Width vs Projection

Two homeowners ask for a four by three metre veranda and mean entirely different structures. One pictures four metres along the back of the house coming three metres out over the patio. The other has it the other way round, and would be surprised by what turned up. The two dimensions are not interchangeable: they are limited by different parts of the structure and priced by different logic, so the mix-up is worth clearing up before you compare quotes.

At Roma Verandas we repeat both figures back with the words attached, because so many enquiries arrive ambiguous. This guide sets out what each dimension means, what physically limits it, why systems come in fixed size steps, what happens when the size you want falls between two of them, and why an extra half metre outwards costs considerably more than an extra half metre sideways.

Which Dimension Is Which

Projection is the depth of the structure out from the house: the distance from the wall to the front beam. Width runs along the house, parallel to the wall, and is also called the span. Most manufacturers quote the pair as width by projection, so a 4000 by 3000 veranda is four metres across the house and three metres out over the patio. The convention is not universal, and it costs nothing to say which is which.

A quick way to keep them straight is to think about which way the water runs. On a lean-to the roof slopes down away from the house into the front gutter, so projection is the direction the roof falls and width is the direction the gutter runs. Everything structural follows from that: the roof beams run in the direction of the projection, the front beam in the direction of the width.

Projection Is Limited by What the Beams Can Span

The roof beams span unsupported from the wall profile to the front beam, with nothing underneath them in between, and that single fact sets the ceiling on projection. As the span grows the load on the beam grows with it, and the amount it deflects in the middle grows far faster still: for a uniformly loaded beam, deflection rises roughly with the fourth power of the span. Double the projection and that beam sags in the order of sixteen times as much. So all else is not left equal — the beam section gets deeper and heavier, and the spacing between beams closes up.

The glazing has its own limit alongside. Each roof panel is a plate supported on its two long edges by the glazing bars, and there is only so far a given thickness of laminated glass or polycarbonate will run before it must be thicker. Going up a thickness adds weight to every square metre of roof, which the beams then carry, which pushes the beam section up again. The loads are real rather than theoretical: the roof is designed for snow as well as its own weight, and for wind trying to lift it off.

The practical result is that most domestic lean-to systems stop at somewhere between three and a half and four and a half metres of projection, with heavier-sectioned systems reaching further. Beyond that you are into a different structure rather than a bigger version of the same one. There is a quieter second limit too: because the roof is pitched, every extra metre of projection raises the fixing height at the wall by roughly 157 millimetres at nine degrees. Plenty of projections are ruled out by the soffit, not the beams.

Width Is Limited by Posts and Panel Sizes

Width behaves differently because it is extensible. The front beam spans between posts, and if you need more width you can in principle keep adding posts and bays along the wall. What limits width is not the total but the clear span the front beam achieves between one post and the next, which is a property of the beam section rather than of your garden.

Clear span is where the choice of system starts to matter. A slim entry-level beam might want a post every three to four metres. A deeper reinforced gutter beam of the sort used in the Nobile carries considerably more, reaching around seven metres on two posts — the difference between an uninterrupted view of the garden and a post in the middle of it. Wide-span models such as the Giallo and Pigato exist for that reason, and the heavier beam usually earns its cost where a post would otherwise land in front of the patio doors.

The other constraint is panel size. A veranda roof is glazed in a series of panels between glazing bars, and those panels come in standard widths — glass roof modules are commonly around a metre; polycarbonate sheet arrives in its own set widths. Width therefore increments in modules, and a system will offer sizes landing tidily on whole panels rather than leaving a narrow off-cut at one end.

Fixed Size Steps, and What Happens Between Them

Customers sometimes find it frustrating that a system offers 3000, 3500 and 4000 millimetres of projection and nothing in between. The reason is that a veranda is not a bespoke fabrication designed from first principles for each house. It is an engineered system of extrusions, brackets and glazing units whose combinations have been calculated and tested as a set, and every published size step has a verified beam section, beam spacing, panel thickness and post size behind it.

Width is the more flexible of the two: a front beam can often be cut between two nominal sizes provided the post spacing and glazing modules still work out. Projection is less forgiving, since changing it changes every roof beam and every glazing panel at once. Hence fine adjustment across the width and coarse steps in depth.

When your ideal depth falls between two steps, the usual answer is to take the next one down, which is more often right than people expect: 3000 rather than 3500 is rarely the difference between the space working and not working. The alternative is to take the larger size and let it oversail, since a roof can extend past the edge of the patio over a lawn or border, provided the posts land somewhere that can be given a proper foundation.

On width, the same question becomes one about an intermediate post. If the width you want exceeds the clear span of your chosen beam, you either move to a heavier beam or accept a third post along the front. Where it lands deserves a decision rather than arithmetic: two equal bays put a post dead centre, which on many houses is directly in front of the patio doors. Setting the bays out from the doors instead, so the post falls beside a window, gives a better result for the same money.

Why Half a Metre Out Costs More Than Half a Metre Across

Adding width is close to a linear exercise. You buy another length of front beam and gutter, one or two more glazing panels, more roof beams, and sometimes an extra post and foundation. Nothing that already existed has to change: the beam sections, the pitch and the wall fixing height are all unaffected. Cost rises broadly in step with the area gained.

Adding projection changes the specification of the whole structure. Every roof beam is longer and may need a deeper section. Every glazing panel is longer and may need to be thicker. The front beam carries a wider strip of roof, so it too may go up a size, pulling the post section and foundations with it. The wall brackets take more load and more uplift, and the roof rises at the wall. It is the difference between adding a leaf to a table and asking the table top to span further unsupported.

Deciding Which One You Actually Need

The most useful thing you can do before choosing is to mark it out. Chalk the width along the wall, run canes at the projection you have in mind, then put the real furniture inside the outline and sit in it. A dining table is usually around 900 millimetres deep and a chair pulled out needs another 750 to 800 millimetres behind it, so a table used from both sides wants close to three metres of projection to sit under cover properly. At two and a half metres you have a good sheltered seating area; at two metres, a generous porch.

In our experience people over-buy width and under-buy projection, largely because width is the cheaper dimension and looks generous on a drawing. What they notice afterwards is rain blowing onto the outer edge of the table. If you tell us what you intend to put under the roof and how you expect to use it, rather than starting from a size, we can work back to sensible dimensions and say honestly where the size steps fall and what the next one up would cost.