DOUBLE THREADED SCREWS FOR STEEL CONSTRUCTIONS

The ULAMEX double threaded screw for steel constructions is a specialist structural fastener engineered for mounting photovoltaic rail systems directly to steel purlins, beams and framework in commercial and industrial buildings. Featuring a self-tapping thread on one end and a standard M10 metric thread on the other, this dual-purpose screw cuts its own thread into the steel substructure while providing a metric bolt interface for attaching mounting brackets, rail connectors and other PV hardware. One fastener replaces the traditional multi-component connection of self-drilling tek screw plus separate threaded stud, significantly simplifying installation on steel-framed roofs and reducing the total parts count per mounting point.

Steel-framed commercial buildings — warehouses, logistics centres, factories, agricultural halls and carport structures with steel frames — present a fundamentally different mounting challenge compared to timber-framed residential roofs. The substructure is thinner but harder, requiring shorter fasteners with a thread geometry optimised for cutting into structural steel rather than gripping wood fibres. The ULAMEX double threaded screw for steel addresses this requirement with available lengths of 80 mm, 125 mm, 150 mm and 160 mm, purpose-matched to typical steel purlin and beam thicknesses encountered in European commercial construction.

Installation Guidelines

Correct installation of double threaded screws for steel constructions is critical for the long-term structural integrity of any commercial or industrial rooftop PV system. The self-tapping end must be driven into the steel purlin or beam flange — never into thin sheet metal cladding alone — to achieve the required pull-out resistance and shear capacity for wind uplift and snow loading forces.

Pre-drilling is mandatory. Unlike wood applications where the thread cuts easily into softwood fibres, steel requires a precise pilot hole to allow the self-tapping thread to engage properly without excessive torque or risk of thread stripping. Use a drill bit diameter of 8.5 mm for standard structural steel purlins (S235 to S355 grade). The pilot hole must be drilled through the full thickness of the steel member at the intended fastening point. Ensure the hole is perpendicular to the purlin surface to prevent the screw from seating at an angle, which would compromise both pull-out resistance and waterproofing.

Drive the screw into the steel purlin using a socket wrench or low-speed impact driver set to the appropriate torque. The self-tapping thread cuts into the pre-drilled hole as it advances, creating a permanent mechanical interlock with the steel. The unthreaded shank section spans the roof cladding layer (trapezoidal sheet, sandwich panel or standing seam), and the M10 metric thread protrudes above the roof surface, ready to receive the mounting bracket or base plate secured with a nut, washer and EPDM sealing washer.

For standard steel-framed commercial roofs, the recommended screw length is determined by adding the steel purlin penetration depth, the cladding thickness and the required metric thread protrusion (typically 30–50 mm above the cladding surface). The 80 mm variant is used for thin-profile purlins with minimal cladding, such as exposed steel beams in carport structures. The 125 mm and 150 mm variants are the most commonly specified for standard commercial roofs with trapezoidal sheet cladding over Z- or C-section purlins. The 160 mm variant is specified for installations with thicker insulated sandwich panels or where the purlin sits deeper below the cladding surface.

Important: Always verify the steel grade and thickness of the purlin before specifying the fastener. Minimum recommended steel thickness for reliable self-tapping thread engagement is 2.0 mm. For purlins thinner than 2.0 mm, consult a structural engineer to confirm that the fastener will achieve adequate pull-out resistance for the design wind and snow loads of the installation.

Applications

Double threaded screws for steel constructions are the standard fastening method for PV installations on commercial and industrial buildings wherever the roof substructure consists of steel purlins, beams or framework. They are used in the following configurations:

• Trapezoidal sheet roofs over steel purlins — the most common commercial PV application in Europe. The screw anchors through the trapezoidal sheet into the underlying steel purlin, and the protruding M10 metric thread receives a base plate or L-bracket that connects to the PV mounting rail. This configuration is standard for warehouses, logistics centres and production facilities with Z-purlin or C-purlin roof substructures.
• Standing seam roofs — for metal roofs with standing seam profiles over steel purlins, the screw provides the structural anchor while dedicated standing seam clamps handle the connection to the seam rib. The shorter 80 mm and 125 mm lengths are typically used where the standing seam sits close to the purlin.
• Sandwich panel roofs — insulated sandwich panels (PIR, PUR, mineral wool core) require longer fasteners to penetrate through the insulation layer and into the steel purlin beneath. The 150 mm and 160 mm variants are specified for typical 80–120 mm sandwich panel thicknesses.
• Steel-framed carport structures — carport PV installations with steel frames use these screws to fasten PV mounting hardware directly to the steel frame members, eliminating the need for separate welded studs or through-bolted connections.
• Agricultural and industrial halls — large-span agricultural buildings, aircraft hangars and industrial halls with steel purlin roof structures are prime candidates for commercial PV installations using these fasteners.

Each mounting point on a commercial PV system typically requires one double threaded screw per bracket or base plate, with brackets spaced at 800–1500 mm intervals along each rail line depending on the structural engineering specification. A medium-sized commercial installation of 100 kWp (approximately 240 modules) on a warehouse roof uses approximately 200–400 double threaded screws, depending on the rail layout, bracket spacing and wind zone requirements.

