Leveling Subfloor With Roofing Felt: Best Practices and Alternatives

Using roofing felt to level a subfloor is a topic that often raises questions among DIYers and professionals. While roofing felt provides a moisture barrier and can serve as a temporary underlayment, it is not designed to compensate for uneven subfloor surfaces. Proper leveling improves floor stability, prevents movement, and extends the life of finished flooring. This article explains why roofing felt is not ideal for leveling, presents safer alternatives, and offers practical steps for addressing subfloor unevenness in a typical American home.

Why Roofing Felt Is Not Ideal For Leveling

Roofing felt is a flexible, thin sheet used to resist moisture and protect roofing assemblies. When placed on a subfloor, its thickness is inconsistent, and it lacks the rigidity required to create a flat, stable surface. Relying on felt to fill dips or compensate for high spots can result in a bouncy floor, squeaks, or a finished floor that shows irregularities over time. In addition, felt can compress under load, especially under heavy furniture or appliances, further diminishing any leveling effect.

Key limitation: Roofing felt does not cure or harden like leveling compounds and is not rated to transfer loads evenly across a large area. For most flooring installations, a stable, rigid substrate is essential, and felt alone cannot provide that stability.

Better Alternatives For Leveling A Subfloor

Several proven methods create a truly level, solid foundation for finished floors. The choice depends on subfloor type (wood, concrete, or a combination), room size, and the expected load. The following options are commonly used in U.S. homes:

• Self-leveling underlayment: A cementitious compound poured over clean, prepared subfloors to create a smooth, level surface. It works well for slight to moderate irregularities and can tolerate passable moisture levels when paired with the right primer and sealant.
• Plywood or OSB overlay with shims: For minor deviations, overlay the subfloor with a new layer of plywood or oriented strand board (OSB) after adding shims beneath high spots. This method increases rigidity and creates a uniform base for laminates and hardwoods.
• Floor leveling compound: Similar to self-leveling underlayment but designed specifically for irregularities in concrete subfloors. It prepares a flat surface ready for tile, vinyl, or other finishes.
• Concrete resurfacing or grinding: On concrete slabs with high spots or unevenness, professionals may grind high points or apply resurfacer to achieve a uniform plane.
• Structural repair: In cases with significant deflection or rot, joist sistering, replacement, or reinforcement may be necessary before any underlayment work.

When selecting a method, consider moisture management, room use, and the type of finished floor planned. For most living spaces, self-leveling underlayment or a plywood overlay offers the most reliable long-term results.

When Roofing Felt May Play A Role

Roofing felt can still be part of the assembly, but as a moisture barrier rather than a leveling agent. It is appropriate to install felt under roof decking or under certain types of flooring as a secondary barrier against moisture intrusion. If roof or plumbing calculations introduce humidity concerns, felt can contribute to a finished system’s longevity when paired with proper ventilation and sealing techniques.

In subfloor preparation, felt should be used in accordance with manufacturer guidelines and local building codes. It should not be relied upon to disguise or correct structural irregularities. If felt is present, assess the underlying surface for high and low points, and plan to address those with a dedicated leveling method before installing the finished floor.

Step-By-Step Guide If You Use Roofing Felt As A Temporary Measure

Not recommended as a primary leveling solution, but if circumstances necessitate a temporary measure while you plan a proper fix, follow these conservative steps. This approach emphasizes caution and long-term planning over quick fixes:

1. Assess the unevenness: Use a straightedge or a long level to identify high and low spots. Mark them clearly with a pencil.
2. Prepare the surface: Remove debris, dust, and protruding nails from the subfloor to ensure any subsequent material adheres properly.
3. Layer strategy: Do not rely on felt alone. If you must use felt, lay a thin, rigid overlayer (such as a properly installed plywood sheet) over the felt to provide a stable surface. Nail or screw the layer, using appropriate fasteners for the subfloor material.
4. Limit thickness buildup: Do not create a multi-layer thickness that exceeds recommended guidelines for your intended finish. Excessive buildup can create more problems than it solves.
5. Plan a proper leveling protocol: Schedule a follow-up, using a leveling compound or plywood overlay, to achieve a truly flat surface before installing finished flooring.

Important: This approach is a temporary workaround. A qualified installer should complete the final leveling with appropriate materials to ensure long-term performance and warranty compliance.

Tools, Materials, And Safety

• Measuring tools: long level, straightedge, chalk line
• Preparation: broom, vacuum, scraper
• Underlayment options: self-leveling underlayment, plywood overlay, floor leveling compound
• Fasteners: corrosion-resistant screws or nails suitable for subfloor material
• Moisture management: moisture meter, primer, sealant or membrane as required
• Safety: dust mask, safety glasses, hearing protection, gloves

Choosing the right materials reduces the risk of future floor movement. Always follow manufacturer installation instructions and wear appropriate safety gear during preparation and installation.

Maintenance And Inspection After Installation

• Check for movement: walk the room and listen for squeaks or creaks, which can indicate movement or a loose subfloor.
• Inspect for moisture: monitor humidity levels and look for signs of moisture intrusion under the finished flooring, especially in basements or crawl spaces.
• Periodic releveling: over time, seasonal wood movement can reveal new high or low spots. Schedule inspection before installing new finishes if the room experiences significant temperature or humidity changes.
• Finish flooring compatibility: ensure the final floor covering is compatible with the leveling method used and does not require a different substrate for heat, moisture, or load considerations.