An L-Foot (also known as a stanchion, bracket, or support) is a fundamental component of a solar mounting system. It is a metal bracket, typically made from aluminum or stainless steel, shaped like an "L" that serves as the primary structural connection between the roof and the rest of the racking system.
L-Feet for metal roof solar mounting structures ( L feet and rails )
Types of L-Feet for Metal Roofs
The specific type of L-foot used is determined by the style of the metal roof. The two main categories are:
1. L-Feet for Through-Fastened (Batten, Corrugated) Metal Roofs
This is the most common application for a traditional penetrating L-foot. Through-fastened roofs have screws that go through the metal panels and into the underlying structural purlins.
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Mounting Method:Direct Penetration.
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How it Works: The base of the L-foot is bolted directly to the roof. This requires the installer to locate the underlying structural purlin (the roof's support beam) and drill through the metal roof panel to secure the foot with a lag bolt or structural screw.
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Key Characteristics:
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Base Plate: Has holes for lag bolts.
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Integrated Seal: Often comes with a pre-applied butyl tape sealant on the bottom.
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Material: Usually aluminum (AA6005-T5) for corrosion resistance and strength.
The Critical Challenge: WaterproofingSince you are creating new holes in the roof, the installation of the L-foot and its waterproofing is the most important part of the job. A proper installation involves a multi-layered sealing strategy:
2. L-Feet for Standing Seam Metal Roofs
On standing seam roofs, the preferred method is a non-penetrating seam clamp. However, an L-foot is still used as the vertical component that connects the clamp to the rail.
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Mounting Method:Clamp-Based, Non-Penetrating.
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How it Works: In this case, the "L-Foot" is often a separate component. A specialized clamp is attached to the vertical seam of the roof. The L-foot then bolts onto this clamp. The solar rail attaches to the vertical leg of the L-foot.
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Key Advantage: This method preserves the roof's warranty and integrity because it requires no penetrations. The clamp simply grips the seam mechanically.
Configuration: Clamp + L-Foot + Rail
Critical Engineering and Installation Considerations for L-Feet
1. Purlins are Everything (For Penetrating Systems)
You MUST hit the structural purlin underneath the metal roof. Hitting only the metal panel is structurally unsafe and will lead to failure. Installers use magnetic purlin finders or carefully measure from the roof's seams to locate the exact center of the purlin.
2. Wind Uplift Calculations
This is the most critical engineering factor. The L-foot, its attachment point (bolt or clamp), and the spacing between them are all calculated by a structural engineer to resist the specific wind uplift forces for that building's location and height. The mounting pattern (how many feet per rail) is not arbitrary; it's based on these calculations.
3. Thermal Expansion and Contraction
Metal roofs expand and contract significantly with temperature changes. The mounting system must accommodate this movement to avoid stress on the roof and the racking.
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On through-fastened roofs, the hole in the base of the L-foot is often a slotted hole to allow for slight lateral movement of the roof panel.
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On standing seam systems, the clamps are designed to allow for movement along the seam.
4. Corrosion Compatibility
Using the correct materials prevents galvanic corrosion:
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L-Foot: Typically aluminum.
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Fasteners (Lag Bolts): Should be stainless steel (e.g., 304 or 316) to avoid reacting with the aluminum foot and the steel purlin.
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Separator Pads: A plastic or neoprene pad is often placed between the aluminum L-foot and the metal roof to prevent direct metal-to-metal contact, further reducing corrosion risk.
Step-by-Step: Proper Installation of a Penetrating L-Foot
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Locate and Mark: Use a purlin finder to locate the center of the structural purlin. Mark the drilling points.
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Prepare the L-Foot: Ensure the pre-applied butyl tape is intact. If not, apply a high-quality sealant tape to the bottom of the foot.
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Drill the Pilot Hole: Drill a pilot hole through the metal roof panel and into the purlin. The hole size is critical—it must match the lag bolt specifications to allow for proper thread engagement without being too loose.
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Install Flashing (if used): Slide a metal or composite flashing under the roof panel's rib, centering it on the hole.
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Place the L-Foot: Position the foot over the hole, ensuring it is aligned correctly.
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Secure with Lag Bolt: Drive the stainless steel lag bolt through the L-foot and into the purlin with an impact driver, torquing it to the manufacturer's specification. Overtightening can strip the hole; undertightening won't create a proper seal.
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Apply Sealant: Apply a high-quality exterior sealant (like polyurethane or silicone) over the head of the lag bolt and around the base of the L-foot where it meets the roof, creating a watertight "dome." This is the secondary weather seal.
Leading Brands and Systems
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IronRidge: Offers the X-FF100 Flashed Foot System, which includes an integrated flashing that is highly regarded for its waterproofing design.
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Unirac: Their RapidRac system includes various L-feet designed for different metal roof profiles.
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Ecolibri: Specializes in metal roof solutions, offering low-profile L-feet with integrated sealing systems.
Conclusion
The humble L-foot is a deceptively simple component that carries immense responsibility. For metal roofs, choosing the correct type—penetrating or clamp-based—and executing a meticulous, watertight installation is non-negotiable for the system's longevity, performance, and the integrity of the building itself. It is the critical link that ensures a solar array will remain securely fastened and leak-free for decades.
