Why a Veranda Never Fits Tight Between Two Walls

One of the more common site problems in this trade is the alcove: a rear elevation with a return at each end, a projecting extension on one side and a garage or boundary wall on the other, and a lovely rectangular space in between that looks as though it was designed for a veranda.

It nearly always can be, and it comes with a rule that surprises people. The assembly manual is explicit: due to expansion of the materials, the veranda may not be placed tightly between two walls; there must be a gap of 1.5 millimetres per metre of width. Not "should ideally have". May not.

Where the Number Comes From

Aluminium expands roughly 0.023 millimetres per metre for every degree of temperature change. That sounds negligible and is not, because the temperature range a dark profile actually experiences is enormous.

A graphite or anthracite extrusion sitting in direct July sun reaches well above air temperature — sixty degrees or more is entirely ordinary on a south-facing elevation. The same profile on a clear January night sits below freezing. Call that a swing of sixty-five degrees, and every metre of width changes length by about a millimetre and a half.

Which is exactly the figure in the manual. The 1.5 millimetres per metre is not a workmanship tolerance or a fitting allowance. It is the calculated movement of the material across a British year, and it has to go somewhere.

What Happens Without It

Build a five-metre veranda hard between two walls in February and you have installed something that wants to be seven and a half millimetres longer in July. It cannot get longer, so it does something else.

What it does is go into compression. The profile bows, usually upwards or outwards where there is least restraint. Glazed edges that were sitting comfortably in their pockets get pushed against metal, and glass is strong in the middle and vulnerable at the edge. Fixings into masonry get worked back and forth every warm afternoon and every cold night, and eventually loosen. Sealed joints that were designed to accommodate a little movement are asked to accommodate rather more than a little.

None of this fails on the day it is built. It fails in the third or fourth summer, which is precisely what makes it worth avoiding at the outset.

Where the Gap Goes

Practically, the structure is made a few millimetres narrower than the opening and the gap sits at one or both ends, closed with a flexible sealant rather than packed solid.

That distinction matters. Sealant in a gap of two or three millimetres, in contact with both faces, is a flexible joint that stretches and compresses with the seasons — which is why neutral cure, low modulus products are used rather than anything that sets hard. Mortar, packers or expanding foam in the same gap defeat the entire purpose, since you have filled the expansion joint with something that does not compress.

The finished result looks like a normal sealed junction. It behaves like a movement joint.

The Same Principle Right Through

Once you know to look for it, deliberate clearance is everywhere in these systems, and it is the most misunderstood thing about them.

Every glazed edge sits in a pocket around twenty millimetres deep with several millimetres of clearance around the pane, so no edge of the glass is ever in hard contact with metal. Glazing sits on rubber gaskets rather than being bedded in mastic, so panes can slide a fraction within their frames. Setting blocks carry weight at defined points rather than along a whole edge. Joints are bolted rather than welded, partly because welding aluminium weakens it and partly because a bolted joint can accommodate a little slip.

The small even gap somebody notices on handover day and worries about is almost always one of these. It is meant to be there.

Getting It Right on Site

Two practical points follow, and both belong to the survey rather than the installation.

The opening has to be measured properly, at the height the structure will sit, and both walls checked for plumb. A rear elevation is rarely square, and two returns are rarely parallel; an opening that measures 4,980mm at the bottom can be 4,940mm at gutter height. The manufacturing width is derived from the tightest point, minus the expansion allowance.

And there is a second rule for the same situation that catches installers out: where concrete bases are used, the posts cannot sit directly against a wall, because the base block is wider than the post. The manual's remedy is to saw the flange off one side of the post and drill two new holes so it can still be fixed in four places. Worth knowing before somebody discovers it with the structure half up.

What to Ask

If your veranda is going between two walls, ask what expansion allowance is being made and how the gap will be closed. The right answer is a specific number of millimetres and a flexible sealant.

Openings That Are Not Square

The expansion allowance assumes the opening is a rectangle. A good many are not, and that is a separate problem sitting on top of the same one.

Rear elevations move. A return that projects two metres can be out of parallel by twenty or thirty millimetres over that distance, and neither wall is necessarily plumb. So the working width is the narrowest point anywhere within the height the structure occupies, and the expansion gap comes off that. It also means the gap will not be equal at both ends, and that the sealed joint at each side will be a different width — which is normal and is why a flexible sealant rather than a rigid packing is specified. The place to establish all of it is the survey, with a tape at gutter height rather than at ground level.

There is a related habit worth adopting on any structure, whether it sits between walls or not. When you notice a small, perfectly even gap somewhere on a newly installed veranda — at the end of a profile, around a pane of glass, where the frame meets the render — the first assumption should be that it is deliberate rather than unfinished. Aluminium systems are designed around movement rather than against it, and the clearances are the mechanism by which that works. If something genuinely is wrong it will usually be uneven, or it will be somewhere water can get in, and both of those are worth raising.

If somebody tells you it will be made to fit exactly, wall to wall, they are either speaking loosely or they have not read the manual. On a glazed structure in particular, that is worth clearing up before anything is manufactured, because the width is fixed the moment the profiles are cut.