Veranda System Tolerances

There is a moment on most handover days when someone runs a hand along the edge of a glass roof panel and finds a gap. It is two or three millimetres, it runs perfectly evenly down the whole edge, and to anyone who has never built anything from aluminium and glass it looks like a job that was not quite finished. It is in fact the reason the roof will still be sound in fifteen years. A veranda is not cabinetry. It is not meant to be assembled to the millimetre, and the gaps, the pockets and the rubber are the parts doing the work.

At Roma Verandas the measurements that a structure is actually manufactured from are taken at survey, by the person who will be responsible for the result, and never lifted from a customer's own tape. This guide explains what tolerance means in an aluminium veranda system: how glass is sized against the opening it sits in, how much of each profile is taken up holding the edge of that glass, why aluminium and glass both have to be allowed to move, and why a structure built too tightly is a structure that eventually fails.

Accurate Is Not the Same as Tight

Two ideas get confused here. Accuracy is knowing the real dimension — the true distance between two posts, on the day, at the height the beam will actually sit. Tightness is building with no clearance. A system can be extremely accurate and still be full of deliberate gaps, and a good one is. Every glazed edge has a designed clearance around it, and the profile is shaped to swallow that clearance so you never see it.

The reason is that nothing else in the situation is precise. A house wall is rarely plumb; twenty millimetres of lean over two and a half metres is unremarkable on a rendered rear elevation, and a bowed brick course can move a fixing line by more than that across four metres. Aluminium extrusion is cut to within a millimetre or so, but it is then fitted to a building assembled by people with a trowel. Clearance is what reconciles the two.

Where the Five Millimetres Goes

Take a single roof panel. The opening it fills is measured to the back of the channels that will hold it — the glass pocket — and the pane is then cut around five millimetres smaller than that opening in each direction. Roughly two and a half millimetres of clearance sits at each side, and roughly two and a half at the top and the bottom. That sounds like sloppiness. It is the opposite: it guarantees no edge of the glass is ever in hard contact with metal.

The pocket itself is generous. In most systems each glazed edge is held in a channel of about twenty millimetres, so even after the clearance is deducted there is something like seventeen and a half millimetres of aluminium covering the edge of the pane. That depth is not decorative. It exists so the glass can shift within its frame, so the gasket has something to seal against, and so the panel stays captured when everything around it has moved. The line you see from underneath is the edge of that pocket, not the edge of the glass.

The same logic sets the width of the profiles. A perimeter bar — the one running down the outer edge of the roof, or against the wall — is typically around sixty millimetres wide. An intermediate bar, which has to carry the edge of a panel on each side and take load from both, is typically nearer eighty. Those numbers turn a measured opening into a set of glass sizes.

Measuring Inner Post to Inner Post

Openings in a veranda are measured inner face to inner face — post to post, not overall width, and certainly not the length of the wall. Everything is then subtracted from that figure. Suppose the clear dimension between the inside faces of two posts is four metres and the roof is to be three panels. Two perimeter bars at sixty millimetres and two intermediate bars at eighty account for two hundred and eighty millimetres, leaving three thousand seven hundred and twenty for the glass, or about one thousand two hundred and forty visible per panel. Add the twenty millimetres of pocket at each side and deduct the five millimetres of clearance, and each pane is cut at around one thousand two hundred and seventy-five.

Follow that arithmetic backwards and the point becomes obvious. An error of ten millimetres in the post-to-post figure does not stay ten millimetres. It is divided across the panels, and it lands as three or four millimetres of extra gap at every edge, on top of clearance that is already there for a reason. Get it wrong the other way and the panes will not go in at all. This is not a difficult problem, but it is one that has to be right before glass is cut, not discovered on the day the roof arrives.

Aluminium Moves, and So Does Glass

Aluminium expands roughly 0.023 millimetres per metre for every degree of temperature change. That figure sounds trivial and is not. A dark profile in a colour like RAL 7016 can sit at well below freezing on a January night and reach sixty degrees or more in direct July sun, and across a sixty-five degree swing a six-metre beam changes length by something close to nine millimetres. Glass moves too, but only about a third as much. So the frame and the panel it holds are constantly growing and shrinking at different rates, in different directions, every day of the year.

That differential movement is why glazed systems are dry glazed on rubber rather than bedded solid. Extruded gaskets, usually EPDM, grip the pane firmly enough to seal and stay put, but they deform rather than resist, so the glass can slide a fraction within its pocket without anything being stressed. Setting blocks carry the panel's weight at defined points near the corners rather than along the whole bottom edge. Joints between profiles are clamped and bolted, not welded, partly because welding aluminium weakens it either side of the weld and partly because a bolted joint can accommodate a little slip.

Remove the clearance and you remove all of that. Glass is strong in the middle and vulnerable at the edge, and a pane whose corner is bearing directly on aluminium has a point of concentrated stress that will be loaded and unloaded every warm afternoon for the life of the structure. Gaskets crushed solid have nowhere to go and stop draining. Beams with no room to grow bow instead. A too-tight veranda does not fail on the day it is built. It fails two or three summers later, which is precisely what makes the mistake worth avoiding.

Gables, Angles and Compounding Error

Tolerance gets harder where the geometry stops being rectangular. The triangular infill at the sloped end of a veranda roof is the clearest example: a glass gable panel has one edge following the roof pitch, and its accuracy depends not only on the width and height of the opening but on the pitch angle being exactly what the drawing says. On a nine-degree roof, an error of half a degree in the pitch shifts the high corner of a three-metre gable by around twenty-five millimetres. There is no pocket deep enough to absorb that.

The same applies wherever the structure meets something irregular — a bay, a conservatory return, a wall that steps out for a kitchen extension. In those situations the surveyor is recording angles and out-of-square as well as dimensions, and the manufacturing figures are derived from both. It is why a veranda that is straightforward on a flat elevation becomes genuinely bespoke once a gable or a corner is introduced.

Why the Surveyor's Figures Govern

None of this makes a customer's own measurements pointless. They are genuinely useful for working out roughly what will fit and whether the project is viable at all, and we would always rather have approximate figures and photographs at enquiry stage than nothing. What they cannot do is drive manufacture, because they do not carry the things that decide glass sizes: the true post positions once the ground is understood, the wall's deviation from plumb, the finished beam height, the pitch actually achievable under the eaves.

So if you notice a small, even gap at the edge of a panel, or a beam that is not quite hard against the render, the honest answer is usually that it is meant to be there. Aluminium systems are designed around movement rather than against it, and the tolerances are the mechanism. If anything about the fit of a proposed structure concerns you — a tight corner, an awkward gable, a wall you already suspect is not straight — that is worth raising at survey, when it is still a drawing and not a delivery.