Planning Movement Joints with Hardwood Siding

Understanding Wood Movement in Hardwood Siding

Hardwood siding, whether tropical species like Ipe or Cumaru, or thermally modified options like Ash or Ayous, will experience dimensional changes due to variations in humidity and temperature. This movement is most pronounced across the grain (width and thickness) rather than along the length of the board. The extent of this movement depends on the wood species, its initial moisture content, and the environmental conditions it will be exposed to.

For example, a tropical hardwood like Ipe is known for its density and stability, but it still requires careful consideration for movement. Thermally modified woods, while engineered for enhanced stability, also exhibit some degree of movement. Ignoring these inherent characteristics can compromise the entire siding assembly, leading to costly repairs down the line.

Factors Influencing Movement Joint Planning

Several factors dictate the necessary planning for movement joints in hardwood siding:

Wood Species and Profile: Different species have varying stability. Dense hardwoods and thermally modified woods generally exhibit less movement than some softer woods. The profile of the siding (e.g., tongue-and-groove, shiplap, rainscreen) also affects how movement is accommodated. Rainscreen systems, for instance, often inherently provide more space for movement due to their open-joint design.

Environmental Exposure: Siding on sun-exposed elevations or in climates with significant humidity swings will experience greater movement. Coastal environments, for example, often have higher ambient humidity.

Board Length and Width: Longer and wider boards will accumulate more overall movement, requiring larger or more frequent movement joints. Shorter boards or narrower profiles might allow for more flexibility in joint placement.

Fastening System: The type of fastener (face-fastened vs. hidden fasteners) and the rigidity of the fastening system can influence how movement is managed. Hidden fastening systems often require more precise planning for movement to prevent stress on the fasteners or the wood itself.

Types of Movement Joints for Hardwood Siding

Movement joints in hardwood siding typically fall into a few categories:

Expansion Gaps Between Boards: This is the most common form of movement accommodation, where a small gap is left between individual siding boards. For open-joint rainscreen systems, these gaps are inherent to the design. For closed-joint profiles, these gaps are often minimal but still critical.

Vertical Control Joints: For very long stretches of horizontal siding, vertical control joints are necessary to break up the continuous run and prevent excessive cumulative expansion. These are often integrated at building corners, changes in material, or at predetermined intervals along a wall.

Perimeter Gaps: All siding assemblies require a perimeter gap where the siding meets other building elements like windows, doors, soffits, or base trim. These gaps allow for movement around fixed elements and are typically sealed or flashed.

Determining Spacing for Movement Joints

The exact spacing for movement joints is project-specific and should be determined based on the chosen wood species, expected moisture content variations, and the siding profile. As a general guideline, manufacturers often provide recommendations for spacing between boards and at perimeter details. Always consult the specific product documentation for your chosen hardwood siding.

For horizontal siding, vertical control joints are typically recommended every 16 to 20 feet of continuous run, or at changes in the wall plane. These joints should extend from the top to the bottom of the siding section and be wide enough to accommodate the cumulative movement of the boards they interrupt. They can be detailed with a trim piece, a channel, or a specialized flashing system.

These gaps allow for both expansion and contraction and are crucial for preventing the siding from binding against other building components. These gaps are typically concealed by trim or sealed with appropriate backer rod and sealant.

Worked Example: Planning Vertical Control Joints

Consider a hypothetical project with a continuous horizontal run of 32-foot long hardwood siding on an exterior wall. The chosen siding profile is a 5.5-inch face-fastened board. To manage cumulative movement, we will plan for vertical control joints.

Given the wall length, a single uninterrupted run of 32 feet would accumulate significant movement. A practical approach is to break this run into two sections. We could place a vertical control joint at the 16-foot mark. This means we would have two 16-foot sections of continuous siding.

For each 16-foot section, the cumulative movement of the boards will be contained within that section and accommodated by the vertical control joint. This joint should be designed to be wide enough to handle the total expansion of 16 feet of hardwood, plus any dimensional changes in the trim components of the joint itself. A 3/4-inch to 1-inch wide vertical control joint, detailed with a weather-resistant trim piece or an open channel, would be a suitable starting point, but the exact dimension should be verified with the siding manufacturer's guidelines for the specific species.

Detailing and Installation Best Practices

Proper detailing of movement joints is as important as their placement. Use weather-resistant materials for any trim or flashing components within the joint. Consider the use of a breathable membrane or flashing behind the joint to ensure water management.

When installing, ensure that the recommended gaps are consistently maintained. This may involve using spacers during installation. For perimeter joints, ensure that sealants are applied over a backer rod to allow for proper movement without adhesion on three sides, which can lead to premature sealant failure. Always follow the manufacturer's specific installation instructions for your chosen hardwood siding product.

For rainscreen applications, the inherent gaps between boards often serve as the primary movement accommodation, but vertical control joints for very long runs and perimeter gaps are still necessary.

Before you order

• Consult product documentation for specific movement allowances of your chosen hardwood species and profile.
• Identify continuous horizontal runs of siding and plan for vertical control joints at appropriate intervals (e.g., every 16-20 feet).
• Determine required perimeter gaps around all openings (windows, doors) and where siding meets other materials (soffits, base trim).
• Select appropriate detailing for vertical control joints and perimeter gaps (e.g., trim, flashing, sealant with backer rod).
• Ensure the fastening system chosen is compatible with the anticipated wood movement and joint design.
• Plan for consistent gap maintenance during installation using spacers or jigs.

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

Keep planning your project