The ULAMEX double threaded screw is the essential structural fastener for mounting photovoltaic rail systems directly to wooden roof rafters. Engineered with a wood thread on one end and a metric thread on the other, this dual-purpose screw creates a permanent anchor point in the timber while providing a standard metric bolt interface for attaching roof hooks, rail brackets, and other PV mounting hardware. One fastener replaces the traditional two-component connection of lag bolt plus threaded rod, simplifying installation and reducing the total parts count per mounting point.
Technical Specifications
| Parameter | Value |
|---|---|
| Material | A2 stainless steel (1.4301 / AISI 304) |
| Metric thread | M10 / M12 |
| Wood thread | Coarse pitch, optimised for softwood rafters (spruce, pine, larch) |
| Available lengths | 200 mm, 250 mm, 300 mm, 350 mm, 400 mm |
| Head type | Hex head (wrench drive) |
| Surface finish | Bright, passivated |
| Corrosion resistance | Suitable for outdoor exposure, EN ISO 3506-1 class A2-70 |
| Thread transition | Unthreaded shank section between wood and metric zones |
| Temperature range | −40 °C to +80 °C continuous service |
Installation Guidelines
Correct installation of double threaded screws is critical for the long-term structural integrity of any rooftop PV system. The wood-thread end must be driven into the centre of the rafter — never into battens or counter-battens alone — to achieve the required pull-out resistance. Pre-drilling is mandatory: use a drill bit diameter of 6.5 mm for M10 and 8 mm for M12, drilling to a depth at least 10 mm deeper than the intended wood-thread penetration.
Drive the screw into the rafter using a socket wrench or low-speed impact driver set to the appropriate torque. The wood thread must be fully embedded in the rafter timber, with the unthreaded shank section spanning the roof tile and insulation layers. The metric thread then protrudes above the roof surface, ready to receive the roof hook or mounting bracket secured with a nut, washer, and optionally a sealing washer to prevent water ingress at the penetration point.
For standard pitched-roof installations, the recommended screw length is determined by adding the rafter depth penetration (minimum 80 mm), the tile and batten thickness, and the required metric thread protrusion (typically 40–60 mm above the tile surface). In most Central European residential roofs with standard tile profiles, 250 mm or 300 mm screws are the most commonly specified lengths. The 350 mm and 400 mm variants are used for thick insulation layers, raised tile profiles, or situations where the rafter sits deeper below the tile surface.
Applications
Double threaded screws for wood are the standard fastening method for pitched-roof PV installations across Europe wherever the substructure consists of wooden rafters. They are used in the following configurations:
- Roof hook systems — the screw anchors into the rafter, and a roof hook slides over the metric thread to sit between or replace roof tiles. The hook is secured with a hex nut and flat washer, then the PV mounting rail bolts to the hook’s upper flange.
- Direct rail mounting — in configurations without separate roof hooks, the metric thread protrudes through a base plate or L-bracket that connects directly to the PV rail. This method is common in trapezoidal sheet metal roofing over wooden purlins.
- Tile replacement brackets — for installations where tiles are replaced by custom flashing plates, the double threaded screw provides the structural anchor while the flashing plate handles waterproofing.
Each mounting point on a residential PV system typically requires one double threaded screw per roof hook, with roof hooks spaced at 800–1200 mm intervals along each rail line. A standard 10-module residential installation with two rail lines uses approximately 12–20 double threaded screws, depending on hook 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. The chromium content of 17.5–19.5 % forms a self-healing passive oxide layer that protects against rain, condensation, and atmospheric pollutants 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 wind uplift and snow loading forces transmitted through the PV mounting system. 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 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 are designed to work seamlessly within the complete ULAMEX photovoltaic mounting ecosystem:
- Roof hooks — adjustable and fixed-height variants, all pre-drilled for M10 or M12 metric bolts
- PV mounting profiles — aluminum rails in 40×40, 45×40, and 80×40 mm cross-sections
- Module clamps — mid and end clamps to secure solar panels to the rail
- Accessories — hex nuts, flat washers, EPDM sealing washers, T-bolts
The M10 variant is the most widely used across residential installations, while the M12 size is specified for commercial-scale projects, heavy-duty roof hooks, or installations in high wind zones (wind zone 4 per DIN EN 1991-1-4) where higher pull-out and tensile loads must be accommodated.
Why Source from ULAMEX
As a B2B wholesale supplier of photovoltaic mounting components, ULAMEX provides double threaded screws in bulk packaging from 50 to 10,000+ pieces per order. All fasteners are manufactured to EN ISO 3506-1 standards and supplied with mill test certificates on request. Our warehouse stocks all standard lengths continuously, enabling same-week dispatch for orders placed by Wednesday.
We supply PV installers, roofing companies, electrical contractors, and solar distributors across Germany, Austria, the United Kingdom, Scandinavia, and the Benelux countries. Whether you are equipping a single residential project or stocking your warehouse for the season, our volume-based pricing scales to match your requirements.
Frequently Asked Questions
- What pre-drill diameter should I use for the wood thread?
- For M10 screws, pre-drill with a 6.5 mm bit. For M12, use an 8 mm bit. Always drill at least 10 mm deeper than the intended thread penetration depth to allow wood chips to clear and avoid splitting the rafter.
- Can I use these screws in hardwood rafters?
- A2 stainless steel screws can be installed in hardwood (oak, beech), but pre-drilling is especially critical. Use a drill bit 1 mm larger than the softwood recommendation and apply a thread lubricant (wax or dedicated stainless steel paste) to prevent galling during installation.
- What is the minimum rafter penetration depth?
- A minimum of 80 mm of wood-thread engagement is recommended for standard residential PV installations. For commercial systems or high wind zones, structural calculations may require 100 mm or more.
- Do I need sealing washers at the roof penetration?
- Yes. An EPDM sealing washer between the roof hook base plate and the tile or flashing is strongly recommended to prevent water ingress along the screw shaft. This is particularly important for interlocking concrete tile roofs where the screw passes between overlapping tiles.
- How many screws do I need per kilowatt of installed PV capacity?
- As a rough guide, a typical residential system uses 1.5–2 roof hooks per module, so a 10 kWp system (approximately 24 modules) requires 36–48 double threaded screws. The exact quantity depends on module dimensions, rail layout, hook spacing, and the structural engineer’s specification for the wind zone.
Ready to order double threaded screws for your next PV project? Contact our sales team for wholesale pricing, volume discounts, and delivery scheduling across Europe.