Wall mirrors cut to size: assessing the wall before you order the glass

Why the wall should decide the order

Most people specify wall mirrors cut to size by settling on a size first and thinking about the fixing on the morning the panel arrives. That is backwards. The substrate changes the thickness you should buy, whether you need safety backing, whether the panel wants holes drilled in it, and occasionally whether you should be fixing a mirror to that wall at all.

It also changes the price, because wall mirrors cut to size for a solid brick wall can be screwed straight up, while the same panel on dabbed plasterboard needs an 18mm ply sub-frame built and skimmed round first. Finding that out after the glass is cut is how a straightforward order turns into a fortnight.

How to find out what your wall is actually made of

Tapping it tells you something but not enough. The reliable test takes five minutes. Drill a 6mm hole somewhere that will be covered by the mirror, go slowly without hammer action, and look at two things: the dust, and how far the bit travels before it meets resistance.

Fine grey-white dust is plaster or render. Behind it, orange or red gritty dust is brick. Pale grey crumbly dust is dense concrete block. Very light, almost fluffy dust that comes out fast is aircrete, the lightweight block used in a lot of post-war and modern construction. Dark grey grit with hair in it is lath and plaster, which is still everywhere in Victorian and Edwardian London. If the bit sinks 12 to 15mm and then falls into a void, you have plasterboard dabbed onto something. If it sinks 50mm into nothing, you have a stud wall.

That depth figure is the single most useful number you can give us over the phone. It tells us more than the age of the property, which is only ever a guess, especially in a house that has been extended twice and converted into flats.

London wall types, and what each will hold

Solid brick, nine inch, pre-1930s

The best case, and common in the inner boroughs. Any fixing works, the wall will hold any domestic mirror you can get through the door, and the only real care needed is to avoid drilling into an old flue or a buried cable run. On these walls we will often tell you the simplest method is also the right one.

Lath and plaster

Timber laths with lime plaster, found on internal partitions, chimney breast returns and ceilings in Victorian and Edwardian houses. The plaster holds nothing. It is brittle, it crumbles around a plug, and a fixing that feels tight on the day lets go quietly a few months later. Everything has to go into the timber studs behind, or into a batten or ply pad spanning two of them.

Plasterboard dabbed onto blockwork

Standard in refurbishments and in a lot of flat conversions. Our home page explains why bonding a mirror onto this build is a bad idea, so we will not repeat it here. The workable answer is to fix through the board and the dab cavity into the block behind, using a fixing long enough to get a proper anchorage depth, or to screw a ply pad through to the block and work from that.

Timber stud

Studs are usually at 400 or 600mm centres but in an older conversion they are wherever somebody put them. Find them, mark them, and do not assume a regular spacing. Noggins give you horizontal lines to fix into but you have to locate them, and a screw that catches only the edge of a stud is worse than useless because it feels solid.

Metal stud, common in new-build flats

Thin galvanised section. It will not take a heavy direct load the way timber will, and a coach screw into a metal stud simply strips. Heavy mirrors on metal stud want a spread load: ply across at least two studs, or proprietary cavity fixings with a published weight rating that you then derate.

Aircrete and lightweight block

Soft enough to drill with almost no pressure, which fools people into thinking it is strong. Ordinary plastic plugs spin in it. It needs the anchors made for the material, and more of them than you would use in brick.

Tiled walls

The tile is a finish, not a substrate. Anything you fix has to pass through the tile and load the wall behind it, and the tile must never carry weight. Drilling is covered in more detail on our bathroom mirrors page, because that is where it usually comes up.

How heavy is the mirror, and how many fixings does it need?

Glass weighs about 2.5kg per square metre for each millimetre of thickness. So 4mm mirror is roughly 10kg per square metre and 6mm is roughly 15kg. A 2,000 by 1,000 panel in 6mm is about 30kg before backing and packaging. A full wall of mirror 2.4m square in 6mm is around 86kg, which is why that job is never one panel.

