Taking a Sun Tunnel Up Through a Flat Roof

On a pitched roof, a sun tunnel dome sits on a flashing kit shaped to match tiles or slates falling away beneath it, and gravity does most of the work of keeping water moving. On a flat roof, the same dome sits over a surface that is never quite level but never steep either, so water can sit against anything that breaks the covering rather than running straight off it. That changes what actually decides whether the job is weathertight: not the flashing skirt itself, but the kerb it sits on, the fall of the roof around it, and how the membrane is dressed up and over the upstand.

A flat roof extension over a kitchen, a garage conversion, a dormer cheek or a bungalow's main roof all take a sun tunnel the same basic way: dome, kerb, a short run of tube, and a diffuser in the ceiling below. What differs from house to house is the roof build-up underneath that dome, and getting that build-up right is most of what separates a flat roof sun tunnel that lasts from one that is back on the ladder within a year.

Every flat rooflight and every flat roof sun tunnel sits on a kerb, a raised timber or GRP upstand that lifts the glazing or the dome clear of the roof surface. On a pitched roof a flashing skirt can sit almost flat against the tiles because water is already moving away downhill. On a flat roof the kerb has to be tall enough that standing water, wind-blown rain and the odd bit of debris sitting against the base never reaches the seal where the membrane meets the upstand. A kerb height of around 150mm above the finished roof covering is the figure installers work to as a minimum, and a roof known to pond in wet weather is often given more.

The upstand also has to be insulated, not just structural. An old timber kerb with no insulation in it becomes a cold bridge straight through the roof build-up, a strip of warm-to-cold contact running the whole way round the tunnel's base, and in winter that shows up as a ring of condensation or even frost on the tube just below the kerb. Modern kerb kits build the insulation into the upstand itself, so the thermal line of the roof carries on unbroken around the opening rather than stopping dead at the kerb's timber.

A flat roof is never actually flat. It is built with a deliberate fall, so rain finds its way to an outlet or a gutter rather than sitting on the covering, and that fall has to be planned around a rooflight or a sun tunnel kerb, not just around the roof's edges. A common design fall for a warm flat roof is around 1:80, allowing for the roof deck to sag slightly over time without water starting to pond against anything raised above it, and a roof with a known ponding problem may be built to a steeper fall again.

• The kerb sits proud of the covering, so a shallow fall running straight at its upstand needs a cricket, a small diverting saddle in the membrane, to split the water round the corners rather than pushing it against the base.
• An outlet or gutter positioned below the dome's run-off point keeps water moving away from the kerb rather than collecting near it.
• A roof already ponding elsewhere, before a tunnel is even considered, is a sign the fall needs attention regardless of what glazing goes in.

None of this is unique to sun tunnels. It is the same reasoning behind

a flat rooflight's upstand and glazing, and a survey on a flat roof checks the same things for a dome as it would for a full flat rooflight: where the water is actually going once it leaves the kerb, not just how the kerb itself is built.

A design fall built into the roof deck also means the kerb itself is rarely sitting perfectly level, and the dome on top of it has to be. A kerb built square to a falling deck would leave the dome tilted very slightly off the horizontal, which does very little to how much light it collects but can leave a shallow pool sitting against one edge of its base after rain. A tapered adaptor between the kerb and the dome's own collar corrects for the fall, so the dome sits level while the kerb underneath it still follows the roof's own slope down to the outlet. It is a small part, easy to miss on a supplier's parts list, and worth confirming is included before a flat roof kerb is ordered rather than after it arrives without one.

Flat roofs are built one of two ways, and which one sits above the room changes where a sun tunnel's insulation has to go. A warm roof carries its insulation above the structural deck, under the waterproof covering, so the timber or concrete deck itself stays close to room temperature. A cold roof carries its insulation below the deck, between or under the joists, leaving the deck itself exposed to outside temperatures with a ventilated void beneath it.

On a warm roof, the kerb's own insulation simply continues the roof's insulation line up and over the opening, which is the simpler case. On a cold roof, the tube passes through a ventilated, unheated void on its way down to the ceiling, much like it would through a loft on a pitched roof, and that stretch of tube needs its own insulated sleeve so the same condensation risk covered on do sun tunnels get condensation does not simply move from the kerb to the tube a little further along. A cold roof also needs its ventilation gaps kept clear around the tube, rather than blocked by the insulation sleeve, so the void it depends on keeps working the way it was designed to.

