Finishing Corners with Thermally Modified Wood

Understanding Corner Assembly Options

When planning exterior cladding with thermally modified wood, corners are critical design elements. They can be understated and seamless, or they can be emphasized as a deliberate architectural feature. The choice of corner detail affects not only the appearance but also the complexity of installation and the potential for long-term maintenance.

The primary options typically involve either joining the cladding boards directly (mitered or butt joints) or covering the transition with a separate trim element. Each approach has specific implications for material usage, labor, and the overall durability of the corner.

Mitered Corner Joints: A Seamless Look

A mitered corner involves cutting each cladding board at a precise angle, typically 45 degrees, so that when joined, they form a 90-degree corner. This method creates a visually continuous surface, giving the impression that the wood wraps around the building without interruption.

Achieving a clean mitered corner requires precise cuts and careful installation. Any inconsistencies in the angle or alignment can become noticeable. Mitered joints are often favored for modern or minimalist designs where a smooth, uninterrupted facade is desired. However, wood movement due to changes in humidity and temperature can sometimes cause mitered joints to open slightly over time, potentially revealing the cut edge or requiring maintenance.

This method is generally more labor-intensive than a simple butt joint or using trim, as it demands a higher degree of accuracy during cutting and fitting.

Butt Joint Corners: Simple and Robust

A butt joint corner is a straightforward assembly where the end of one cladding board meets the face of another at the corner. One board runs continuously to the corner, and the adjacent board butts into its end. This creates a distinct visual line where the two boards meet.

This approach is simpler to execute than a mitered joint, as it requires fewer precise angle cuts. It can be a robust option, less prone to showing movement than mitered joints, as the primary expansion and contraction occur along the length of the boards. However, the exposed end grain of the butting board may be more susceptible to moisture absorption if not properly sealed or detailed.

When using a butt joint, consider which side of the corner will have the exposed end grain. Often, the less prominent or less visible side is chosen for this detail, or a small reveal is incorporated to manage water run-off.

Trim-Enhanced Corners: Defined and Protected

Using trim pieces at corners provides a distinct architectural detail and can simplify installation. Corner trim, often called corner boards or battens, covers the ends of the cladding boards where they meet, protecting them from weather and impacts. This method is common in traditional and contemporary designs, offering a clean, finished appearance.

Lumber Plus offers thermally modified wood trim options that can match or complement your selected cladding. For example, the NORX 1x4 (3/4" x 3-1/2" actual) or 1x6 (3/4" x 5-1/2" actual) trim boards in thermally modified Ash or Pine can be used for this purpose. These trims can be installed flush with the cladding or create a slight reveal, depending on the desired aesthetic.

Choosing trim pieces allows for a less precise cut on the cladding boards themselves, as the trim covers the joint. This can reduce installation time and complexity. When selecting trim, ensure its dimensions are appropriate for covering the cladding edges and that it provides adequate protection and visual balance.

Consider the fastening method for trim. It should be securely attached to the underlying structure, typically with appropriate fasteners that account for wood movement. Proper flashing and sealing behind and around trim pieces are crucial for preventing water intrusion.

Worked Example: Designing a Corner for a Shed Cladding Project

Imagine cladding a small garden shed with thermally modified wood, measuring 8 feet by 10 feet. For this example, we'll use NORX thermally modified Pine cladding, which typically comes in 5-1/4" actual face width. We want to achieve a clean, modern look while ensuring durability.

**Option 1: Mitered Corners**

For a seamless appearance, mitered corners are chosen. Each cladding board at the corner will be cut at a 45-degree angle. For the 8-foot wall, boards will be cut to length, and the last board on each side of the corner will have a 45-degree miter. The same applies to the 10-foot walls. This requires careful measurement and cutting for each board to ensure a tight fit. For an 8-foot wall using 5-1/4" face width boards, you'd need approximately 19 boards (96 inches / 5.25 inches per board = 18.28, rounded up to 19 for full coverage plus cuts). The boards at the corner would be cut to meet precisely. This method prioritizes aesthetics but demands high precision during installation.

**Option 2: Trim-Enhanced Corners**

To simplify installation and add a protective element, we opt for NORX thermally modified Pine 1x4 trim boards for the corners. The cladding boards on both sides of the corner will be cut square (90 degrees) to butt against each other. The 1x4 trim (3/4" x 3-1/2" actual) will then be installed over the joint, covering the exposed ends of the cladding. This means the cladding boards would run to within about 1/2" of the corner, leaving space for the trim to cover them completely.

For each vertical corner, two 1x4 trim pieces would be installed, one on each face, overlapping slightly to form a neat corner. For an 8-foot tall corner, you would need an 8-foot length of trim for each face, totaling 16 linear feet per corner for two sides. This method offers easier installation and added protection for the cladding ends, creating a more defined corner aesthetic.

This example illustrates how the choice of corner detail influences both the cutting and installation process, as well as the final visual outcome.

Considerations for Different Profiles

The profile of your thermally modified wood cladding can also influence corner detailing. For example, a tongue-and-groove profile may lend itself well to mitered corners, as the interlocking nature can help maintain alignment. However, the miter cut still needs to be precise across the entire profile.

Shiplap profiles, with their overlapping nature, might require a slightly different approach for mitered corners to ensure the overlap is maintained. For butt joints or trim-enhanced corners, the profile's specific overlap or recess needs to be considered to ensure a clean transition and proper water shedding.

Always review the manufacturer's recommendations for corner details specific to the cladding profile you select. This ensures compatibility and adherence to best practices for that particular product.

Before you order

• Desired Aesthetic: Do you prefer a seamless, continuous look (mitered) or a more defined, trimmed appearance?
• Installation Skill Level: Are you comfortable with precise angle cuts required for mitered joints?
• Budget: Does your budget allow for the potentially higher labor costs of mitered corners or the material cost of trim pieces?
• Cladding Profile: How does the specific profile of your thermally modified wood affect corner detailing?
• Durability and Maintenance: How important is protecting exposed end grain from moisture, and are you prepared for potential future maintenance of mitered joints?
• Available Trim: Are matching or complementary thermally modified wood trim pieces available for your chosen cladding?
• Water Management: How will each corner detail manage water runoff and prevent moisture intrusion?

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

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