Barrel vault and northlight rooflights

Most domestic rooflights are flat or lightly pitched rectangles. The commercial world is different, because the buildings are bigger, the roofs span further, and the people underneath are working rather than passing through. That produces two forms you rarely see on a house but constantly on a warehouse, a school, a retail unit or a leisure building.

The barrel vault is a rooflight bent into an arc. Seen from the end it reads as a semicircle or a shallower segment of one, glazed in curved panels and carried on a series of ribs. It covers openings that a flat panel could not span cleanly, and it does it while shedding rain and adding height to the space below. You find it over shopping arcades, entrance canopies, swimming pool halls, glazed links between two buildings, and the top-lit corridors of schools and offices.

The northlight is the older idea and the cleverer one. It is a run of small pitched rooflights, each with a steep glazed face pointing north and a shallow opaque back facing south, repeated across a wide roof like the teeth of a saw. It was the standard way to light a Victorian weaving shed or engineering works, and the reasoning behind it is exactly the reasoning this business applies to every south-facing job in Southend. The glass faces away from the sun on purpose.

A barrel vault is defined by its span and its rise. Span is the width of the opening it bridges, measured across the base. Rise is how far the top of the curve sits above that base. The ratio between them sets the whole character of the roof. A true semicircular vault has a rise equal to half the span, so a three metre span rises 1.5 metres, which is a lot of height and a lot of surface area. Most commercial vaults are segmental instead, a flatter arc with a rise of perhaps a fifth to a quarter of the span, which sheds water and snow while keeping the profile lower and the glazed area down.

The glazing itself comes in three broad forms. Curved glass is the premium option, thermally formed to the radius, giving a smooth uninterrupted sweep and the best clarity, at the highest cost and weight. Curved multiwall polycarbonate is the workhorse: cold-bent to the radius on site or supplied pre-curved, light, tough, cheaper, and available in thicknesses from around 10mm up to 35mm. Faceted vaults use flat glass panes set between angled bars to approximate the curve, which suits glass because glass does not like being bent, and gives a segmented look rather than a true arc.

Polycarbonate earns its place here for reasons beyond price. It weighs a fraction of glass, so the supporting structure can be lighter. It resists impact, which matters on a roof that might be hit by hail or foot traffic. And a multiwall sheet carries its own insulating cavities, so a thick sheet reaches a respectable thermal figure without a sealed glazing unit. The trade is clarity: polycarbonate diffuses light rather than transmitting a clear view, and it can yellow over decades unless it carries a proper UV-protective surface. For the mechanics of the material and where it beats glass, our page on polycarbonate rooflight installation goes into the build-ups in detail.

The curve is not decoration. It does three practical things at once. It spans a wide opening without an intermediate support, because an arch carries load in compression along its length. It lifts the ceiling line, which makes a narrow space feel generous and lets light reach deep into the plan. And it sheds water and debris naturally, because there is no flat area for either to pool on.

That combination suits particular buildings. Circulation spaces are the classic: a corridor, a covered walkway, the link between an old building and a new extension. Retail and leisure use them over arcades and pool halls, where the height and the sweep of light do real work for the atmosphere. Entrance canopies use short barrel vaults as a signal of the way in. Across south Essex you see them on the newer retail parks and business units around Basildon and the A127 corridor, and on school and college buildings where a top-lit atrium ties several teaching blocks together.

The catch is orientation, and it is the same catch as everywhere else. A barrel vault running east to west presents one long face to the south. That face behaves like any large area of south-facing glass: it collects solar energy through the middle of the day and can drive the space below towards overheating. A vault running north to south spreads its exposure more evenly across the day but catches the low morning and evening sun on its flanks. There is no orientation that removes the question, only ones that change its shape, which is why the glass specification matters as much as the frame.

Here is the physics the Victorians worked out without a single calculation of G-value. In the northern hemisphere, a surface that faces due north receives almost no direct sunlight. The sun tracks across the southern half of the sky, so it never shines squarely onto a north-facing slope except for a brief low glance early and late on midsummer days. What a north-facing pane does receive is diffuse light: the soft, shadowless illumination scattered across the whole sky dome.

For a working building that is close to ideal. Diffuse north light is even from wall to wall, it does not throw hard shadows across a bench or a desk, it does not glare off a screen or a machine, and it changes very little from morning to evening. And because there is no direct beam, there is almost no solar gain through it, so the space stays cool without blinds, shading or air handling fighting the sun all afternoon. A weaving shed needed exactly that: flat, reliable light on the looms and a floor that did not cook in July. A modern warehouse, sports hall, print works or open-plan office wants the same three things.

The steep back slope of each tooth, the one facing south, is not glazed at all. It is an insulated, opaque roof surface. That is the deliberate part. The design gives up the free winter heat a south slope could have collected in exchange for never having to deal with the summer gain and glare it would also have delivered. On a building that runs machinery, computers or a lot of people, all of which generate their own heat, that is usually the right trade.

The original northlight was structural. The whole roof was built as a sawtooth, with trusses shaped to carry the alternating steep and shallow slopes, and the glazing was single-glazed steel-framed lights that leaked heat freely. Nobody builds that way now, because the thermal performance is unacceptable under current Building Regulations and the maintenance burden of that much single glazing is heavy.

