Thermally Modified Ash at the Base of a Wall: Clearance and Drainage

Why Ground Clearance Matters for Thermally Modified Ash

Thermally modified ash offers enhanced stability and resistance to moisture compared to untreated wood, making it suitable for exterior applications like cladding. Maintaining adequate ground clearance serves several key purposes:

First, it prevents the cladding from wicking moisture directly from the ground, which can lead to swelling, discoloration, and, over time, degradation. Second, it allows for proper airflow behind the cladding, aiding in the drying process and reducing the risk of moisture accumulation within the wall assembly. Third, it helps keep dirt, mulch, and other landscaping debris from accumulating against the wood, which can trap moisture and create an environment for mold or mildew growth. Finally, sufficient clearance can deter pests that might otherwise use the cladding as a bridge to enter the structure.

Recommended Ground Clearance Guidelines

This measurement should be taken from the lowest point of the ground adjacent to the wall, considering any landscaping, paving, or other elements that might raise the effective grade. In areas where splash-back from rain or snowmelt is a significant concern, or where landscaping is particularly dense, a greater clearance may be prudent.

Managing Drainage at the Wall Base

Effective drainage is just as important as ground clearance. Even with ample clearance, poor drainage can lead to water pooling near the foundation, increasing humidity and potential splash-back onto the cladding. Consider the following:

Slope the grade away from the building: Ensure the ground slopes away from the foundation at a minimum rate of 1/4 inch per foot (2%). This directs water away from the wall.

Install perimeter drainage: For areas with heavy rainfall or poor soil drainage, a perimeter drain (e.g., a French drain) can help collect and divert subsurface water.

Avoid planting directly against the wall: Keep shrubs, plants, and mulch several inches away from the cladding to allow for air circulation and prevent moisture retention. If using a planting bed, consider a gravel border directly at the base of the wall.

Address downspouts: Ensure all downspouts discharge water well away from the foundation, ideally into a drainage system or onto a splash block that directs water at least several feet from the wall.

Worked Example: Planning Ground Clearance for a Clad Wall

Consider a new residential addition with a perimeter of 40 linear feet (a 10 ft by 10 ft square). The design calls for horizontally installed thermally modified ash cladding. The lowest cladding board is 5.5 inches wide (nominal 1x6 Plus S4S E4E End Match profile).

The project site has a finished grade that includes a 2-inch layer of decorative gravel around the foundation. The client wants to ensure maximum durability for the cladding.

Step 2: Calculate the required height of the foundation wall or base trim. If the foundation wall extends 12 inches above the original soil grade, and there's a 2-inch gravel layer on top, the top of the gravel is 12 inches above the original soil.

Step 3: Specify the starting point for cladding installation. The first course of thermally modified ash cladding should begin at least 7 inches above the finished gravel grade.

Step 4: Confirm drainage. The landscape plan includes a 1/4-inch per foot slope away from the foundation for at least 6 feet, and all downspouts are directed into an underground drainage system. This ensures water will not pool near the base of the clad wall.

Thermally Modified Ash Profiles for Cladding

Thermally modified ash is available in several profiles suitable for cladding, allowing for various aesthetic and installation approaches. For applications at the base of a wall, common profiles include S4S E4E End Match in nominal sizes like 1x6 Plus or 5/4x6. These profiles offer a smooth, uniform surface that can be installed horizontally or vertically, depending on the desired architectural effect and rainscreen design.

The end-matched feature of some profiles can help minimize waste and simplify installation by allowing boards to be joined between studs, though careful planning for ground clearance remains essential regardless of the profile chosen. Always confirm the specific profile and its intended application to ensure it aligns with the project's design and performance requirements.

For trim and molding applications around the base or other details, smaller S4S E4E profiles like 1x10, 2x2, 2x4, or 2x6 are also available, offering a cohesive look with the main cladding material.

Before you order

• Have I determined the finished grade level around the entire perimeter of the building?
• What is the minimum ground clearance required by local building codes or best practices for my climate?
• Does the proposed landscaping (mulch, plants, paving) impact the effective ground level?
• Is there sufficient slope away from the foundation (at least 1/4 inch per foot for 6-10 feet)?
• Are all downspouts directed away from the foundation, or into a drainage system?
• Have I considered a gravel strip or other non-absorbent barrier directly against the foundation?
• What is the height of the foundation wall or base trim that will be exposed below the cladding?
• Is the chosen thermally modified ash profile suitable for exterior cladding and compatible with the intended installation system?
• Have I accounted for any potential splash-back from heavy rain or snowmelt in the clearance measurement?
• Will there be adequate airflow behind the cladding to promote drying?

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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