Profiles
Profiles are the aluminium mounting rails that carry photovoltaic modules between the roof anchors and spread the module load into the structure. Choose by rail height against the span between fixings, by single or double rail per module row, and by the T-slot that has to match your clamps. They join into continuous runs and are used across pitched roofs, flat roofs and the wider solar structures range. Guide
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Rails: section height, span and the T-slot
A mounting rail is a structural beam, so what matters most is not its length but its section height measured against the span between fixings. A taller profile carries more load over a longer span, letting the installer follow the anchor spacing the roof has instead of adding fixings it cannot take. The second property is the T-slot: the channel geometry decides which end clamps, mid clamps, joiners and hooks engage in it, and mixing slot systems is the usual cause of parts that will not seat. Around those two decisions sit the details that decide whether the array lasts outdoors: rail joiners, stainless fasteners paired with the aluminium, expansion gaps in long runs, and the cable channel that keeps DC leads off the roof.
FROM THE FIELD
"Cutting rails gets treated as scrap handling, and it is not. Every cut end has to be deburred and the swarf brushed off the roof, because filings left on a membrane or in a gutter corrode into stains that never come out, and a burr in a T-slot ruins the clamp."
Ladislav Proc · Mounting systems specialist, ONSA Plus
How to choose mounting rail profiles
Start from the fixing spacing the roof allows, because that sets the span the rail has to bridge. Section height follows from the span and the load, rail quantity from the module layout, and clamps, joiners and end caps from the T-slot you have chosen. Picking the rail first and finding the spacing afterwards forces redesign on site.
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Span, joiners and continuous runs
The rail is loaded by module weight, snow and wind uplift, and deflects between the anchors. Shorten the span or raise the section and deflection drops. Where a run is longer than one rail, a joiner continues it, and the run behaves as a continuous beam only if the joint does not fall at midspan.
- Section height: match it to the span and snow load. A low profile is cheaper and neater, a tall profile bridges wider anchor spacing without extra roof penetrations.
- Rail joiners: place them near a fixing, not mid span, and never inside the length a module clamp has to grip.
- Thermal expansion: aluminium moves roughly 2.3 mm per metre of rail over a 100 K swing, so long runs need an expansion gap at the joint and a sliding connection at one end.
- Corrosion pairing: use A2 or A4 stainless fasteners with aluminium rails. Avoid direct contact with copper and untreated steel.
- Cable management: route DC leads and bonding inside the rail channel with UV resistant clips, so nothing rests on the covering or hangs in a loop.
Tip from practice
Check the T-slot before ordering anything else. Clamps, joiners, hooks and earthing parts are only interchangeable within a slot system, and a rail that fits the roof perfectly is useless if your clamps and hooks cannot slide into it. Order rail, clamps and joiners as one set.
One rail or two per module row
Modules can sit on a single rail per row, clamped near the frame centre, or on two rails at the mounting points the datasheet specifies. The choice is driven by load, module size and the roof beneath.
| Property | Single rail per row | Double rail per row |
|---|---|---|
| Load path | Concentrated on a single line | Shared across two clamped lines |
| Snow and uplift capacity | Lower, suits sheltered low load sites | Higher, standard for heavy loads |
| Typical use | Small arrays, light modules, low pitch | Large formats, exposed sites, flat roofs |
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Frequently asked questions about mounting profiles
How far apart can the rail fixings be?
The maximum span depends on the rail section height, the module weight and the snow and wind loads at the site. A taller profile spans further. Take the permitted spacing from the system load table for your load case, not from rafter spacing alone.
Do I need one rail or two per module row?
Two rails is the normal arrangement, because the module is clamped in the zones its datasheet specifies and the load is shared. A single rail is only acceptable where the module manufacturer permits centre clamping and the loads are low.
Can I mix rails and clamps from different systems?
No. The T-slot geometry differs between ranges, so clamps, joiners, hooks and earthing parts only engage reliably within their own system. Order rails, clamps and joiners together, and check the slot profile before combining with existing stock.
How do I join two rails into a longer run?
With a rail joiner made for that profile, positioned close to a fixing rather than at midspan and clear of any module clamp. On long runs leave an expansion gap at the joint so thermal movement is absorbed, not passed into the anchors.
Do aluminium rails corrode against stainless bolts?
The pairing is standard practice and the galvanic difference between aluminium and A2 or A4 stainless is small enough to accept outdoors. What to avoid is direct contact with copper or untreated steel, and leaving swarf or unprotected cut edges on the roof.
Can I run the DC cables inside the rail?
Yes, and it is good practice. The rail channel keeps solar cable and bonding off the covering, out of standing water and away from wind flapping. Use UV resistant clips and keep connectors accessible, not buried behind a panel.
Rail section and span between fixings follow the wind and snow load at the site and the module format you are mounting.
About ONSA Plus
Why installers across Europe order rails 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 rail spans, joiners and clamp compatibility from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
