Understanding Thermally Modified Wood Properties
Thermally modified wood undergoes a high-temperature heat treatment process that alters its cellular structure. This process typically enhances dimensional stability, reduces equilibrium moisture content, and can improve resistance to decay and insect attack. These changes also influence how the wood interacts with fasteners.
Specifically, the reduced moisture content and altered cell structure can make thermally modified wood slightly more brittle than untreated wood, especially certain species. This characteristic necessitates careful fastener selection to prevent splitting during installation and to ensure long-term holding power without causing stress concentrations.
Material Compatibility: Preventing Corrosion and Staining
The most critical factor in fastener selection is material compatibility. Dissimilar metals in contact, especially in the presence of moisture, can lead to galvanic corrosion. This phenomenon not only degrades the fastener itself but can also cause unsightly black staining on the wood surface.
Grade 304 stainless steel offers good corrosion resistance for many exterior applications, while grade 316 stainless steel provides superior resistance, particularly in coastal or high-moisture environments. Hot-dipped galvanized fasteners can be an alternative for some applications, but stainless steel is often preferred for its longevity and reduced risk of staining.
Fastener Type and Application
The type of fastener chosen depends heavily on the specific application (e.g., siding, decking, soffits) and the desired aesthetic. Common fastener types include screws, nails, and hidden fastening systems.
Screws offer excellent holding power and are often preferred for their ability to be removed if necessary. Self-tapping screws with appropriate threads for wood can minimize splitting. Nails, while quicker to install, provide less pull-out resistance than screws. Hidden fastening systems are popular for maintaining a clean, unblemished surface, but require specific plank profiles and installation methods.
For siding and cladding, face fastening with screws or nails is a common approach, where the fastener head is visible. Countersinking and plugging can conceal face fasteners for a cleaner look. Hidden clip systems, often used with tongue-and-groove or specific profile thermally modified wood, attach to the back or edge of the board, leaving the face clear.
Pre-drilling: A Critical Step
Due to the increased brittleness of some thermally modified wood species, pre-drilling pilot holes for fasteners is often essential. Pre-drilling significantly reduces the risk of splitting the wood, especially near board ends or edges.
The diameter of the pilot hole should be carefully matched to the fastener. For screws, the pilot hole should be slightly smaller than the screw's core diameter to allow the threads to grip. For nails, the pilot hole should be slightly smaller than the nail shank. Always test on a scrap piece of wood to determine the optimal pilot hole size for your specific thermally modified wood species and fastener.
Worked Example: Fastener Selection for a Thermally Modified Siding Project
Consider a hypothetical residential project requiring 200 linear feet of 1x6 thermally modified wood siding for an exterior wall. The project is located in a moderately humid environment, and the client desires a clean, contemporary look with minimal visible fasteners.
Given the aesthetic requirement and exposure, a hidden fastening system or face fastening with concealed plugs would be suitable. For this example, let's select a hidden clip system designed for 1x6 tongue-and-groove siding. These clips typically require stainless steel screws for attachment to the furring strips.
The furring strips are 1.5 inches deep. Each 16-foot siding board will require approximately 12 clips, spaced 16 inches on center along the furring strips.
It is crucial to verify that the specific hidden clip system is rated for exterior use and compatible with the thermally modified wood profile. Always refer to the manufacturer's installation guidelines for both the wood and the fastening system for precise spacing and fastener specifications. For this project, we would also consider sourcing complementary products like a high-quality rainscreen material and durable furring strips to ensure the entire wall assembly performs as intended. Products like Norx rainscreen materials and Maximo Accoya wood may be considered for similar projects.
Key considerations for this example:
1. **Environment:** Moderate humidity, justifying Grade 304 stainless steel.
3. **Wood Profile:** Tongue-and-groove 1x6 siding, compatible with a clip system.
4. **Substrate:** Furring strips for rainscreen assembly.
6. **Pre-drilling:** Essential for attaching clips to furring strips and for any face-fastened trim pieces.
Before you order
- Identify the specific thermally modified wood species and its manufacturer's recommendations.
- Determine the exposure conditions (interior, exterior, coastal, high-moisture).
- Select the fastener type (screws, nails, hidden clips) based on application and aesthetic preference.
- Verify the fastener length and diameter are appropriate for the wood thickness and substrate.
- Confirm if pre-drilling is required and determine the correct pilot hole size.
- Account for thermal expansion/contraction if using face fasteners; ensure fasteners allow for slight movement.
- Review the manufacturer's installation guides for both the thermally modified wood and the fastening system.
- Calculate the required quantity of fasteners, including a buffer for waste or errors.
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
Can I use regular galvanized nails with thermally modified wood?
This is due to the enhanced corrosion resistance of stainless steel, which helps prevent black staining on the wood and ensures long-term integrity of the fastening.
Is pre-drilling always necessary for thermally modified wood?
Pre-drilling is highly recommended for most thermally modified wood installations, particularly when fastening near board ends or edges. The thermal modification process can make the wood more brittle, and pre-drilling significantly reduces the risk of splitting, ensuring a cleaner installation and better holding power. Always test on a scrap piece first.
How do I choose between face fastening and hidden fastening?
The choice depends on your aesthetic preference and the specific wood profile. Face fastening, where fasteners are driven through the face of the board, is straightforward but leaves visible fastener heads. Hidden fastening systems, which attach to the edges or back of the board, create a clean, unblemished surface but often require specific tongue-and-groove or shiplap profiles and can be more complex to install. Consider the overall design intent for your project.
Ready to Plan Your Thermally Modified Wood Project?
Request a material quote with your project dimensions, desired profile, and ZIP code. Our team can help confirm current availability and service options for all your thermally modified wood and fastener needs.
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