Can rooflights be fitted to a standing seam roof?

A standing seam roof keeps water out with raised, interlocking seams that run down the slope. Cut a hole in it for a rooflight and you interrupt those runs, so the job is to close the interruption cleanly rather than to caulk over it. That is done with an upstand, a raised timber or metal kerb built around the opening and dressed with the same metal as the rest of the roof.

The panels either side of the hole are trimmed and turned up the kerb, the seams are terminated into the upstand, and the metalwork is welted so that any water hitting the kerb runs back onto the roof and away. The rooflight then lands on top of the finished kerb on its own perimeter flashing, sitting proud of the roof surface. Height matters here: a kerb of at least 150mm above the finished metal is the sensible minimum, and lower than that on an exposed estuary roof is asking the wind to drive water over the top.

The one detail people skip is the upslope side. Water coming down the roof arrives at the back of the kerb and has to be split around it, not dammed against it. On anything but the smallest opening that means a back gutter, sometimes called a cricket, formed behind the upstand to divert the flow to either side. Leave it out and the metal behind the rooflight becomes a puddle every time it rains.

Standing seam earns its keep on low pitches. Some systems weather down to around three degrees, far shallower than tile will manage, which is exactly why architects reach for it on flat-looking modern roofs. That low pitch changes how you think about the rooflight.

Glass wants a fall on it. On a shallow roof a flat rooflight will hold surface water and collect dirt in the corners, and standing water reads as streaks and staining within a season. On a near-flat standing seam roof we would treat the opening the way we treat a flat roof: either a rooflight built with a slight fall across the glass, or a unit set on a tapered kerb so the pane sheds towards the low side. A little fall keeps the glass clean and stops the seals sitting in water.

If the roof is genuinely pitched, closer to fifteen degrees or more, the same principles that govern any pitched roof skylight installation apply, and the kerb detailing is more forgiving because gravity is already moving the water for you. The lower the pitch, the more the drainage detail earns its money.

A metal roof at a shallow pitch points at the sky rather than at a compass direction, and that is the same situation as a flat roof: it collects the high summer sun almost square on, whichever way the building is turned. Southend faces south across the Thames Estuary with an open horizon and reflected light coming off the water, so a low, sky-facing rooflight over a room here gathers a lot of solar energy across the cooling season.

This is where the specification does the real work. Ask for the G-value, the figure that tells you how much of the sun’s heat the glass lets through, not just the U-value that governs winter heat loss. Clear double glazing sits around 0.6 and passes better than half the sun’s energy into the room. Solar-control glazing around 0.3 roughly halves that while still letting plenty of daylight down. On a south-facing or near-flat metal roof, that difference is the gap between a bright room and one that runs like a greenhouse by mid-afternoon. It is the whole basis of our energy-efficient skylight installation work, and it applies to a standing seam roof exactly as it does to felt or fibreglass.

Standing seam comes in zinc, aluminium, coated steel and copper, and all of them expand and contract with temperature more than a tiled roof does. The kerb detailing has to allow for that movement, which is another reason the weathering is kept in the metalwork and clipped rather than fixed rigid against the glazing. A good upstand accommodates the roof breathing without opening a gap at the rooflight.

Most standing seam roofs are also warm roofs, with the insulation above the structural deck. When you form the opening you carry that build-up up the kerb so there is no cold bridge around the rooflight, otherwise the perimeter is where condensation shows first. It is worth checking the make-up before anyone cuts, because a rooflight dropped into a warm metal roof without insulating the kerb will drip in winter even when the roof itself is dry.

You see these roofs most on commercial buildings, workshops and retail units, and increasingly on architect-designed house extensions. On the commercial side the openings are often larger and repeated across a roof, which is its own discipline: we cover that under commercial skylight installation. The physics of the kerb is the same at any scale, but the drainage between multiple units on a big low-pitch roof needs setting out properly rather than unit by unit.

If the roof is already down and watertight, replacing or adding a rooflight is straightforward work: the opening is formed, a kerb built and dressed, and the metal panels terminated into it. What you want to settle first is the pitch of the glass, the height of the kerb and the glass specification for the way that roof faces. If the roof runs shallow or faces anywhere near south, plan on solar-control glazing from the start rather than wishing for it in July.

One thing to rule out early: if your problem is water already getting past an existing metal roof, that is a roofing job before it is a glazing one, and a metal roofer is the right first call. Where you are adding new glass or swapping a failed unit, tell us the roof material, the pitch and roughly which way it faces. Ring 01702 898232 or request a quote and we will specify the kerb and the glass together, so the opening is weathered by the roof and the light in the room is the light you actually wanted.