Introduction #
Ganged windows are two or more windows set close together in the same wall, each one its own separate unit with its own frame, separated by wall framing and drywall in between. Each window still counts as a separate window. We case them differently: as one continuous unit, with a mullion strip (a strip of trim that covers the gap between two window units) in the center between each pair, so the finished casing reads as one wide window instead of several small ones set side by side.
This guide assumes you already know how to case a single window. It covers only what changes when several windows share one casing job: keeping the group in line, and where the mull between two windows replaces the casing legs that would normally face each other. It does not re-teach the picture-frame method itself. It also does not re-teach how to build or install the mull post that goes between two ganged windows. That piece follows the approved casing style for the job and the plans, the same as any other spec this guide does not have on hand.
The frames are separate. The finished look is not. Even when the windows were not set perfectly in line during framing or install, the outside edges of the casing still need to line up across the whole group. That is our job, not the framer’s, and it is the one thing this guide is really about: cheating the reveal (the strip of jamb you leave showing between the jamb and the casing) a little on one window so the group reads as one straight, tied-together unit.
How to Case Ganged Windows Step by Step #
Step 1: Inspect the Group and Find the Shared Line #
Treat the whole run of windows as one job before you cut a single piece of casing. Check each window the way you would check one on its own: run your hand along every jamb edge to confirm it is flush with the drywall, and open, close, and lock each window to confirm it works. Fix any problems now, before any casing goes on.
Once every window checks out on its own, step back and look at the group as a whole. You are not grading any single jamb here. You are comparing how far each window sits out of line with its neighbors, the same call the core guide’s own troubleshooting makes for a bowed jamb. Split the difference so the group reads straight, instead of chasing a perfect measurement on any one window. That read is what tells you how much you will need to cheat the reveal in Step 2 to get the group tied together.
Step 2: Mark Reveal Lines, Cheating Where the Group Needs It #
Set your combination square to 3/16″ and mark the reveal line the same way you would on a single window. Mark it on every jamb edge that will get its own casing, the top and bottom of each window, and the outside jamb of the first and last window in the group. Keep your pencil marks light. On stain-grade material, pencil marks show through the finish every time: sand them off clean, or set your mark about 1/32″ past 3/16″ so the casing just covers it instead of leaving it exposed.
Where a window sits slightly out of line with its neighbor, use the read you took in Step 1: hold your reveal mark a little tighter or a little wider than 3/16″ on that jamb, whichever keeps the outside edge of the casing running straight across the group. A reveal that varies slightly window to window is fine. An outside casing line that jogs where two windows meet is not.
Do not mark a full reveal line on a jamb that faces a neighboring window, the side where two windows share the wall between them. That gap gets a mull in Step 4, not a matched pair of casing legs, so leave it for now.
Step 3: Case the Outside of Each Window #
Every window still gets its own header and bottom casing, and its own casing on any side that faces open wall instead of another window. Measure, cut, and install those pieces the same way as the core guide: 45-degree miters cut on the miter saw about 1/16″ long so you can back-bevel and close the joint tight, with glue on every miter. An end-of-group window does not have two full sides to pair up the way the core guide’s single window does. Its inside leg becomes the mull instead of a casing piece, and the core guide never covers that case directly. Case the header and that one outside leg together anyway, the same pairing principle applied to a case the core guide does not have: two pieces to fine-tune instead of one. A window in the middle of a group of three or more has no outside leg at all, just a header and a bottom, so case those together on their own too.
Nail with 18-gauge, 1-3/4″ brads, two rows per piece, the same pattern as a single window. Glue does the holding at every miter. Don’t cross-nail it.
Step 4: Install the Mull Between Each Pair of Windows #
Where two windows in the group share a wall, the gap between them gets one mullion strip, not two facing casing legs. Dry-fit it against both windows’ casing before you commit to it, and check the outside edge of the whole group one more time. The alignment call already got made back in Step 2. This is a final check, not a place to start correcting a group that is still out of line. How the strip itself gets built and fastened to the framing between the two windows is not something this library documents yet: follow the approved casing style for the job and the plans.
Glue every joint where the mull meets a window’s casing, the same as any other miter in this guide, and don’t cross-nail it. The width of the mull, and whether it is a single flat strip or a built-up piece, depends on the approved casing style for that job and on the plans: check both before you cut. If the gap between one pair of windows in the group is not the same as the gap between the next pair, that is also a call for the approved style and the plans, not a fixed rule. Ask if it is not obvious.
Quality Check After You Case Ganged Windows #
Before you call the group finished, check it against this list:
- The outside edges of the casing run in one straight line across the whole group, even where the reveal was cheated to get there
- Every miter is tight and glued, including where a mull meets a window’s casing
- Reveal holds at 3/16″ everywhere it was not cheated for alignment
- Every window in the group still opens, closes, and locks, with nothing rubbing on a mull or a casing leg
- Every brad is set below the surface, nothing proud to the touch
- Sweep up: collect the cutoffs and leave the sill and floor clear
Troubleshooting #
Even careful work can run into small problems on a ganged group. Here are the most common ones and how to fix them.
Windows Don’t Line Up Across the Group #
This is normal on a lot of framing, and it is why you read the group before marking any reveal lines in Step 1. Fix it by cheating the reveal on the jambs closest to the problem, tighter on one window, wider on its neighbor, until the outside casing edge runs straight again. Holding an exact 3/16″ reveal on every single jamb matters less than the group tying together visually.
Mull Won’t Sit Flat Against One Side #
This is usually the same cause as a single casing piece rocking: the drywall is proud of the jamb somewhere along that edge, sticking out past the jamb face. Pound the drywall back with a hammer right along the jamb, the same fix as a single window, so the mull can seat flat and tight on both sides.
Gap Where the Mull Meets the Casing #
Same fix as any miter that won’t close: take the piece back to the saw and back-bevel it, or bump the saw angle half a degree, until the front face closes tight. Glue it, hold it until the glue grabs, and wipe any squeeze-out off with your finger while it’s still wet. Don’t cross-nail it to force it shut, glue does the holding.
Related Guides #
Casing ganged windows uses the same skills as a single window, just applied across the group. Start with the core guide if you have not cased a window before. The mullion strip guide is listed below too, but it covers a different job: dividing the glass inside a single window with muntin strips. That is not the structural mull that goes between two ganged units, so do not follow its steps for that piece.
