Fabrication Guide

How custom mirrors are measured, fabricated and mounted

A custom mirror looks effortless once it's on the wall, but getting there depends on a few unglamorous decisions made well before fabrication: when the wall is measured, how much tolerance is built in, and what the mirror is actually going to hang from. This guide walks through that process from measurement to mounted, finished panel.

Written by the Gloria Glass installation team

Why mirrors are measured last

Mirror glass cannot be trimmed, notched or drilled once it leaves the shop, so the measurement it's built from has to reflect the wall exactly as it will be when the mirror goes up — not as it's planned to be. That means tile, paint, cabinetry, sconces, outlets and any other fixture the mirror will sit against or clear should be in final position before we measure. Measuring against drawings or a wall that's still mid-build introduces a gap between the plan and reality that a fixed piece of glass cannot absorb later.

Out-of-square walls, bowed walls and tile lippage

Very few walls are truly flat, square and plumb, and mirrors — being rigid, unforgiving panels — reveal those imperfections more than most finishes. A wall can be out of square at the corners, bowed slightly across its width, or have tile lippage where individual tiles sit at a slightly different plane. Two common approaches manage this: template or scribe fabrication, where the panel edge is shaped to follow an irregular boundary such as an uneven ceiling line or a curved wall, and deliberate reveals, where the mirror is sized intentionally smaller than the opening so a consistent margin is visible on every side regardless of how the wall varies. Which approach fits depends on the wall condition found during measurement.

Tolerance: why exact-fit cuts are a risk, not a goal

It's tempting to assume a tighter fit is a better fit, but a mirror cut to the precise nominal dimension of an opening is gambling that the opening is exactly that size everywhere, on a wall that almost never is. A small deduction — a deliberate, planned gap — gives the panel room to install cleanly even where the opening runs slightly under its nominal size, and avoids a mirror that binds against one edge while gapping visibly at the other. That gap is then covered by trim, a reveal detail, or simply left as a clean, consistent shadow line, depending on the design.

Cut-outs and reference-point discipline

Outlets, sconces, wall-mounted faucets and recessed TVs each require a cut-out or notch positioned precisely relative to the mirror's edges. Because that position is fixed permanently at fabrication, it's recorded from multiple fixed reference points on the wall — not estimated from a single measurement that could itself be slightly off. The shop drawing generated from that layout is the document worth reviewing carefully before approval, since a cut-out that's a fraction of an inch out of position on a finished mirror cannot be adjusted afterward.

Edge treatment

Any mirror edge left exposed rather than concealed behind a frame or trim is finished — polished, bevelled or otherwise — both for appearance and to remove the sharp edge left by cutting. Which finish suits a given installation is largely an aesthetic decision, and we cover the options and their tradeoffs in more depth in our edge finishes guide.

Backing and substrate: what the mirror is going onto

Drywall on studs, drywall without solid backing, plaster over lath, tile set on cement board, and masonry all take fixings differently. Drywall alone, without a stud or added blocking behind it, is generally not adequate to carry a mirror of meaningful size or weight on its own — the fixing needs something more substantial to bite into. Tile over cement board can usually take mechanical fixings through the tile, but drilling through finished tile carries its own risk of cracking, which is a factor in choosing between a mechanical and an adhesive approach. Identifying the actual substrate, not just assuming it from the wall's appearance, is part of the measurement visit.

Adhesive vs. mechanical mounting

Safety backing films

A backing film applied to the rear of a mirror is intended to hold fragments together if the glass is ever broken, rather than letting them fall as loose pieces. It's a consideration weighed against the mirror's location, size and the mounting method, rather than something applied to every mirror by default, and whether it's appropriate for a given installation is worth discussing at the specification stage.

Oversized mirrors, seams and access

Very large mirrors run into a practical ceiling well before an aesthetic one: a single panel has to physically get from the shop to the wall, which means fitting through doorways, stairwells and elevators, and being safely carried and lifted by the number of people available on site. Where a single panel isn't feasible, a panelised layout — two or more mirrors set out with deliberate, evenly spaced joints — achieves the same coverage while staying within safe handling and access limits. Joint placement is planned so it reads as intentional rather than as a limitation of the install.

Humidity, ventilation and desilvering

Mirror backing is vulnerable at the edges, where moisture can reach behind the reflective coating and begin breaking it down — visible as the dark, cloudy edge damage often seen on old bathroom mirrors, known as desilvering. It tends to start at an unsealed or worn edge and creep inward over time. Adequate bathroom ventilation, edge sealing appropriate to a wet location, and avoiding standing water at the bottom edge all reduce that risk, though none of them make a mirror immune to it indefinitely.

Gym and studio wall mirrors

Continuous mirrored walls for gyms and studios bring their own planning questions: panel joints should land at heights and positions that don't interrupt the sightline where people actually look, mounting height needs to suit the activity, and the bottom edge often benefits from protection against impact from equipment or floor-level traffic. Wall flatness is the real limiting factor on these runs — an uneven wall produces visible distortion across a large mirrored surface that no mounting method can correct, so it's assessed during measurement rather than discovered after installation.

Care and cleaning

A non-abrasive glass cleaner applied to a cloth, rather than sprayed directly onto the mirror, keeps liquid from pooling at the edges where it can work behind the backing. Abrasive pads and harsh chemical cleaners can dull polished edges or damage certain decorative coatings, so a gentler routine generally outlasts a more aggressive one.

Pre-measure checklist

• Tile, paint and any wall finish behind the mirror are complete
• Outlets, sconces, faucets and cabinetry are installed in final position
• Rough dimensions of the intended mirrored area, if known in advance
• Preferred edge treatment, if there's an existing preference
• Whether a single panel or a panelised layout is expected, given access
• Any existing mirror that will need removal, and its current fixing method
• Whether the space is a wet area requiring moisture-appropriate backing

Full detail on custom mirror applications, edge options and what we need for an estimate is on the custom mirrors service page. If your project also involves a frameless shower enclosure, mirror and shower glass are often measured and specified in the same visit.

Frequently Asked Questions

Why can't a mirror be cut to the exact size of the opening?

Walls are rarely perfectly square or perfectly flat, so a mirror cut to the nominal opening dimension can bind against an out-of-square corner or leave an uneven gap on the opposite side. A small, deliberate tolerance or reveal is built in so the panel installs cleanly against the real, imperfect wall rather than the drawing's idealized one.

When should a wall be measured for a mirror?

After tile, paint, cabinetry and any fixtures the mirror needs to clear are in their final position. Measuring earlier risks a mismatch between the shop drawing and what actually gets built, and because mirror glass cannot be trimmed on site, that mismatch cannot be corrected after fabrication.

How is a large mirror mounted without visible hardware?

Options include a bottom J-channel with top clips that grip the edges, standoff hardware that holds the panel slightly off the wall, or a concealed cleat system that hangs the mirror on a hidden rail. The choice depends on the mirror's weight, the wall substrate, and whether the mirror needs to be removable for future access.

What causes the black edge damage I sometimes see on old mirrors?

That's typically desilvering — moisture reaching the reflective backing at an unsealed or worn edge and breaking it down over time. It generally starts at the perimeter and creeps inward, which is why edge sealing and keeping standing water off the bottom edge in wet areas matter for longevity.

Ready to talk through your wall and layout? Reach us through the contact page with rough dimensions and photographs of the space.