Roman shades and cellular blinds are mounted the same way in only one respect: both go inside the window recess or outside against the wall or trim, and that single choice drives nearly everything else. Inside mount asks for depth, roughly an inch for a slim cellular headrail and closer to two inches once a Roman shade's folded stack is counted, while outside mount attaches to the casing or the wall and needs no recess at all. Settle the mount first, then the lift mechanism, because a Roman that can't clear the recess and a cellular shade with the wrong operating system both leave you staring at a window you can't cover the way you pictured.
I keep a tape measure in the drawer under the toaster, and it comes out every time a friend or a neighbor's kid asks me whether the shades they're eyeing will actually work. The first thing I do isn't measure the width. I put a finger into the top of the recess, flat against the back of the opening, and feel how much room sits between that back plane and the front edge of the trim. That gap decides the whole job. Everything after it is bookkeeping.
If your finger can't get past the first knuckle before it hits the sash or the window hardware, inside mount is already in trouble.
The depth number settles inside versus outside before anything else
A recessed window only holds what the recess can swallow. Cellular shades are the forgiving ones. Most of their headrails are slim, and because the fabric collapses into a honeycomb that compresses nearly flat, a recess in the neighborhood of an inch often has enough room for the rail and the compressed stack above the glass. Roman shades are a different animal. They don't compress into a thin cell; they fold. A Roman's fabric folds back on itself in layers, and those layers are thickest at the top where they gather. That's where the two inches usually comes from. Not the hardware. The fabric.
So the honest test isn't "how deep is my window." It's "how deep is my window at its shallowest point, and how much does this particular treatment need to stack." A window can have plenty of depth in the middle and almost none at the outer corners where the casing meets. I've watched a perfectly deep opening fail at one corner because of how the trim was built out, and nobody catches that standing across the room.
Two things I check while my hand is still in there:
- Is the recess square? An out-of-square opening leaves a visible sliver of daylight along one side of an inside-mounted shade. Nobody notices it on a drawing. Everybody notices it at 7 a.m.
- What's already mounted in the recess? Crank handles, tilt hardware, and old storm-window fittings steal depth that a tape measure run down the middle of the opening never sees.
If either answer is bad, outside mount stops being a compromise and starts being the correct answer.
Outside mount is not the lesser option, it's a different trade
Hanging outside the recess means the shade mounts to the wall above the opening or to the face of the trim itself. No depth requirement. Nothing trapped between the glass and the sash. That's why outside mount rescues shallow recesses, crank-out windows, and openings where hardware already claims the space.
The cost is visible. An outside-mounted shade sits proud of the wall and covers a wider footprint than the glass. That's usually good for light control, because a Roman hung outside the opening overlaps the frame and kills the light leak around the edges that an inside mount simply cannot reach. But it also means the shade now lives in front of your trim. If the casing is the thing you love about that window, a full-width outside-mounted Roman will cover its top rail and soften the whole picture. That's a real trade, not a footnote, and it's the one people regret.
There's a width consideration that gets missed, too. Outside mount has to be wider than the opening to do its job. Mount it at exactly the width of the glass and the shade reads as a mis-sized panel with light pouring past both sides.
The mechanism is where Roman and cellular stop being interchangeable
This is the part that decides whether you still like the thing in three years.
Cellular shades run on several different systems. A cordless version uses a spring or tension mechanism inside the rail. You pull down, you push up, and the shade stays where you leave it. A cord loop version runs a continuous loop through a clutch. Motorized versions do the work for you. Cellular is also where top-down/bottom-up lives. The rail can drop from the top, rise from the bottom, or both, which is genuinely useful on a street-facing window where you want daylight without giving away the room.
Roman shades lift differently because the fabric is heavy and the whole point is the fold. Most Romans run on cords through a cord lock, or on a continuous loop with a clutch. The fabric has to gather in a controlled way or the folds go crooked, and once they're crooked they tend to stay crooked. Motorized Romans exist and they're lovely, but a motor pushing a heavy fabric stack is doing more work than a motor pulling a compressed honeycomb, and wide, weighty Romans are the treatments that push that hardware hardest.
Top-down/bottom-up is available in cellular far more often than in Roman. If that flexibility is the reason you started looking, it may push you toward cellular on its own.
Cord, cordless, and the safety question that isn't optional
Corded window coverings are a strangulation hazard for young children. The operating cords and the inner cords on older or cheaply built treatments are the specific problem. Loops, dangling pull cords, and any cord that can form a slack loop are the same category of risk, and a shade that's merely inconvenient to operate is not in the same conversation.
That's why cordless matters more than any aesthetic consideration on a window in a child's room, a guest room kids actually use, or any house with pets that climb. Cordless cellular shades solve it cleanly. Cordless Romans exist too, but heavy fabric on a cordless mechanism is a harder engineering problem, and the choices thin out as the shade gets bigger.
If a corded treatment is already on the window and you're not replacing it yet, the practical moves are keeping cribs and beds off the window wall and keeping cords gathered well out of reach. But if you're buying anyway, buy cordless on every window a child can reach.
Weight and width decide the hardware, so decide them early
The last thing I look at is how big and heavy the treatment will actually be. It matters more than the fabric swatch.
A wide window with a tall drop is a lot of material. Cellular fabric is light for its size. Roman fabric is not. A Roman spanning a wide opening carries real weight on the mounting hardware and on whatever raises it, and that weight decides whether the hardware is adequate and whether the mechanism feels smooth in year two or gritty. Slim hardware on a heavy Roman is a slow-motion failure, and it usually announces itself as sagging folds long before anything comes off the wall.
So the order I'd give anyone: figure out the depth, pick inside or outside mount, then pick the mechanism that matches the size, weight, and safety needs of the house. Width and drop come last in that sequence, but they feed straight back into the first three decisions, which is why picking them first is where most of the trouble starts.
The mistake I see most often is falling for a fabric, ordering it, and only then discovering the recess is three-quarters of an inch deep or a crank handle lives in the way. The fabric was never the decision. The window was.
