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Plan How to Install an Interior Combination Unit

Introduction to Interior Combination Units

This video explains how to design and install custom combination door and window units that fit existing openings without major construction.

Video Summary

Combination units use doors, sidelites, and transoms to fill large wall openings without structural remodeling.
These systems can transform spaces, such as converting a dining room opening into a bedroom or office.
Units are custom-sized, shipped ready for assembly, and designed for quick installation—sometimes in a single day.
Double doors (French doors when glass is used) and sidelites expand the width of an opening.
Transom windows and flanker transoms are used above openings to fill vertical space and enhance design.
The video series teaches how to measure, design, and assemble these components to fit an exact opening.

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Your Choice of Arrangements

We can design arrangements of doors and windows to fit nearly any opening. Our online calculation tool will give you the best solutions for your opening.

Video Summary

Arrangements can include single doors, double doors, or pass-through openings, with optional sidelites on either side.
Depending on the height of the opening, transoms can be added above the doors or sidelites, including multiple transoms ganged together.
The best arrangement is determined by the purpose of the space and the shape and size of the opening—not just aesthetics.
Larger, wider openings often work best with double doors to keep sidelite sizes practical and balanced.
Square or taller openings may be suited to either a single door with sidelites or double doors without sidelites.
Finding the right arrangement may take a few iterations, our online calculation tool will present all the alternatives.

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Bottom Row of the Arrangement

In the arrangement, the building blocks of the bottom row span the width of the opening -- with a small gap on each side to allow for proper installation.

Video Summary

The bottom row is made up of two parts: a door area and a fill area that together span the width of the opening.
The door area can be a single door, double door, or a cased opening if no door is desired.
The fill area is made up of sidelite windows, which may appear on both sides, one side, or not at all, depending on the remaining width.
The size of the door you choose directly determines how much fill area is left for sidelites.
To keep sidelites looking balanced, the door must leave enough space—generally at least 10 inches per side for sidelites to look proportional.
A small gap is intentionally left around the unit to allow it to be set square, plumb, and level, then shimmed and later covered by casing.

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Top Row Aligns with Bottom Row

The building blocks of the top row also span the width of the opening -- but the blocks sized to align with the bottom row.

Video Summary

The size and layout of the top row are driven entirely by the choices made in the bottom row.
The available height for the top row is determined by the fixed height of the bottom row, leaving the remaining space for a transom.
In most cases, about 10 inches of clear height is ideal for a transom; if less space is available, trim can be used instead of a window (see "Loose End" video below).
While the top row spans the full width of the opening, its building blocks look best when their widths line up with the door and sidelites below.
To achieve this alignment, the top and bottom rows use matching unit dimensions, ensuring a clean and symmetrical design.
Small gaps are intentionally left at the sides and top of the unit to allow it to be set square, plumb, and level during installation.

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Unit Dimensions versus Ordering Dimensions

The unit dimension of a building block equals its ordering dimension plus 3/4" on each side to account for the jamb.

Video Summary

A window sash and door slab are the core building blocks in a combination unit.
Walls are much thicker than doors and windows, so a jamb is required to adapt them to the wall depth.
Jambs also serve as the structure that connects individual building blocks together into a single combination unit.
Once a jamb is added, a sash or slab becomes a window or door unit, and its overall size is called the unit dimension.
Because jambs are made from 3/4-inch material, they add 3/4 inch per side, making window units 1-1/2 inches wider and taller than their ordering dimensions.
Ordering dimensions are used to purchase doors and windows, while unit dimensions are used to size the combination unit for your opening.

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Unit Dimentions for Doors

Door unit dimensions work similar to windows but check double door width carefully.Only watch this video if you plan to use a door on the bottom row.

Video Summary

A door with a jamb is called a door unit, and the outside-to-outside jamb measurement is the unit width used for planning.
For a standard single door, the unit width is calculated by adding 3/4 inch per side (1-1/2 inches total) to the door slab width.
A standard double door without a lock is sized the same way: add 3/4 inch on each side of the combined slab widths.
Interior double doors that lock use a T-astragal, which fixes one door in place so the operating door can latch against it.
Manufacturers handle T-astragals differently, so the actual unit width can vary even when slab sizes appear identical.
Because of this variation, the only foolproof way to confirm the unit width of a locking double door is to measure the assembled unit on site or verify manufacturer-listed unit dimensions.
If you plan on supplying your own door, you can enter the measured unit width into our calculation tool so the system can size the remaining building blocks correctly.
For interior doors, the top of the door jamb aligns at 81-3/4 inches above the finished floor, based on an 80-inch door slab plus jamb thickness.
The jambs of sidelites align flush with the door jamb, and sidelites should be ordered with an 80-inch sash height to match the door.
Any space above 81-3/4 inches in the opening is filled with transom windows in the top row.
Sidelites are intentionally held 1/4 inch short at the bottom to allow door jambs to be trimmed for floors that are not perfectly level.
This clearance ensures the door unit can be set square, plumb, and level without interference from the sidelites.

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Cased Opening Dimensions

A cased opening uses jambs to create a door-sized opening where people can pass-through.Only watch this video if you plan to use a cased opening on the bottom row.

Video Summary

A cased opening allows passage without a door and offers more flexibility in width than a standard door.
For visual consistency, cased openings are planned to match the standard door opening height wherever they are used.
The target clear opening height is 81 inches above the finished floor, matching a typical door jamb.
A cased opening is constructed from 1x jamb material formed into an upside-down “U” with two side jambs and a top jamb.
Because jambs add 3/4 inch on each side, the unit width of a cased opening is 1-1/2" wider than its clear interior width.
When sidelites are included, the tops of the sidelite align with the cased opening and are ordered with 80-inch sash heights to match door proportions.

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Determine Your Starting Point

Watch this video to identify the starting point for your project, and make your selection in the box below. Your starting point impacts the sizing recommendations for your combination unit.

Video Summary

Most projects begin with a rough opening, which is an unfinished wall opening created during construction or renovation.
A rough opening is identified by its exposed wall cross section, typically showing studs and drywall edges.
If you have not yet framed an opening, you can start with a new opening and plan the framing later.
In completed homes, a finished opening connects two rooms and can be converted into a combination unit.
Cased openings have trim and finished jamb material around the opening. Only smaller cased openings can often accept a combination unit without modification.
Drywalled openings have no casing and are fully wrapped in drywall, including the interior faces of the opening.
Selecting the correct starting point ensures the system recommends the right arrangements and gap sizes for your opening.