Roof lantern sightlines and glazing bar thickness

What a sightline actually measures, and the two ways it is quoted

There is no single agreed definition, which is the root of most confusion.

Some manufacturers quote the face width of the rafter: the visible width of the aluminium as seen straight on. Others quote glass to glass: the gap between the edge of one pane and the edge of the next, which includes the rafter plus the two gasket lines either side of it and is therefore always the larger number. A system honestly described as a 52mm sightline on face width may be 68mm glass to glass. Both figures are true and they are not comparable.

Ask for both, in writing, with a note of which face they are measured on. A supplier who cannot produce those two numbers in an email is quoting from a brochure rather than from a technical sheet.

External face width, internal face width, and why they differ on the same rafter

A thermally broken lantern rafter is not one section. It is an outer aluminium extrusion, a polyamide thermal break, and an inner extrusion, and the outer and inner are usually different widths on purpose.

Outside, the section is kept as narrow as the glass support allows, because that is the face the neighbours and the planning officer see. Inside, the section is often wider, because it has to carry the glazing pocket, the drainage channel and the fixing for the internal cover trim. A rafter reading 52mm from the garden can be 75mm from the sofa. Since the internal face is the one anybody actually spends time looking at, the internal figure is the one worth pressing for.

Rafter depth carries the span, and it is not the number in the brochure

Width and depth are separate dimensions and they do different jobs. The width is what you see. The depth, the dimension running down from the glass into the room, is what resists the bending load from wind, snow and the weight of the glass itself.

A slim rafter is slim in width and usually deep in section: 50mm wide by 100mm or more deep is a common profile, and on a long rafter run it goes deeper still. That depth is invisible from outside because you are looking at the narrow face, and unmissable from inside because it hangs into the room. On a lantern with a shallow pitch the deep rafters read as a coffered grid overhead, which some people like and some do not, and it is worth looking at a real section rather than a drawing before signing off.

Ridge width, hip width, and the corner where three sightlines meet

The rafters are the thin part. Everything else on the frame is wider.

A ridge extrusion on a domestic lantern is typically 70mm to 110mm across the top, wider still where a steel core has been inserted to control deflection over a long span. A hip rafter runs diagonally and has to receive glass on both faces at an angle, so it is broader than a common rafter, commonly 90mm to 130mm. At each top corner of a hipped lantern the ridge, two hips and sometimes a jack rafter converge, and that junction is the widest area of metal on the whole roof. On a small hipped lantern the four corner junctions can account for more visible aluminium than all the common rafters put together, which is one of the quiet arguments for a gable-ended unit on a narrow footprint.

Structural glazing against an external cap, measured in millimetres

Two ways of holding the glass down, and about 35mm of difference between them.

A capped system lays the glass on the rafter, then screws an aluminium pressure cap over the joint through the glass line. It is robust, it is cheaper, and the cap adds its own width on top of the rafter beneath, typically taking an 80mm rafter to 90mm or 95mm visible outside. The cap also stands proud of the glass, so it collects grit and moss along its upper edge and it interrupts the water running down the slope.

A structurally glazed system bonds the pane to the rafter top with a structural tape or silicone and leaves only a weathered joint between panes. Outside you see glass and a shadow line of around 12mm to 20mm, with the rafter visible only as a darker band beneath it. It is the reason modern lanterns read as glass from the garden. It costs more, the tolerances in manufacture are tighter, and the panes are not individually removable in the way a capped one is.

Counting the bars before choosing the sightline

Rafter spacing on a lantern is set by pane weight, not by taste. Panes are commonly spaced 500mm to 1000mm apart, because a double glazed lantern unit runs about 32 kilograms per square metre and a wider spacing takes a single pane past a two-person lift on a scaffold.

So the bar count comes first. A 2000mm long lantern at 500mm spacing needs three intermediate rafters each side, at 1000mm it needs one. Four extra rafters at 62mm each is 248mm of aluminium added to the same footprint, which swamps the difference between two systems’ sightlines. Anyone comparing lanterns on sightline alone, without comparing pane spacing, is comparing the wrong number.

The eaves beam is the widest sightline on the lantern

Every discussion of sightline concentrates on the rafters, and the widest piece of metal is at the bottom.

The eaves beam or base ring runs the whole perimeter, it carries the rafter feet, it resists their outward thrust, and it houses the drainage channel that takes away condensate and any water crossing the glazing joints. It is commonly 80mm to 120mm wide seen from inside, and on a structurally reinforced version it is wider again. Add the internal plaster reveal below it and the visual band round the base of a lantern is often 150mm or more. That is why a lantern over a small opening can feel heavy even with slim rafters: the perimeter is fixed and the glass area is not.

