Understanding the Role of the Substructure
The substructure for hardwood siding acts as the intermediary layer between the building's structural wall and the exterior cladding. Its primary functions include creating a level surface for installation, providing a secure anchor for fasteners, and, critically, forming a ventilation and drainage plane.
For rainscreen assemblies, which are common with hardwood siding, the substructure creates an air gap that allows moisture to drain away and promotes airflow, helping to dry out the wall assembly. This gap is essential for preventing moisture accumulation, which can lead to issues like mold, mildew, or wood degradation over time. Without a properly designed substructure, even the highest quality hardwood siding may underperform.
Furring Strips: Creating the Air Gap and Level Surface
Furring strips are typically the most visible component of the substructure. They are battens, often made of wood or metal, that are attached to the structural wall or sheathing. Their main purpose is to create the necessary air gap behind the siding and provide a plumb and level surface for fastening the hardwood boards.
When selecting furring strips, consider their material, dimensions, and treatment. Wood furring should be dimensionally stable and, for exterior applications, often pressure-treated or naturally durable to resist rot and insects. Metal furring systems offer an alternative, particularly for non-combustible assemblies or where greater dimensional stability is desired. The spacing of furring strips must align with the intended siding profile and fastener requirements to ensure adequate support for each board. For vertical siding, furring is typically installed horizontally; for horizontal siding, furring is installed vertically.
Fastening Systems: Secure Attachment to the Substructure
The choice of fastening system directly relates to the substructure. Fasteners must be appropriate for the hardwood species, siding profile, and the furring material. Stainless steel fasteners are generally recommended for exterior hardwood siding to prevent corrosion and staining, especially with species known for their extractive content. Hidden fastening systems are often preferred for a clean aesthetic, but they require specific furring strip dimensions and precise installation.
Exposed fasteners, while visible, can offer a robust attachment method and may be simpler to install. Regardless of the type, fasteners must penetrate sufficiently into the furring strips to ensure a strong hold. The spacing and pattern of fasteners are determined by the siding manufacturer's recommendations and local building practices, taking into account wind loads and the movement characteristics of the wood.
Substrate and Weather-Resistive Barrier (WRB)
Beneath the furring strips and behind the entire substructure assembly lies the structural substrate and a weather-resistive barrier (WRB). The substrate is typically the sheathing (e.g., plywood, OSB) attached to the building's framing. The WRB, such as house wrap or a fluid-applied membrane, is installed over the substrate to provide a secondary line of defense against moisture intrusion. It's crucial that the WRB is properly lapped and sealed around openings to effectively shed water.
The substrate and WRB work in conjunction with the furring strips and siding to create a comprehensive wall assembly that manages moisture and protects the building's interior. For rainscreen systems, the WRB is designed to be the primary waterproof layer, with the siding and air gap acting as a protective screen.
Worked Example: Planning a Horizontal Siding Substructure
Consider a 10-foot tall by 20-foot wide wall intended for horizontal hardwood siding with a 6-inch face width. The structural wall has OSB sheathing.
For horizontal siding, the furring strips will be installed vertically. These strips will be attached directly to the OSB sheathing, ensuring they are plumb and spaced to align with the siding's fastening points. This ensures that each siding board has a secure attachment point across its length.
For a 20-foot wide wall, you would need approximately 15 vertical furring strips (20 feet / 16 inches per strip = 15 strips, rounding up for the end). Each strip is 10 feet long. Therefore, you would need to purchase 15 pieces of 10-foot furring strips, or equivalent lengths to achieve this coverage with minimal waste. The WRB would be installed over the OSB sheathing first, lapped correctly, before the furring strips are attached. Stainless steel fasteners would then be used to attach the hardwood siding to the furring strips.
Connecting with Hardwood Siding Products
Lumber Plus offers a range of hardwood siding options, including Thermally Modified Wood, Maximo Accoya, and various Brazilian hardwoods. These materials can be specified with a variety of profiles suitable for rainscreen assemblies. When planning your substructure, consider the specific requirements of the chosen hardwood species and profile.
For example, thermally modified ash and clear radiata pine offer stability and durability (S1). Maximo Accoya, an acetylated wood product, also provides enhanced performance for exterior applications (S1). Brazilian hardwoods like Ipe, Cumaru, and Garapa are known for their natural density and resistance (S1). Each material may have specific recommendations for fastener type and spacing, which will influence your furring strip layout. For composite options, NORX composite cladding provides an alternative that may have different substructure requirements than natural hardwood (S1).
Explore our full selection of siding and cladding materials at our product page: [Hardwood Siding and Cladding](/siding-and-cladding).
Before you order
- Confirm the required air gap dimension for your climate and local codes.
- Determine if horizontal or vertical furring is needed based on siding orientation.
- Select furring strip material (wood, metal) and treatment (e.g., pressure-treated).
- Calculate furring strip spacing based on siding profile and fastener requirements.
- Choose appropriate fasteners (e.g., stainless steel, hidden vs. exposed) for the hardwood species.
- Verify the type and proper installation of the weather-resistive barrier (WRB).
- Consider any specific manufacturer recommendations for your chosen hardwood siding.
- Account for detailing around windows, doors, and corners, as these may require additional blocking or furring.
Confirm installation details, approved uses and warranty terms in the current instructions for the exact product and profile. Your installer should verify the substrate, structure and applicable project requirements.
Questions to help you decide
What is a rainscreen assembly and why is it important for hardwood siding?
A rainscreen assembly is a type of wall construction where the exterior cladding (like hardwood siding) is separated from the weather-resistive barrier by an air gap. This gap allows for drainage of any moisture that penetrates the siding and promotes ventilation, helping the wall assembly dry out. It's important for hardwood siding because it significantly reduces the risk of moisture-related issues such as rot, mold, and dimensional instability, thereby extending the life and maintaining the appearance of the facade.
Can I use standard dimensional lumber for furring strips?
Yes, standard dimensional lumber, such as 1x3 or 1x4 boards, can often be used for furring strips. However, it's crucial to select lumber that is suitable for exterior use. For many applications, pressure-treated lumber is recommended to resist moisture, rot, and insect infestation. Ensure the lumber is straight, free from significant defects, and dimensionally stable to provide a consistent and level surface for your siding. The actual thickness of the lumber will define the air gap.
How do I calculate the quantity of furring strips needed?
To calculate the quantity of furring strips, first determine their orientation (vertical for horizontal siding, horizontal for vertical siding) and the required spacing. Then, multiply this number by the height of the wall to get the total linear footage. Always add a percentage for cuts and waste, typically 10-15%, and round up to the nearest full board length available. Remember to account for any additional furring needed around openings or corners.
Ready to Plan Your Hardwood Siding Project?
Request a material quote with your project dimensions, exact profile preferences, and ZIP code to confirm current availability and service options. Our team can help you specify the right materials for your substructure.
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