Leveling Subfloor With Roofing Felt: A Practical Guide

Leveling a subfloor is essential for a smooth, durable finish for any floor covering. Roofing felt is sometimes considered as a quick and inexpensive method to address minor irregularities or high spots. This guide explains when roofing felt is appropriate, how to use it safely, and practical alternatives to ensure a stable, long-lasting floor.

Assessing The Subfloor And Setting Realistic Expectations

Before choosing roofing felt for leveling, inspect the subfloor thoroughly. Look for loose boards, squeaks, soft spots, gaps, or water damage. Roofing felt can help address slight variations but is not a substitute for structural repairs or significant unevenness. If the subfloor shows large dips, warping, or movement, these issues should be corrected with proper subfloor repair and, if needed, a structural assessment. Key point: roofing felt works best for minor high spots or small discrepancies, not major leveling needs.

Choosing The Right Type Of Roofing Felt

Roofing felt comes in different weights and applications. For leveling purposes, standard asphalt-saturated felt (15-pound or 30-pound) or synthetic roofing underlayment can be used as a shim-like layer. However, synthetic felts tend to be thinner and more flexible, which can affect uniformity. Do not confuse roofing felt with concrete or self-leveling compounds; felts provide a passive adjustment and can compress over time. Tip: avoid using old or degraded felt that has absorbed moisture, as this can promote mold and compromise rigidity.

Materials And Tools You’ll Need

To level with roofing felt, assemble the following:

• Roofing felt rolls (asphalt-saturated or synthetic underlayment, sized for your area)
• Scissors or utility knife
• Measuring tape and chalk line
• Heavy-duty stapler or roofing nails and hammer
• Spirit level or laser level
• Plywood shims or thin plywood sheets (optional for thicker adjustments)
• Safety gear: eye protection, gloves
• Adhesive intended for felt (if recommended by the manufacturer)

Step-By-Step: How To Level A Subfloor With Roofing Felt

The following steps focus on achieving a smoother surface for the next layer, typically underlayment or finished flooring. Work in small sections to maintain accuracy.

1) Prepare And Clean Clear the area and remove debris. Sweep or vacuum thoroughly to ensure the felt has consistent contact with the subfloor. A clean surface prevents creases and air pockets that can telegraph through to the finished floor.

2) Identify High Spots And Measure Using a straightedge and level, locate high spots and deviations. Mark these areas with chalk so the felts can be applied strategically. Plan to distribute compression evenly across the floor to avoid creating new bumps.

3) Cut And Dry-Fit Felt Cut strips or sheets of felt that will cover the irregular areas. Lay them in a dry-fit pattern, ensuring no overlaps exceed recommended widths. If using strips, stagger seams like a parquet floor to reduce potential creasing.

4) Secure The Felt Fasten the felt to the subfloor with a staple gun or nails, depending on the subfloor type. Space fasteners according to manufacturer guidance. Avoid tearing or stretching the felt during installation, which can create tension and future gaps.

5) Check Flatness Re-measure with a level across the treated areas. The goal is to achieve a flatter, more uniform plane when you place the next layer. If high spots persist, apply additional felt or consider supplementary shims to balance the surface.

6) Add A Thin Patch Layer If Needed For stubborn irregularities, you can layer a very thin patch of felt or a compatible underlayment product. Ensure the combined thickness does not create an edge lift when the final floor is installed.

7) Install The Next Layer Once the felt is secured and the surface reads true, install the chosen underlayment or finish flooring per its installation instructions. Avoid products that demand a perfectly pristine substrate if the felt layer remains slightly uneven.

Practical Considerations And Limitations

Leveling with roofing felt is a minimal intervention. It is not a cure for significant structural issues or for floors with substantial variances. The method can help in some cases, but professionals often prefer engineered solutions when precision is critical, such as for tile, hardwood, or stone installations. Important: roofing felt compression can change over time, especially in humid conditions or high-traffic areas, potentially affecting long-term flatness.

When Roofing Felt Is A Suitable Option

Roofing felt may be appropriate in the following scenarios:

• Minor, consistent high spots across a small area where major leveling compounds are not practical
• As a temporary solution in renovation projects where quick flattening is needed before final flooring
• As a spacer for low-height installations where adding a full underlayment is not feasible

In these cases, document the limitations and plan for a more permanent fix in future projects if possible.

Common Mistakes To Avoid

• Using old or damp felt that can promote mold or decay
• Overlapping seams or stretching the felt, which creates ridges
• Relying solely on felt for major leveling without addressing structural causes
• Ignoring manufacturer guidelines for staple or nail spacing and acceptance of underlayment products

Alternatives To Roofing Felt For Leveling

For more pronounced unevenness, consider:

• Self-leveling underlayment compounds that create a smooth, ready-to-tile surface
• Thin plywood shims or an additional plywood layer to build up low areas
• Spot grinding or planing of high spots where the subfloor is wood
• Engineered subfloor panels or new subfloor installation for significant deviations

Each alternative has different costs, drying times, and compatibility with finished floors. It is essential to match the method to the final floor covering and the subfloor material.

Safety And Best Practices

Wear safety gear when cutting and handling materials. Ensure good ventilation if adhesives or heavy underlayment are involved. If moisture issues exist, address them before applying any leveling approach to prevent mold and substrate damage. Follow local building codes and manufacturer instructions for the specific underlayment and flooring products used in the project.