Folded sheet metal
Folded sheet metal roofs, meaning standing seam coverings in zinc, copper, aluminium or coated steel, are the one common roof you can build a photovoltaic array on without making a single hole. Seam clamps grip the upstanding fold and are tightened to a defined torque, so load passes through the seam into the structure below. Choose the clamp by seam profile: double lock, angled seam or snap lock. Add rails and accessories. Guide
Related seam roof categories
- Trapezoidal sheet roofs
- Clamps and roof accessories
- Rail profiles
- Pitched roof systems
- Solar structures
Standing seam: mounting with no penetration
On a standing seam roof the covering is formed from long metal trays whose edges are turned up and folded together into a raised seam, held to the deck by hidden clips that let the metal slide as it expands. That gives the mounting system something no other roof offers: a continuous structural upstand running up the slope that a clamp can grip. A seam clamp is a machined aluminium block whose setscrews pinch the fold, and rails bolt directly to it. Nothing is drilled and nothing is sealed, which is why heritage zinc and copper roofs and warranted metal coverings are often approved only for this method. The trade-off is that the array hangs from the seams, so the clamp must match the seam profile and the seams must carry the load into the building.
FROM THE FIELD
"Photograph and measure the seam before ordering anything. A 25 mm double lock, a 38 mm angled seam and a snap lock look similar from the ground and take three different clamps. A clamp that does not fit the fold grips on the tips of the metal and either slips or creases the seam open, and creasing a zinc seam is a roofing repair, not an adjustment."
Ladislav Proc · Mounting systems specialist, ONSA Plus
How to choose a standing seam mounting system
Everything follows from the seam. Identify its profile and height, confirm the tray material and how the trays are clipped down, then pick the matching clamp and let the seam spacing set the rail layout. The array is only as strong as the folds it hangs on.
Show the full guide Close the guide
Matching the clamp to the seam profile
Seam clamps are made to a profile, not a size range, and the fit must be positive along the whole jaw. Identify the seam before specifying, with a photograph and a caliper measurement of height and thickness.
- Double lock standing seam: the classic zinc and copper detail, both edges folded twice into a tight upstand around 23 to 25 mm high. The stiffest profile for clamping.
- Angled seam, single lock: one fold, so a leaner and more open upstand. It accepts a clamp well but deforms more easily, so torque discipline matters more.
- Snap lock: trays snapped together over a hidden clip rather than mechanically seamed. A badly matched clamp can push the joint open, so use only clamps released for snap lock and check the reduced load values.
- Tray material: zinc, copper, aluminium and coated steel differ in softness and galvanic behaviour. Aluminium clamps on copper need isolation, and soft zinc marks under a hard jaw.
Torque, seam deformation and load transfer
The clamp works by friction between its jaws and the metal of the fold, so setscrew torque is a designed value: too low and the clamp creeps down the seam under snow load, too high and the seam is crushed, its lock sprung open or the metal indented so it fatigues there. Use a torque wrench and the datasheet figure, never feel. Just as important is what happens below: uplift travels seam, clip, deck, then structure, so the fixing of the trays to the deck must be capable of the array load. On an older roof with widely spaced clips the clamp count comes from a calculation and may exceed what the module layout suggests.
Tip from practice
A metal roof breathes along its length and the clamps travel with the trays they grip, so never rail rigidly between bays without allowing movement. Follow the system's guidance on where to lock and where to let rails float, and keep clamps clear of seam ends and transverse joints.
Layout, spacing and finishing
Seam spacing, typically 400 to 600 mm depending on tray width, sets the grid you can clamp on, so it is the equivalent of rafter spacing on a tiled roof. Cross rails span between clamps on adjacent seams, and modules are held with standard mid and end clamps inside the frame's permitted clamping zone. Two detailing points remain: cable entry, which still has to be made somewhere sealed such as a ridge or existing service penetration, and bonding, since anodised rail joints are not a reliable electrical path. Both are covered in roof accessories.
Frequently asked questions about standing seam mounting
Can solar panels be fitted to a standing seam roof without drilling?
Yes, and it is the normal method. Seam clamps grip the upstanding fold and rails bolt to them, so the covering is never pierced and no sealing detail is needed. It is the only common roof type that can be mounted on with no penetration.
How do I identify which seam clamp I need?
Measure the seam height and thickness and photograph the profile. Double lock, angled single lock and snap lock seams each take a different clamp, and a mismatched jaw grips only on the tips of the fold.
Can a clamp damage the seam?
Yes, if it is overtightened or the wrong profile. Excess torque crushes the fold, can spring the lock open and indents soft metals such as zinc. Use a torque wrench set to the clamp datasheet figure, not feel.
Will the seams carry the weight of an array?
Load passes from the clamp into the seam, then into the hidden clips and the deck below, and that whole chain has to be checked. Wide clip spacing or an older deck usually means more clamps than the module layout needs.
Does the clamp work on snap lock panels?
Only with clamps released for snap lock, and at the reduced load values the supplier states. Snap lock trays are pressed together over a hidden clip rather than mechanically folded, so an unsuitable clamp can push the joint open.
How is thermal movement handled if nothing is fixed down?
The trays slide on their clips and the clamps move with them, so the mounting must not lock separate bays rigidly together. Follow the system's guidance on which rail joints are fixed and which float.
Seam clamps must match the exact seam profile on your roof, and the load path runs through the seam into the structure below.
About ONSA Plus
Why installers across Europe order seam clamps from us
ONSA Plus is a European B2B distributor of photovoltaics, battery storage, heat pumps and EV charging stations. In our own EU warehouses we keep over 1,500 products from 25+ brands, including Huawei, Solis, SolaX and Dyness.
We are not just a trading company. We design and service energy systems ourselves, so we answer questions on standing seam clamps and load transfer from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
