Standing Seam Metal Roofing
A standing seam roof is not watertight because it is sealed: it is watertight because it is folded. The joint between two bays is an upstand folded over twice, standing well above the plane the water runs on, and not a single fastener pierces the covering. That shifts the design question: not "how do we close it", but "how do we let a metal that grows several millimetres per metre in summer move freely".
Overview
TAV. 00A double-lock standing seam roof is sheet metal folded on site: bays of copper or zinc-titanium joined by a continuous upstand, with no fasteners through the sheet and no sealant working in watertightness. The result is a very durable, light covering able to follow curved, conical and irregular roof planes as no other covering can. The difficulty, though, moves to two points that site work routinely underestimates. The first is the deck: metal does not correct its substrate, it copies it, and every hollow becomes a permanent shadow on the finished surface. The second is thermal movement: a bay is an element several metres long anchored to a substrate that does not move, and if it is held at both ends it distorts in summer and pulls on its fixings in winter. The phases below follow that logic: first the deck and its ventilation, then the edges that set the reference, then how the bays are anchored, and finally the seam itself and the singular points, which is where these roofs actually leak.
The site score
TAV. HThe detail of the craftthe plane the metal will copy
The deck: a continuous plane and a ventilated cavity
Above the structure the ventilation cavity is formed with battens running continuously from eaves to ridge, then closed with boarding or structural panels. The resulting plane must be continuous, dry and flat. Alignments and edge levels are checked, and above all that air can enter at the eaves and leave at the ridge without obstruction.
«Metal does not forgive the deck: what you get wrong here you will see for thirty years, every time the sun rakes low across it.»site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
The rule behind everything
Metal moves: deciding where it stays put
A metal roof does not have a strength problem, it has a movement problem. Every bay grows in the heat and shrinks in the cold, by a small amount per metre but not a negligible one over the full length of a roof plane. The design cannot prevent that movement: it can only decide where it starts from. A limited zone is chosen where the clips genuinely hold the sheet, and everywhere else clips are used that keep it down while letting it slide. The expansion then discharges towards the free edges, where the upstands and laps were designed to absorb it.
Indicative expansion figures, useful as an order of magnitude; bay lengths and clip layout follow the requirements of the system actually specified.
The mistake nobody sees
Not touching is not enough: what matters is who is upstream
Two dissimilar metals in contact with moisture set up a corrosion cell in which the less noble one is consumed to protect the other. On a roof, though, direct contact is only half the problem, and not the more insidious half. Water running over a noble metal carries traces of it away and deposits them on everything it meets further downstream: a less noble flashing or downpipe is eaten away even if it touches nothing, simply because it sits below. Hence the way to read a roof: look at the direction of fall before you look at the details.
A simplified nobility scale, sufficient for the commonest detailing choices; critical pairings should be checked case by case, also against the environment, whether marine or industrial.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QHow long can a single bay be?
With sliding clips, eight to ten metres is comfortably achievable, and some systems go further. The limit is not a single number, though: it depends on the metal, which expands by different amounts, on the roof's real temperature swing, which on a dark south-facing plane is far wider than the air's, and on the clip type, which has a finite travel. Past that travel the bay no longer grows freely and starts pushing again. When the roof plane is longer than one bay can handle, you do not force it: a transverse step is introduced, a change of level that breaks the continuity and creates two independent runs, each with its own fixed point.
Single or double lock: when is a single lock enough?
The difference is one extra fold, and that fold makes the water climb once more before it can get in. On steep pitches water runs off quickly and has neither the time nor the means to climb: a single lock is sufficient and quicker to execute. As the plane flattens, water dwells longer on the sheet, wind can push it uphill and capillary action starts working in the water's favour: from there on the double lock is needed. Below a certain pitch even a double lock is not enough on its own, and a gasket is added inside the fold. That choice is made at design stage, not on site, because it changes how the bays are roll-formed.
Can you walk on it?
Yes, but not everywhere and not freely. You walk on the flat of the bay, ideally over a clip, where the metal is supported by the deck; you never walk on the seam, because it is the one element holding the watertightness and a crushed fold never recovers. The real problem, though, is not the occasional trip, it is the recurring maintenance of whatever sits on the roof, typically plant, panels and aerials. If those are planned, a dedicated access route must be planned too: walkways and anchor points decided at design stage and integrated into the covering, not added afterwards by drilling through something that was just built without a single hole.
Copper turns green: how long does it take, and what happens meanwhile?
Copper does not go straight from red to green: it passes through a long brown phase running from warm chestnut to nearly black, during which the building looks very different from the render. That phase lasts years, and the true green patina arrives on timescales that vary enormously with climate: quickly in marine or humid polluted air, far more slowly in dry climates and clean air, where it can take decades. Two things should be said to the client beforehand rather than afterwards. The first is that the colour will develop unevenly, because the more washed areas change first. The second is that in the early years copper runoff leaves traces on the surfaces below, so the design of edges and drainage must keep copper away from pale stone, render and glazing, not because of corrosion but because of staining.