The thermal break sits inside the sightline and costs width

The polyamide strip separating the cold outer extrusion from the warm inner one is typically 14mm to 34mm across, and every millimetre of it is inside the sightline you are measuring.

That is the trade nobody mentions. A wider break gives a warmer internal face, a better whole-unit U-value and less risk of condensation running down a rafter in January. A narrower break gives a slimmer bar. Sections built for very slim sightlines sometimes achieve it by shortening the break, which shows up not in the brochure but in the whole-unit U-value and, on a cold morning in a kitchen, on the aluminium itself. Ask what the break depth is alongside the sightline, and read the two together.

Sightline from the room below is not sightline from the garden

Perspective does most of the work here. From the garden you see the lantern obliquely, so the rafters foreshorten and the caps or shadow lines read as a set of parallel strokes. From directly underneath, you see the internal faces close to square on, from a distance of two to three metres, for several hours a day.

The consequence is straightforward. Money spent narrowing the external sightline improves a view you take a few times a week. Money spent on the internal face width, the internal colour and the depth of the rafter improves what you look at every evening. On most Gloucestershire jobs the internal face is where the specification should be argued.

Why a pale bar looks thinner than a dark one of the same width

Perceived width is not measured width. A dark internal frame against a white plastered ceiling gives a hard tonal edge, and the eye reads the whole grid as a strong object. The same section finished in an off white or a light grey merges into the ceiling, the edges soften, and the glass becomes the thing you notice.

The effect is large enough to beat a real 15mm difference in section width. A 70mm internal face in RAL 9010 white reads slimmer than a 55mm face in anthracite. Dual colour frames, dark outside and pale inside, exist precisely for this, and the finish options are set out on the roof lantern frame finishes page.

Shadow gaps, gaskets and the black line beside every pane

Between the aluminium and the glass on both sides of every rafter runs a gasket, and on a structurally glazed system there is a weather joint between panes on top as well. These read as black lines and they belong in the sightline arithmetic.

A typical arrangement adds 6mm to 10mm of visible gasket each side of an internal rafter face, so a stated 70mm internal sightline is nearer 85mm of visually continuous dark band. On a capped system the joint between the cap and the glass adds a similar line outside. None of this is a defect. It is simply the difference between the number in the specification and the width your eye measures from the dining table, and it is why the glass-to-glass figure is the more honest one to ask for.

Sightline against pane weight, because the two pull opposite ways

Slimmer bars mean wider panes for a given footprint, and wider panes mean heavier glass. Glass weighs about 2.5 kilograms per square metre for every millimetre of thickness, so a 6mm toughened outer with a 6.8mm laminated inner is around 32 kilograms per square metre before the frame is counted.

Take a 2000 by 1200 lantern from four panes per slope to three and each pane grows by a third. A 1.0 by 1.4 metre pane at 32 kilograms per square metre is about 45 kilograms and a two-person lift. Push the same slope to two panes and the individual unit is past 65 kilograms, which on a Chalford or Rodborough plot reached by a flight of steps is a mechanical lift and a different day’s work. Triple glazing takes the same pane past 85 kilograms and forces the spacing back down, which puts bars back into a design chosen for having few of them.

Matching a lantern sightline to bifold doors already in the opening

A rear extension usually has bifold or sliding doors across the garden elevation, and the lantern sits directly above them in the same field of view.

Bifold stiles are commonly 90mm to 120mm and slider interlocks can be 30mm to 50mm, so a lantern with 50mm rafters above a door set with 110mm stiles reads as two unrelated products. The frame that looks best is usually the one whose visible widths sit within about 20mm of the doors, and whose finish is the same RAL reference from the same coater where possible. Where the doors are already in and the lantern is being added later, take the door section width with a tape and specify to it rather than to a preference.

Conservation work, where a wider bar is the condition rather than the compromise

In Cheltenham’s conservation areas and on listed rear elevations through the wolds villages, the slim aluminium argument runs the other way. A conservation officer looking at a rear extension behind a Regency terrace, or a garden room beside a Cotswold stone cottage at Painswick or Northleach, is frequently looking for a frame that reads as joinery.

What gets consented in those cases is often a painted timber or timber-cored lantern with 75mm to 100mm sections in a dark heritage colour, deliberately not the slimmest thing available. That is a legitimate design position, and arguing for a 50mm structurally glazed frame on a listed rear elevation usually costs more time than it saves. Establish the constraint with pre-application advice before a sightline is chosen, since it settles the whole specification in one move.