Material Properties and Corrosion Resistance

The A2 stainless steel grade (Werkstoff 1.4301, also known as AISI 304 or 18/10 chromium-nickel steel) provides excellent corrosion resistance for outdoor rooftop applications on commercial and industrial buildings. The chromium content of 17.5–19.5 % forms a self-healing passive oxide layer that protects against rain, condensation, industrial pollutants and the chemical residues commonly found in factory and agricultural environments throughout the 25–30 year expected service life of a photovoltaic installation.

A2-70 classification per EN ISO 3506-1 indicates a minimum tensile strength of 700 MPa and a proof stress of 450 MPa, providing ample structural capacity for the wind uplift and snow loading forces encountered on large commercial roof areas. These forces are typically higher than on residential roofs due to the greater exposure and larger tributary areas of commercial building rooftops.

The self-tapping thread end undergoes additional hardening treatment to ensure it can cut cleanly into structural steel grades S235 through S355 without deformation. This localised hardening is carefully controlled so that the thread tip achieves the cutting ability needed for steel penetration while the overall fastener retains the ductility and corrosion resistance characteristic of A2 stainless steel.

Unlike carbon steel alternatives that require zinc coating or hot-dip galvanising, stainless steel fasteners maintain their corrosion resistance even when the surface is scratched during installation — a significant advantage in the field where tools, steel swarf and handling inevitably contact the fastener surface. For installations in coastal environments or areas with high chloride exposure (within 5 km of the sea), consider upgrading to A4 stainless steel (1.4401 / AISI 316) fasteners with added molybdenum for enhanced pitting resistance. Contact our technical team for availability.

Compatibility with ULAMEX PV Systems

These double threaded screws for steel constructions are designed to work seamlessly within the complete ULAMEX photovoltaic mounting ecosystem, with particular emphasis on the components used in commercial and industrial-scale installations:

• Mounting brackets and base plates — adjustable and fixed-height variants for trapezoidal, standing seam and sandwich panel roofs, all pre-drilled for M10 metric bolts
• PV mounting profiles — aluminium rails in 40×40, 45×40 and 80×40 mm cross-sections, with the 80×40 mm heavy-duty profile recommended for commercial installations with longer spans between brackets
• Module clamps — mid and end clamps to secure solar panels to the rail, available for all standard module frame thicknesses
• Accessories — M10 hex nuts, flat washers, EPDM sealing washers, spring washers and mounting plates for a complete fastening solution

The M10 thread size is the standard for commercial steel-construction PV fasteners, providing the tensile and shear capacity required for the higher wind loads typical of exposed commercial and industrial rooftops. This screw is designed to pair directly with the ULAMEX mounting plate (product 448) and M10 hex nuts (product 424) for a complete bracket connection, with washers (product 464) completing the assembly.

Frequently Asked Questions

What is the difference between the steel version and the wood version of double threaded screws?

The wood version (product 422) features a coarse wood thread designed to grip softwood rafter fibres, with longer lengths of 200–400 mm to penetrate through thick timber. The steel version (this product, 423) features a self-tapping thread with a hardened tip designed to cut into structural steel, with shorter lengths of 80–160 mm because steel purlins are thinner than wood rafters. Both share the same M10 metric thread on the opposite end for mounting PV hardware. Choose the wood version for residential timber-framed roofs; choose this steel version for commercial buildings with steel purlin substructures.

What pre-drill diameter should I use for steel purlins?

For standard structural steel purlins in grades S235 to S355, use an 8.5 mm drill bit. The pilot hole must pass completely through the steel member. For higher-strength steels or hardened sections, consult the fastener torque/pull-out data sheet or contact our technical team for specific recommendations. Always drill perpendicular to the purlin surface.

What is the minimum steel thickness for reliable thread engagement?

A minimum steel thickness of 2.0 mm is recommended for reliable self-tapping thread engagement with adequate pull-out resistance. For thinner steel members (1.5–2.0 mm), the screw will still engage but the pull-out capacity is reduced — have a structural engineer verify that the reduced capacity meets the wind and snow load requirements for your specific installation. Steel members thinner than 1.5 mm are generally not suitable for this type of fastener.

Do I need sealing washers at the roof penetration?

Yes. An EPDM sealing washer between the base plate and the roof cladding surface is essential to prevent water ingress at the screw penetration point. This is particularly critical on trapezoidal sheet roofs where rainwater flows along the troughs and any unsealed penetration becomes a direct path for leaks. ULAMEX supplies compatible EPDM sealing washers as part of the fasteners and accessories range.

How many screws do I need for a commercial PV installation?

A typical commercial installation uses one double threaded screw per mounting bracket, with brackets spaced at 800–1500 mm along each rail line. As a rough guide, a 100 kWp commercial system (approximately 240 modules in a two-rail-per-row layout) requires 200–400 screws depending on bracket spacing, rail layout and the structural engineer’s specification for the wind zone and snow load. Contact our sales team with your project details and we will help calculate the exact quantity.

Ready to order double threaded screws for steel constructions for your next commercial PV project? Contact our sales team for wholesale pricing, volume discounts and delivery scheduling across Europe. Call us directly at +48 504 424 761 or browse the complete PV fasteners and accessories range.