Introduction
When planning or installing a solar energy system, most people focus on panels, inverters, or racking structures. Yet, beneath the visible components lies a category of hardware that often goes unnoticed but is essential: fasteners. Without reliable fasteners, no solar array—no matter how advanced—can stay secure over time.
Among the many types of fasteners, the solar hanger bolt stands out as a key connector in both solar pitched roof mounting systems and solar metal roof mounting systems.
It anchors mounting hardware firmly into the building’s structural elements, such as rafters or steel purlins, while also sealing the roof against water ingress. A properly selected and installed hanger bolt ensures that solar panels remain in place for decades, unaffected by wind, rain, and snow.
At Suzhou Bilateral Import & Export Co., Ltd., we specialize in delivering solutions that combine quality, durability, and adaptability. We supply you with almost all types of solar hanger bolts, no matter whether used in a wood or metal roof.
1. What is a Solar Hanger Bolt?
A solar hanger bolt is a dual-purpose fastener with two distinct ends:
Wood thread (lag screw side): Designed to grip deeply into wooden rafters or beams.
Machine thread (metric side): Allows secure attachment to brackets, L-feet, or rails above the roof surface.
By combining these functions, the bolt creates a direct structural bridge between the solar mounting system and the roof frame. This bridge ensures that forces generated by wind uplift, snow accumulation, or the dead weight of solar modules are safely absorbed by the building’s structure.
In short, while small in size, a solar hanger bolt plays a large role in structural safety, durability, and waterproofing.
2. Types of Solar Hanger Bolts
Wood-thread solar hanger bolt
This is the most common type, designed for pitched roof installations where timber rafters are present. The deep wood threads deliver strong pull-out resistance and long-term reliability.
Practical example: Residential rooftops in suburban areas often use this type, as most houses rely on timber framing.
Solar Hanger Bolt(Sharp Point with Coarse Thread)
Commonly Used: Wood/Softwood
What’s Special
- ST8 can effectively prevent wood from cracking during installation.
- ST3/8 is applicable to most softwoods, but you must create reserved holes before installation.
Solar Hanger Bolt(Self-Drilling)
Commonly Used: Metal Roof
What’s Special: It can be installed directly into metal sheets without pre-drilling. Can save labor costs. When used on wooden structures, a pre-drilled hole is required.
Flat-End Solar Hanger Bolt
This version is designed to sit flush with adapter plates, preventing unnecessary protrusion above the roof surface. It is often used where aesthetics or space constraints are important.
Practical example: Buildings with visible rooftop solar systems—such as schools or office buildings—benefit from flat-end designs to maintain a neat appearance.
Solar Hanger Bolt (Flat end with relief groove)
The Relief Groove design provides additional benefits: it distributes stress more evenly, reduces the risk of timber cracking, and improves the waterproof seal around the bolt. This makes it the preferred choice for heavy-duty applications in demanding environments.
Practical example: Solar arrays in coastal regions or alpine climates, where strong winds and heavy snow are common, typically adopt this type to ensure extra safety and durability.
Solar Hanger Bolt(Fine Thread with Sharp Point)
What’s Special: Pre-drilling required. Enhanced thread grip delivers a stronger, more secure fastening.
3. How Solar Hanger Bolts Work
The working principle of a solar hanger bolt is simple yet highly effective. The wood-thread side anchors into the structural layer of the roof, while the machine-thread side protrudes above the surface to connect with brackets and rails. With the addition of EPDM sealing washers and flashing plates, the connection becomes both structurally sound and waterproof.
This design ensures that all external loads—whether dynamic (wind and snow) or static (panel weight)—are transferred downward into the roof’s load-bearing members. In other words, the solar hanger bolt is the invisible channel through which the system’s safety is guaranteed.
4. Installation Process
Installing solar hanger bolts requires precision, as both safety and waterproofing depend on correct execution:
Identify rafters or purlins using a stud finder or structural drawings.
Drill pilot holes when working with wood-thread types to prevent splitting.
Insert the hanger bolt, driving the wood-threaded end into the structural member.
Apply waterproofing measures, including EPDM washers, flashing plates, and sealant.
Attach mounting brackets or rails to the exposed machine-thread end.
Tighten with torque control, ensuring the right balance (e.g., 20–25 Nm for M10).
A well-installed bolt ensures decades of leak-free and stable solar operation.
