.

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Starting Point: Finished, cased opening (Spacers).

Material Needed: Mullion. We recommend 1-1/4" symmetrical mullion for this installation.

Tools Required: Saw, nail gun.

Mullion

• Purpose of Mullion: Mullion is a narrow, thin piece of symmetrical trim stock that covers the joint between the two spacers added below the transom sash. The mullion runs horizontally covering the seam and is sandwiched between the two pieces of casing on each side.
• Mullion for Your Installation: Measure the gap between the two pieces of casing. Cut mullion to match. You should have some reveal on the top and bottom of the mullion, but not necessarily the same reveal as the casing. The left and right end of the mullion should directly abut the casing. Install mullion with 1-1/4” trim nails.

Starting Point: Finished, drywalled opening (In Wall).

Material Needed: Casing.

Tools Required: Miter saw, nail gun.

Casing

Most people know about casing - the trim pieces that form a perimeter around doors, windows, cased openings and interior transoms.

Casing profiles (that is, their cut or shape) differ regionally, so we leave it up to our customers to match the casing in their home.

• Purpose of Casing: When a house is first built, casing functions to conceal the gap between the edge of the rough opening and the jamb of the door, opening or window. Since we all associate casing with this "finished look", Transoms Direct prescribes adding casing whenever it is practical tp do so — even if there is no gap to cover. So if your starting point for today’s installation is a finished, drywalled opening, we recommend you add casing around the opening even if it is not technically needed. Casing just makes the final installation look more complete.
• Reveal for Casing: Common industry practice leaves 1/4" of the face of the jamb showing (on the inner side) with the remainder of the jamb covered by the casing (outer side). This area of jamb left showing is called a "reveal." This "reveal" differs regionally, so we suggest that you measure the reveal if you have a completed home and match accordingly.
• Casing for your installation: For all 4 sides, carefully measure each piece with the reveal you have chosen, then miter cut the corners. Attach casing with 2” trim nails — first to the jamb, then to the drywall. You may need to place small shims behind the casing to compensate for the slight bow that the drywall corner bead creates in the wall.

Pro-Tip: If the installation of your upper piece of casing runs into the ceiling or conflicts with crown moulding planned for above, consider replacing the casing on the top with a piece of 1x material with no miter. The side pieces of casing should kill nicely into the 1x piece, and you can wrap the crown over the top of the 1x for a very nice, integrated look.

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Material Needed: Paint, sandpaper, masking tape.

Tools Required: Paint brush.

Finishing our products, for the most part, does not require any special technique or skill. Hints for each type of finish are provided below.

Pro-Tip: We have found that it is usually easier to paint and clean many of our more intricate transoms before installation. Lay transom flat while painting.

1. Sanding. All of our products have been sanded to a uniform, smooth finish before shipping. If you would like to touch-up the sanding job, we recommend light "hand sanding" with fine (150-220 grit) sandpaper. Be sure to sand "with the woodgrain" to avoid sanding marks. Note: Do not allow sandpaper to contact glass as this will damage the surface.
2. Masking. This step is not required for clear glass — only for textured or etched glass. For clear glass, you will clean the glass with a razor in a later step. Other types of glass have an irregular surface and cannot be cleaned with a razor. Irregular glass can be covered with non-transfer type painter’s tape — leaving about 1/16” perimeter of glass showing around the edges (see tip on “setting the glass” below). Ensure that the edges of the tape are tightly adhered to glass to prevent finish from being drawn under tape by capillary action.
3. Priming, Painting or Staining. Hints are provided below for most all finishing options.
• Priming. Any suitable wood primer works well with our products. After primer application, sand areas of "standing" wood grain -- to provide a smooth and more uniform final finish. Pro-Tip: Primers tend to bond to glass better than paint (making scraping harder), so try to avoid getting very much primer on the glass.
• Painting. Paint sash, stop and jamb with trim brush. Pro-Tip: You may note that glass lites fit loosely in the frame — moving around a bit in the slot created by the stiles, rails and muntins. This is normal. You may “set the glass” by allowing some of the paint to lap over onto glass (usually 1/8 to 1/4 inch). The thicker the better as this will make final scraping easier. If you have masked the glass, leaving a 1/16” gap around the perimeter will allow you to flood the gap and set the glass — even with the glass protected.
• Staining. For initial preparation, fine sanding of frame and muntin components is recommended to ensure uniform grain and porosity before staining. We have also found that wood conditioner will help to provide a more consistent stain base and minimize color variations due to grain. During stain application, any residual glue from manufacturing (usually at the joints) will appear as light spots. When these spots occur, simply sand the light area and reapply stain until stain penetrates into woodgrain. When possible, we recommend staining transoms before installation. This makes it easier to control the finish process and clean up any residue on the glass.
• Clear Finishes. We strongly recommend masking glass before applying any clear finish — even for clear glass. Clear finishes can adhere much more tenaciously and dry much harder than paints — so they are much harder to scrape from glass. Just as with paints and primers, clear finishes may be used to “set the glass” as described under painting section above.
4. Cleaning the Glass. When properly applied, paint can be easily scraped from glass after finishing. The best technique is to use a sharp razor to trace the perimeter of all components. This creates a clean cut line between the wood parts and glass surface. After the cut line is completed, hold razor edge flat against glass at a shallow angle and drag it under the paint to remove. If you have masked the glass, you may want to use a razor scraper to clean up the edges of the flood coat used to set the glass. Pro-Tip: Using a glass cleaner to pre-wet the surface will help razor to slide along glass surface and avoid possible scratches. Keep applying through the cleaning process to keep surface lubricated.

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