The important point is not the number, it is the direction the load acts in. A mirror should be supported from below, so the weight sits in shear on a channel, a batten or a pair of screws, with the top restrained to stop it tipping forward. Panels that hang in tension off the top, or that rely on adhesive to resist a downward pull, are the ones that come off.

Every fixing manufacturer publishes a load rating, and every one of those ratings assumes the correct material, the correct hole, the correct embedment and a load in the right direction. Treat the published figure as a ceiling you never approach, use more fixings than you think you need, and never count the plasterboard as part of the structure.

Fixing methods, and when each is right

There are four in common use. Drilled holes with mirror screws and domed caps, which is simple, reversible and fine on solid walls. Mirror clips or a channel set, where a J-section takes the bottom edge and clips or a top channel hold the head, which is strong, reversible and does not require a single hole in the glass. Adhesive combined with mechanical support, which is the normal method for large flush panels. And a ply sub-frame fixed to the structure, with the mirror bonded and supported on the ply, which is what you use when the wall itself cannot be trusted.

Adhesive is where most of the damage happens, and it is a chemistry problem before it is a strength problem. Ordinary building silicones, solvent-based grab adhesives and most general-purpose tubes will attack the backing paint and then the silver. The result does not show up for months. It appears as a grey halo or black spotting that spreads outward from exactly where the adhesive sat, and it is permanent. Use a neutral-cure mirror adhesive or a proper mirror mastic and nothing else. If a fitter turns up with a tube of all-purpose grab adhesive, stop him.

The second adhesive rule is that the back of the mirror has to be able to breathe. Beads run vertically, spaced, with the ends open, so any moisture that gets behind the panel can dry out. A mirror buttered all over its back onto a cold wall traps damp against the backing and rots it from behind.

The walls we tell people not to mirror

A wall with active damp, salt staining or a tide mark. Put a mirror over it and you have hidden a problem that is still happening, and you will lose the mirror as well as the plaster. Fix the wall, let it dry out properly, then mirror it.

A chimney breast with a working fireplace or a live flue, because of the heat and the movement. A freshly plastered or freshly skimmed wall, until it is genuinely cured rather than merely touch dry. And any position where a single panel would bridge two different substrates, such as plasterboard meeting brick, or a run that crosses a boxed-in soil pipe. The two materials move at different rates and the mirror sits across the join taking the strain.

Sometimes the honest answer is not to fix it to the wall at all. A large mirror leaning on a floor plate, with a discreet restraint strap at the top, is a perfectly legitimate solution in a rented flat or where the wall is hopeless. It costs less than a sub-frame, it takes ten minutes, and you can take it with you when you move.

Where wall mirrors cut to size actually fail

Four ways, in roughly this order of frequency. Adhesive incompatibility, which is slow and invisible until it is not. Moisture reaching the edges, which blackens the silver inward from the perimeter and is covered on our bathroom page. Movement where the panel bridges a junction, which produces a crack running diagonally from somewhere near the join.

And the one nobody expects: over-tightened mirror screws. A drilled hole in annealed mirror has no give at all. The screw needs a soft grommet or washer in the hole, it needs to be finger tight plus a nip, and it needs the wall to be flat behind it. Someone pulling a mirror screw up hard against a bump in the plaster will crack the panel at the hole, usually a few hours after they leave.

What to challenge in a fitting quote

If the method described is bonded to wall with mirror adhesive and the panel is over about a square metre, ask two questions. What holds the weight while the adhesive cures, and what holds it if the adhesive ever lets go? If the answer to either is nothing, that is not a fixing method, it is an optimistic one.

The challenge runs the other way too. Do not let anybody sell you a steel sub-frame and a structural survey for a 700mm hall mirror on a solid brick wall in a Victorian terrace. Two holes, two plugs and two mirror screws is the correct specification there, it will outlast the house, and we will tell you that rather than quote for the frame.