The Importance of Zero Gap Between Siding and Wall Sheathing

When discussing the installation of siding, a common question arises about the acceptable gap between the siding and the wall sheathing. While some professionals might advocate for a gap, my view is that there should be zero acceptable gap. This article will explain why a zero gap is crucial for preventing moisture-related issues and ensuring the longevity of the siding and overall structure.

Why Zero Gap is Essential

When evaluating the installation of siding, it's important to consider the potential for moisture penetration. Martin's answer, while potentially well-intentioned, puts the emphasis on what is 'acceptable' rather than focusing on what is truly safe and effective. What is more critical is understanding why there should be no gap between the siding and the wall sheathing at all.

Preventing Moisture Penetration: A gap between the siding and the sheathing creates an opening that allows for water movement. Any moisture that penetrates the siding can easily find its way inside the wall cavity and onto the wall sheathing. This is particularly problematic because moisture can lead to wood rot, mold growth, and other structural issues. Path of Least Resistance: Water always seeks the path of least resistance. If there is a gap, the water that penetrates the siding is more likely to move down the gap rather than run over the surface. This means that even small gaps can lead to significant problems, including moisture accumulation behind the sheathing. Structural Integrity: A tight seal between the siding and the sheathing helps maintain the structural integrity of the wall assembly. Without a gap, the siding provides a continuous barrier, which helps prevent any moisture from getting inside the wall.

What Would a Gap Accomplish?

The primary argument for a gap often revolves around the presence of additional layers such as an air control layer, vapor barrier, or insulation. However, these layers do not necessitate a gap. In fact, the gap can undermine the effectiveness of these layers. Here’s why:

No Gap vs. With Gap

Without a Gap: In a perfect installation, the siding is flush with the sheathing. This creates a continuous moisture barrier that helps prevent any moisture from reaching the sheathing. Even when some moisture does penetrate the siding, it has nowhere to go but to escape the wall assembly through proper drainage and ventilation.

With a Gap: If a gap is allowed, it creates a potential entry point for water and a place where water can pool and sit. The gap can accommodate capillary action, which can pull moisture deeper into the wall assembly. This can lead to significant damage and reduce the effectiveness of other layers such as the vapor barrier.

Creating a Zero Gap

Creating a zero gap between the siding and the wall sheathing is typically a straightforward process. The key steps are as follows:

Prepare the Wall Sheathing: Ensure that the wall sheathing is clean, dry, and free from any debris that could interfere with a proper seal. Install Flashing: Use flashing materials to ensure that the edge of the siding is properly sealed. This can be done by using self-adhering membranes or applying waterproof sealant. Apply Sealant: Apply a high-quality sealant around the edges of the siding to ensure a water-tight seal. This sealant should be capable of withstanding the elements and maintaining its integrity over time. Use Gasket Strips: For a more precise fit, use gasket strips to fill any gaps and provide additional insulation. These strips should be placed around the edges of the siding to ensure a tight seal. Ensure Proper Ventilation: While the focus is on preventing gaps, it’s also important to ensure proper ventilation to allow any moisture that does enter the wall cavity to escape.

Conclusion

It’s clear that a zero gap between the siding and the wall sheathing is essential for maintaining the integrity of the wall assembly and preventing moisture-related issues. By understanding the risks associated with gaps and implementing the proper techniques to achieve a tight seal, homeowners and professionals can ensure a more reliable and long-lasting siding installation.

The use of air control layers, vapour barriers, and insulation does not justify the presence of a gap. These layers are intended to improve the performance of the wall assembly, not to create an opening for moisture. Proper installation techniques should aim for a zero gap to ensure the best possible outcomes.