The modern version is a prefabricated northlight rooflight: a self-contained unit that sits on a flat or low-pitch roof deck and reproduces the sawtooth geometry in miniature, repeated in rows across the roof. Each unit has an insulated south-facing back, a glazed north-facing face, upstands to lift it clear of the roof membrane, and a sealed glazing unit or insulated polycarbonate rather than bare single glass. It gives a flat-roofed building the daylight behaviour of a northlight roof without the structural roof shape, and it can be specified to hit the U-values Part L now demands.

The orientation still has to be got right on the ground. A northlight rooflight only delivers north light if it is installed with its glazed face genuinely pointing north, so the survey has to establish true north across the roof, not magnetic north and not the direction the building happens to face. Get the rows the wrong way round and you have built a south-facing solar collector on a building that was supposed to stay cool, which is the exact failure the design exists to prevent.

Once the shape is settled, the glass or polycarbonate specification does the rest of the work. Two numbers carry it. The U-value measures how fast heat escapes in winter, in watts per square metre per degree, and lower is better. The G-value measures how much of the sun’s energy passes through as heat, on a scale from 0 to 1, and on any surface that faces the sun, lower keeps the space cooler. Solar-control glazing, the kind we specify wherever a roof plane faces south, uses a coating to cut the G-value while keeping most of the daylight, and the reasoning behind it is set out on our page about solar-control rooflight glazing.

The two rooflight forms pull the specification in different directions. A barrel vault, with at least one face exposed to the sun, wants solar control on the sunlit side. A northlight, glazing pointing away from the sun, has little direct gain to manage, so it can run a clearer, higher-G glass and simply collect all the diffuse daylight going. The table below gives typical published figures for the common commercial build-ups. Treat them as the shape of the market rather than a fixed price list, because exact numbers vary by manufacturer and by whether a figure is quoted centre-pane or whole-unit.

Read the pattern rather than the individual rows. The polycarbonate options run cheaper and lighter but diffuse the view and reach a modest U-value only at the thick end. The glass options give clarity and a wider spread of G-value, so you can choose a clear high-G pane where a northlight faces away from the sun, and a low-G solar-control pane where a vault faces towards it. On a building with both forms on the same roof, the two units should not carry the same glass, and anyone quoting a single specification across the whole roof has not thought about which way each face points.

Both forms have to sit within sensible geometric limits, and the numbers below are the ranges you will meet in practice rather than absolute maxima.

Structure is where a commercial rooflight stops resembling a domestic one. A barrel vault carries wind uplift across a large curved surface, so the ribs and their fixings back to the roof structure have to be engineered, not assumed. Snow load matters on the shallower segmental vaults, where a flatter arc holds more of a fall than a steep one sheds. Thermal movement matters on any long run, especially in polycarbonate, which expands and contracts noticeably with temperature, so continuous vaults need expansion joints designed in from the start. A northlight roof spreads its load across many small units on a regular grid, which is kinder to the structure, but each upstand still has to be flashed and insulated properly to keep the thermal line continuous across the roof.

A large area of overhead glazing on a working building brings duties that a single loft window does not. Ventilation comes first, because a top-lit space stratifies: warm air rises to the highest point and sits there. A barrel vault has its warmest air trapped along the crown, so opening vents at the ridge, actuated electrically, clear that layer far faster than anything lower down. Northlight rooflights can be specified with opening lights in the glazed face for the same reason. Glass controls what arrives through the roof, and opening controls what leaves.

Safety glazing is not optional overhead. Any glass over a space people occupy needs a laminated inner pane, so that if it fractures the interlayer holds the fragments rather than dropping them into the room, and polycarbonate is inherently suited to overhead use for the same reason. Access for cleaning has to be designed rather than improvised: a curved vault and a rooftop grid of northlights both need a safe route across the roof and a way to reach the glass, and that is easier to build in at the start than to bolt on afterwards.

New and enlarged rooflights on any building are notifiable under Building Regulations. Part L sets the thermal standard the glazing has to meet, and Part K covers the safety glazing and any guarding at roof level. We make that notification to Southend-on-Sea City Council, or to the relevant local authority for the building’s address, as part of the installation. The full picture of specifying and installing large-format rooflights on commercial buildings sits on our commercial skylight installation page.

The choice usually settles itself once you name what the space is for. If the building needs steady, even, glare-free light across a wide floor and wants to stay cool through the summer, the northlight is the honest answer, and it is the one to reach for over a workshop, a warehouse, a studio, a sports hall or a deep open-plan office. If the building needs height, drama and a continuous sweep of light over a corridor, an atrium or an entrance, the barrel vault is the form that delivers it, and the specification job is then to control the solar gain on whichever face meets the sun.

The questions to put to whoever is quoting are the same ones that separate a considered specification from a catalogue order. Which way does each glazed face point, established from true north on the actual roof. What is the G-value of the glass proposed for the sunlit faces, and is it low enough for a surface that collects afternoon sun. What is the U-value, and is it quoted whole-unit rather than centre-pane. How is the vault or the rooflight grid ventilated, and are the opening lights at the highest point where the hot air collects. How is the glass reached for cleaning once the building is in use. And who notifies Building Control, which for our installations is us.

Skylights On Sea has been installing rooflights across south Essex for over fifteen years, every installation backed by a ten-year workmanship guarantee, and the orientation survey is where each job starts. If you are weighing a barrel vault against a northlight for a commercial roof, ask us for a specification that names the G-value and the U-value for each face separately, so the glass is matched to the way the roof actually meets the sun.