Walls That Aren't Straight: What Polycarbonate Forgives and Glass Does Not

Almost no house wall is straight. Rendered rear elevations bow, brick courses wander, extensions meet original walls at not quite the same plane, and a wall that reads as flat to the eye can be twenty or thirty millimetres out over the length of a veranda. That is entirely normal and it is not a defect in your house.

What it does mean is that the roof material you choose changes how forgiving the installation is. Polycarbonate can absorb a wall that wanders. Glass cannot, and understanding why is the difference between a specification that works and one that arrives on site and does not.

How Much Deviation Is Normal

Worth calibrating, because people either assume their wall is perfect or assume it is a disaster.

Twenty millimetres of lean over two and a half metres of height is unremarkable on a rendered elevation. A bowed brick course can move a fixing line by more than that across four metres of width. Where an extension meets the original building, a step of ten or fifteen millimetres between the two planes is routine.

None of that stops a veranda going up. All of it has to be measured at survey rather than assumed, which is why the manufacturing dimensions come from somebody standing at the wall with a straightedge and a level at the height the profile will actually sit — not from a tape run along the skirting board inside.

Why Polycarbonate Forgives

Multiwall polycarbonate is light, flexible and — crucially — cuttable on site with ordinary tools.

That gives an installer two ways to deal with a wall that is not straight. The sheets can be trimmed to follow the actual line rather than the theoretical one, taking a few millimetres off along the length where the wall bulges. And any remaining discrepancy hides under the wall profile and its trim, provided there is enough profile depth and enough spare trim to cover it.

That second point is the one to check when ordering. A polycarbonate installation into an irregular wall wants a bit more trim than the drawing strictly calls for, because the cover pieces are what conceal the variation. Running out of trim on the last two metres is a small problem that becomes a visible one.

Two cautions. A cut edge in multiwall sheet leaves the flutes open, and open flutes fill with water, dust and algae, so every cut edge needs taping or capping the same way the factory edges are. And trimming a sheet reduces how much of it sits under the cover strip, so there is a limit to how much you can take off before the fixing detail suffers.

Why Glass Does Not

Glass cannot be cut, drilled or trimmed after manufacture. Toughened glass gets its strength from a compressed surface skin locked in during manufacture, and any work on it afterwards releases that and destroys the pane. Laminated glass cannot be cleanly cut on site either.

So a glass panel arrives at exactly the size it was ordered at, and the only accommodation available is what the frame provides.

That accommodation is real but finite. Every glazed edge sits in a pocket around twenty millimetres deep, with a designed clearance of roughly five millimetres overall — two and a half per side, two and a half top and bottom — so no edge of the glass is ever in hard contact with metal. That clearance exists to allow for thermal movement and manufacturing tolerance, not to absorb a wandering wall.

Use it up on a wall deviation and you have removed the movement allowance the system needs. A pane whose corner is bearing on aluminium has a concentrated stress point that will be loaded and unloaded every warm afternoon for the life of the structure, and glass is strong in the middle and vulnerable at the edge.

What Actually Happens on a Wonky Wall With Glass

The accommodation is made at the wall, not in the glazing. The wall profile is packed and shimmed to a true line, and the gap behind it is closed with a flexible sealant rather than being taken up by moving the glass.

That is why the manuals insist the surface where the wall profile sits must be firm, even and flat, and why making good a hollow before fixing is part of the job rather than an optional refinement. A profile bridging a dip has a gap behind it that sealant is being asked to span, and a bead spanning eight or ten millimetres sags while curing and pulls away from one face within a season or two.

On a badly out-of-true wall this can mean a genuine making-good exercise before the veranda starts — packing, a bedding mortar, or in extreme cases a render repair. That is a cost, and it is far better identified at survey than discovered on installation day.

The Second Rule People Forget

Where a structure goes between two walls, there is a separate requirement that interacts with all of this: it may not be fitted tightly, and there must be a gap of 1.5 millimetres per metre of width to allow for thermal expansion.

So on a five-metre alcove you are already giving up seven and a half millimetres of width deliberately, before any allowance for walls that are not parallel. The working width is the narrowest point anywhere within the height the structure occupies, minus the expansion allowance — and both walls need checking for plumb, not just measuring at the bottom.

What This Means for Choosing

None of it is a reason to avoid glass. It is a reason to be honest about the wall before ordering it.

If your elevation is reasonably true, glass is straightforward and the tolerances built into the system handle everything they were designed to handle. If your wall visibly bows, or an extension meets the original building mid-run, expect either some making-good or a slightly different design approach — sometimes the neat answer is to treat the run as two systems meeting at a coupling rather than one continuous piece, so each section is set out to its own bit of wall.

Getting a Survey That Catches This

Everything above is settled at survey or it is settled on installation day, and one of those is much cheaper than the other.

A survey that catches wall irregularity takes measurements at the height the wall profile will actually sit rather than at ground level, checks both ends of the run for plumb, and puts a long straightedge horizontally along the wall to find bows. It records the deviation as a number rather than a general impression, and it notes where the render changes, where an extension meets the original wall, and anything fixed to the elevation that will be in the way. If you are buying supply-only and confirming your own dimensions, those are the checks to make — and if anything comes back further out than about twenty millimetres, say so when you order rather than hoping the tolerances will absorb it.

The single most useful thing you can do before asking for a quotation on a glazed veranda is hold a long straightedge against the wall at roughly the height the profile will sit and see what the gap looks like. It takes five minutes, it costs nothing, and it turns a surprise into a line item.