Introduction
In recent years, rooftop PV installation has become the preferred solution for energy-saving upgrades in residential and commercial buildings. According to predictions by the IEA (International Energy Agency), by 2030, global rooftop PV capacity will account for over 70% of distributed PV. More than half of these rooftop PV systems are installed on pitched roofs, especially those with concrete tile and metal tile structures.
In pitched roof systems, the Solar Hanger Bolt is the critical connecting component between the mounting system and the roof structure. It must continuously withstand stresses from wind loads, snow loads, and temperature cycles throughout its 20-30 year lifespan. Improper selection of the hanger bolt size can lead to:
Minor Issues: Unstable installation, reduced system efficiency;
Major Issues: Roof leaks, loose modules, or even detachment of entire panel arrays.
Therefore, determining the appropriate hanger bolt size for solar has become a key issue that cannot be ignored in the design and construction phases. This article will combine practical cases and engineering standards to help you make a scientific decision.
1. Solar Hanger Bolt Specifications and Design Points
1.1 Diameter
Common diameters: M8, M10, M12.
M8: Suitable for lightweight systems, low-wind areas, such as single-row small residential roofs.
M10: The most commonly used specification, balancing load capacity and installation convenience, often used on concrete tile roofs.
M12: Highest strength, commonly used for metal tile roofs, large commercial roofs, or areas with high snow/wind loads.
👉 Engineering Reference: According to DIN EN 1991 (Eurocode for Loads), in areas with wind pressure exceeding 0.8 kN/m², prioritizing M12 is recommended.
1.2 Length
The hanger bolt length must satisfy:
Penetrating the roof covering (tile/metal sheet + waterproofing layer + wooden board);
Embedding into the load-bearing structure (rafter/purlin) by at least 40-60mm.
Case Analysis:
Concrete Tile Roof: Tile thickness 25mm + wooden board 20mm → Recommended bolt length ≥ 200mm.
Metal Tile Roof: Tile thickness 5mm + purlin → Bolt length 120-150mm is sufficient, but a larger diameter is needed to ensure pull-out strength.
1.3 Thread Type
Wood Thread (grips wooden rafters, large pitch).
Machine Thread (for connecting to mounting brackets, compatible with nuts/washers).
Self-Drilling Type (suitable for metal purlins, eliminates pre-drilling).
1.4 Material
SUS304 Stainless Steel: Standard choice for residential roofs.
SUS316 Stainless Steel: Suitable for coastal, high-humidity, or industrial areas.
Galvanized Steel: Low cost, but shorter lifespan, not suitable for long-term PV projects.
Pro Tip: In pitched roof PV mounting systems, the hanger bolt material must match the corrosion resistance of the racking rails; otherwise, a “weakest link effect” may occur.
2. The Influence of Roof Thickness and Tile Type
The selection of the hanger bolt size is closely related to roof thickness and tile type.
Common Roof Scenarios
| Roof Type | Covering Thickness | Recommended Bolt Length | Characteristics |
|---|---|---|---|
| Concrete Tiles | 20-40mm | 200-300mm | Tiles are thick, often used with L-feet. |
| Metal Tiles | <10mm + Purlin | 100-200mm | Self-drilling/drill-tip required, waterproofing is critical. |
| Flat Tiles | 10-20mm | 200-250mm | Common in European residences. |
| Asphalt Shingles | 5-10mm + Wood Board | 100-200mm | Prevalent in North America, lightweight roof. |
Selection Logic
Concrete Tile Roof → Focus on “penetration capability,” requiring long bolts + L-feet brackets.
Metal Tile Roof → Focus on “pull-out strength,” emphasizing diameter and sealing performance.
3. Application Differences of Common Sizes M8 / M10 / M12
| Model | Load Capacity | Typical Application | Advantages | Limitations |
|---|---|---|---|---|
| M8 | Low | Small Residential Pitched Roofs | Small drill hole, fast installation | Not suitable for high snow/wind areas |
| M10 | Medium-High | Mainstream Concrete Tile Roofs | Balances strength & installation ease | It may be insufficient in extreme conditions |
| M12 | High | Metal Tiles, Large Roofs | Strong pull-out strength, withstands harsh environments | Large drill hole requires professional installation |
Practical Application Experience:
European Residences: Mostly use M10;
Southeast Asian Coastal Commercial Projects: Often use M12;
North American Lightweight Asphalt Shingle Roofs: M8 still has significant application space.
4. Installation Precautions and Risk Mitigation
Core Steps
Locate Load-Bearing Structure: Use a stud finder to ensure fixation onto rafters/purlins.
Pre-Drilling: Avoids wood splitting, ensures vertical alignment.
Waterproof Sealing: Pitched roof projects must use EPDM washers + sealant + flashing.
Torque Control: Over-tightening damages threads, under-tightening causes loosening.
Common Mistakes and Risks
Mistake 1: Bolt not driven into the load-bearing structure, only fixed into the wooden board → Prone to loosening.
Mistake 2: No waterproofing treatment → Roof leaks, long-term structural erosion.
Mistake 3: Using low-grade steel → Corrosion appears after 2-3 years, requiring full replacement.
👉 For medium and large projects, it is recommended to involve a structural engineer for a secondary review to ensure each bolt meets load requirements.
5. The Role of Hanger Bolts in Pitched Roof Mounting Systems
A pitched roof mounting system typically consists of:
L-feet base
forming a complete structure.
Concrete Tile Roof System: Bolt penetrates the tile → fixed to the rafter → connects to L-feet and rails.
Metal Tile Roof System: Bolt goes directly through the tile surface → fixed to the purlin → used with a waterproof cap, forming a reliable waterproofing system.
This means the hanger bolt is not just a fastener, but a core component for “mechanical load transfer” and “waterproof barrier” in the pitched roof system.
Conclusion
Selecting the appropriate solar hanger bolt size is a core step in ensuring the safe and stable operation of a pitched roof PV system.
M8 → Lightweight residential projects;
M10 → Mainstream concrete tile roofs;
M12 → Metal tile roofs, large commercial projects, and high wind load areas.
In practical application, the bolt’s diameter, length, and material must match the roof thickness, tile type, and the overall racking system design.
In other words, solar hanger bolt selection is not an isolated action, but part of the overall design of the pitched roof PV mounting system.
About Us
Suzhou Bilateral Import & Export Co., Ltd.
As a professional supplier of solar mounting systems and stainless steel fasteners, we provide global customers with:
A full range of Solar Hanger Bolts (M8/M10/M12), compatible with concrete and metal tile roofs;
Components for pitched roof mounting systems like L-feet, rails, mid-clamps, end-clamps;
Integrated solutions for both rooftop and ground-mounted PV mounting systems.
Our products have been widely used in residential, commercial, and large-scale ground-mounted power plants, and exported to markets in Europe, North America, Southeast Asia, etc.
👉 Learn more by visiting our website: www.szbolts.com
