Flush L-frame vs. Z-frame: the two kinds of inside mount shutters

Both are inside mounts with a 1¼-inch jamb inside the window opening, and both shrink the window. A flush L-frame has nothing wrapped around the opening, so existing casing stays untouched. A Z-frame wraps 1 to 3 inches onto the wall, which gives a drywall-wrapped window a casing look.

The two kinds of inside mount: flush L-frame and Z-frame

Inside mounts come in two types. Both start with the same 1¼-inch jamb fastened to the inside of the window opening. The difference is what happens at the edge of the opening, on the wall you see.

Flush inside mount with an L-frame

The flush mount uses an L-frame and nothing wraps around the opening. It is the choice when the window already has casing, the molding around the window, and the goal is not to touch that casing. The shutter sits inside, and the existing trim stays exactly as it is.

Inside mount with a Z-frame

The Z-frame uses the same 1¼-inch jamb, but the frame wraps around the outside of the opening, typically 1 to 3 inches onto the wall. It is the choice when the window is wrapped in drywall with no casing around it. The Z that wraps onto the drywall gives the window a casing look it did not have before.

"They both shrink the window," says Dave. The Z-frame adds a trim look on the wall, but the jamb still sits inside the opening, so the panels are smaller than the window either way.

Does a flush inside mount need caulking?

Often, yes. Most windows are not perfectly square, and a flush mount sits inside the opening. The shutter can fit tight against the jamb at the bottom and leave a gap of up to ¼ inch at the top. Homeowners notice that gap, so the installer runs a thin bead of caulk along it.

Caulk comes in basic colors, mostly white and off-white. On a white shutter it blends in. On a custom paint color or a stained shutter, the caulk line can show, which is worth weighing before you choose a flush mount on an out-of-square window. A 1-inch Z-frame avoids the caulk line altogether.

When does a 1-inch Z-frame make sense?

A 1-inch Z-frame is the bridge between a flush mount and an outside mount. It is the choice when you want an inside mount but the window is too shallow or out of square. It does two things at once.

• It pulls the shutter out about ¼ inch, which gains the clearance a shallow window is missing.
• Its small lip covers the edge of the opening and hides an out-of-square window, so no caulk line is needed. On a window with casing, it overlaps the casing only a little.

At every consultation, Dave checks each window for square first. Then he shows the homeowner a flush L-frame and a 1-inch Z-frame side by side, using real samples, so they can see how much each one covers. Few shutter companies carry those samples or explain the difference.

Are L-frames and Z-frames standard stock sizes?

Through a broker or dealer, yes. They order standard frames and can't make adjustments or build their own. We often modify our L-frames, Z-frames and outside mount frames. We can cut down or narrow the L-frame, add a sill plate at the bottom, or change the top and bottom rails, because we do the design ourselves, not a computer program or a fill-in order form.

Before you buy, ask who is designing your panels: the width, the height and the number of louvers. On narrow windows especially, ask how much window you will lose. Dave sees the same problem with replacement windows, where glass that once ran almost to the jamb lost 4 inches of width and height. Homeowners find that unsettling, and the same thing can happen with shutters if nobody plans for it.

This is part of our inside vs. outside mount guide. To see the options in person, visit our factory and showroom in Lake Forest, or call 949-951-0600 for a free in-home consultation.