5. Materials and Standards
Most solar hanger bolts are manufactured from stainless steel, chosen for its strength and corrosion resistance.
SUS304 (A2-70): Standard choice for urban and inland rooftops; cost-effective and durable.
SUS316 (A4-80): Premium option for coastal or industrial environments where salt or pollutants accelerate corrosion.
International standards such as UL 2703, IEC 61730, and ISO 9227 guarantee that the bolts meet both mechanical and environmental requirements. Choosing certified fasteners not only protects installations but also ensures compliance with building codes and warranties.
6. Load Capacity
Load performance depends on bolt size, embedment depth, and installation quality. Typical values include:
| Spec | Pull-Out Strength | Shear Strength | Working Load |
| M8 | 5–6 kN | 4–5 kN | ≤ 1.5 kN |
| M10 | 7–8 kN | 6–7 kN | ≤ 2.0 kN |
| M12 | 9–10 kN | 7–8.5 kN | ≤ 2.5–3.0 kN |
Always design with a safety factor of 3 or higher and ensure embedment of at least 60 mm into timber structures.
7. How to Choose the Right Bolt
Selecting the correct solar hanger bolt requires evaluating multiple factors:
Roof type:
- Wooden roofs (e.g, wooden beams/purlins) → wood-thread;
- Metal roofs (e.g, thin steel sheets/metal purlins) → self-drilling or flat-end.
Load requirements: Residential → standard bolts; Utility-scale → Relief Groove heavy-duty bolts.
Material environment: Inland → SUS304; Coastal/industrial → SUS316.
Aesthetic needs: Use flat-end bolts for low-profile or visually sensitive projects.
Certification: UL/TÜV/ISO ensures compliance and warranty protection.
By considering these factors, installers and EPCs can avoid mismatches and ensure both safety and cost efficiency.
8. Common Mistakes to Avoid
Anchoring into the roof decking only, instead of rafters or purlins.
Selecting bolts that are too short or excessively long.
Skipping waterproofing measures leads to leaks over time.
Over-tightening can damage threads or seals.
Using uncertified fasteners in projects where compliance is required.
These mistakes may seem small, but they often result in costly rework or warranty issues.
Suggestions for use
- Solar Hanger Bolt(Sharp Point with Coarse Thread): ST8 thread is used for wooden purlins, which can prevent the wood from cracking. ST3/8 is suitable for softwood, and a reserved hole is needed.
- Self-drilling solar hanger bolt can be used for both wooden and steel purlins. When used in wood, holes need to be reserved; otherwise, a lot of heat will be generated due to friction during use, which will lead to wood carbonization and even combustion, and destroy the wood structure and the screw itself.
- Both steel structure and flat tail need reserved holes to be installed, and both can be used in wood or metal in theory.
- It is suggested that the solar hanger bolt with a different pitch should be selected according to the back thickness of the roof tiles, and the matching solar hanger bolt should be selected according to the purlin material to avoid affecting the roof and system life.
9. FAQs
Q1: Can solar hanger bolts be reused?
No. Reuse weakens grip strength and reduces waterproofing efficiency. Always use new bolts for each installation.
Q2: Why choose a solar hanger bolt (with Relief Groove)?
It spreads the load more evenly, reduces timber cracking, and enhances sealing. This makes it ideal for projects with high mechanical stress.
Q3: Which stainless steel is best?
SS304 is reliable for inland projects. SS316 is better for coastal or industrial sites due to superior corrosion resistance.
10. Future Trends
The solar fastener market continues to evolve with innovations such as:
Pre-assembled hanger bolt kits that reduce installation time.
Advanced Relief Groove designs are capable of handling extreme weather.
Smart hanger bolts with torque sensors for real-time monitoring.
Eco-friendly stainless alloys designed for sustainability goals.
These trends point toward faster, safer, and greener installations.
Conclusion
The solar hanger bolt is far more than just a piece of hardware—it is the backbone of any secure solar mounting system. Whether for a solar pitched roof mounting system with timber rafters or a solar metal roof mounting system with steel purlins, choosing the correct type of hanger bolt—wood-thread, self-drilling,flat-end —is vital for long-term performance.
At Suzhou Bilateral Import & Export Co., Ltd., a company committed to offering one-stop solar panel mounting solutions and accessories to you.
Discover more in our Solar Mounting Products Catalog or request a free consultation today.
