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Yes, you can use fiber cement board as an underlayment for a deck, but its suitability depends on the specific application and it’s not a solution for every situation. Here’s a breakdown of the key factors to consider:

When Fiber Cement is a Good Choice for a Deck Underlayment

• When installing a rigid surface: Fiber cement is an excellent choice when you plan to install ceramic tile or natural stone on your deck. It provides a stable, rigid, and solid surface that prevents the movement and flexing that can cause tiles and grout to crack.
• In wet areas: Fiber cement is highly resistant to water damage, rot, and moisture. This makes it ideal for decks in wet environments, such as those near a pool or in a humid climate. While it’s not completely waterproof, it won’t swell, rot, or degrade when exposed to moisture.
• For durability and low maintenance: Fiber cement is a long-lasting material that is resistant to termites, rot, and fire. When used as an underlayment, it creates a durable base that requires little upkeep.

Important Considerations and Limitations

• Not a drainage system: Fiber cement board is water-resistant, but it is not a waterproof barrier. Water will seep through the seams, screw holes, and edges. If your goal is to create a dry space underneath your deck, a purpose-built under-deck drainage system is the correct solution. These systems are designed with integrated channels to collect and direct water away.
• Weight: Fiber cement is a heavy material. You must ensure that your deck’s substructure is strong enough to support the added weight. Your deck’s framing may need to be specifically engineered to handle the load.
• Proper installation is critical: Incorrect installation can lead to problems. You must follow the manufacturer’s instructions for things like:
  • Proper fasteners: Use corrosion-resistant screws designed for fiber cement.
  • Expansion gaps: Leave the correct gaps between boards to allow for expansion and contraction with temperature changes.
  • Substructure support: Ensure the joists are correctly spaced to prevent sagging or warping.
  • Ventilation and drainage: Proper airflow and drainage are essential to prevent moisture from getting trapped and causing mold or rot.
• Health and safety: Cutting fiber cement can create dust containing crystalline silica, which is harmful if inhaled. Always wear a proper dust mask or respirator and work in a well-ventilated area.

In summary, if you are looking to create a durable, tiled deck in a wet area, fiber cement underlayment is an excellent option. However, if you are simply looking for a material to create a dry space underneath a deck, a dedicated drainage system is the more appropriate and effective solution.