Many flat roofs sit directly over a room with no loft space above it at all: a single-storey kitchen extension, a garage with a room built over it, or a dormer's flat cheek tucked in above a landing ceiling. In that case the run from kerb to ceiling can be short enough that it barely counts as a tube run, sometimes no more than a spacer between the kerb and a diffuser fixed almost straight to the plasterboard below it. The detail is covered fully on fitting a sun tunnel with no loft space, and a flat roof extension is one of the two situations, alongside a room built hard against the rafters of a pitched roof, where that short run comes up most often.

A short run changes very little about the kerb, the fall or the flashing, all of which still have to be right regardless of how far the tube travels afterwards. What it does change is how much choice there is over the tube's diameter, because a short, straight run loses very little light on the way down, so a smaller dome can still deliver a bright result where the same diameter over a long, bent loft run would leave a room disappointingly dim.

Getting the collar wrong for the covering is the point where a flat roof job most often lets water in later, which is the flat-roof version of the same warning covered for pitched roofs on flashing a sun tunnel into a slate roof. On a flat roof, the wrong collar rarely fails on day one; it tends to fail slowly, as UV and weather work at a joint that was never properly bonded to the membrane it sits against, which is why the collar and the covering are matched at survey stage rather than chosen off a shelf on the day.

Where a flat roof runs up to a parapet wall rather than a simple open edge, as it does on plenty of the flat-roofed extensions built onto Fletton-brick terraces and new-town semis across Bretton and Orton, the kerb also has to keep a sensible distance back from the upstand at the wall itself. Too close, and the two upstands compete for the same run-off, with water backing up in the gap between them in heavy rain rather than reaching the outlet either was built to serve.

A sun tunnel through a flat roof is a roof penetration in the same way a rooflight is, and it is treated the same way under current Building Regulations: the concern is the weathertightness and insulation of the finished opening, not anything about the room it lights. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, which on most flat roof sun tunnel jobs comes down to confirming the kerb, the insulation continuity round it and the flashing detail, all covered in full on what Building Control expects of a sun tunnel.

Where the work sits within a larger flat roof recover or replacement, the same notification generally covers the roof and the tunnel together, since both are part of the one finished detail being signed off, rather than two separate pieces of paperwork for what is really a single job.

A flat roof survey for a sun tunnel spends more time on the roof itself than on the room below it. That means lifting a corner of the existing covering where it can be checked without damage, establishing which of warm or cold construction is underneath, measuring the fall across the area the kerb will sit in, and confirming there is nowhere nearby, an outlet, an upstand at a parapet, or a rooflight already in place, that the new kerb would interrupt.

Only once that is settled does the loft-side question of the route through to the ceiling come into it, and on a flat roof that route is often the shortest, simplest part of the whole job precisely because the roof structure above the room is so shallow. The parts of the sun tunnel itself, the dome, tube and diffuser, work no differently on a flat roof than on a pitched one; it is only the roof they sit on that changes the specification, and a survey that treats a flat roof kerb as an afterthought to a standard flashing kit is the one most likely to need revisiting.

Does a flat roof sun tunnel need a bigger kerb than a pitched roof flashing?

Usually yes in height, though not necessarily in footprint. A pitched roof flashing can sit close to the tiles because the pitch itself moves water away. A flat roof kerb has to lift the seal clear of standing water and wind-blown rain on its own, which is why 150mm or more above the covering is the figure worked to rather than a slimmer, low-profile skirt.

Can an existing flat roof rooflight opening be reused for a sun tunnel instead?

Sometimes, if the existing kerb is sound, correctly insulated and the right size, but it is checked rather than assumed. A kerb built for a rooflight is not always the right height or the right diameter for a dome and collar, and reusing one that has already started to fail simply carries the same fault into the new fitting.

Is a flat roof sun tunnel more prone to condensation than a pitched roof one?

Not inherently, provided the insulation round the kerb and, on a cold roof, along the tube, is continuous. The risk comes from a broken thermal line, not from the roof being flat, and a well-detailed flat roof kerb performs the same as a well-detailed pitched roof flashing.

Does the flat roof's covering need replacing to fit a sun tunnel?

Not necessarily, but a covering already near the end of its life is worth replacing at the same time rather than cutting a fresh opening into material that will need lifting again in a few years regardless.