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Roofing Felt Overlap: A Complete Guide for Homeowners and Builders
When it comes to roofing installation, one of the most critical steps is understanding roofing felt overlap. This detail may seem small, but it plays a huge role in keeping a roof watertight, durable, and capable of withstanding the elements. Overlapping roofing felt properly ensures that water sheds correctly, prevents leaks, and extends the life of the roof. Whether you are tackling a DIY roofing project or just want to understand what your contractor is doing, knowing how roofing felt overlap works can save you from costly mistakes.
In this guide, we’ll break down what roofing felt is, why overlapping matters, how to install it correctly, common mistakes, and best practices used by professionals.
What Is Roofing Felt?
Roofing felt, often called tar paper or underlayment, is a protective barrier that goes between the roof decking and the shingles or other final roofing material. Its primary function is to provide an extra layer of moisture protection.
- Materials: Traditionally made of a base of wood cellulose, fiberglass, or polyester soaked in asphalt.
- Types: Comes in different thicknesses, typically 15-pound or 30-pound felt.
- Purpose: Creates a temporary waterproof barrier before shingles are installed and serves as a secondary defense against leaks.
Why Roofing Felt Overlap Matters
The way roofing felt is installed is just as important as using it in the first place. Roofing felt must overlap to prevent water from sneaking through seams.
- Water Resistance: Proper overlap ensures water runs down the slope without seeping under the underlayment.
- Wind Protection: Overlap prevents strong gusts from lifting the felt.
- Durability: A good overlap minimizes the chance of premature wear or gaps.
- Shingle Support: Even though shingles are the first line of defense, overlap in felt ensures backup protection.
How Much Should Roofing Felt Overlap?
The amount of overlap depends on the slope of the roof and the type of felt being used.
- Standard Overlap (Horizontal):
- For 15-pound felt: typically 2 inches.
- For 30-pound felt: typically 3–4 inches.
- Vertical Overlap (End Laps):
- Minimum 4 inches for all types of felt.
- Low-Slope Roofs:
- Require more overlap (at least 19 inches) to provide added protection against standing water.
- Steep-Slope Roofs:
- Standard overlaps work since water drains quickly.
Step-by-Step Guide to Installing Roofing Felt with Proper Overlap
Step 1: Prepare the Decking
- Ensure the roof decking is clean, smooth, and dry.
- Remove nails, splinters, or debris that may puncture the felt.
Step 2: Start at the Bottom
- Always begin at the eaves and work your way upward.
- This ensures each overlap directs water downward without entering seams.
Step 3: Roll Out the Felt
- Lay the first course horizontally along the eaves.
- Secure with roofing nails or staples, placed about every 6–8 inches along edges.
Step 4: Overlap Correctly
- Horizontal laps: maintain 2–4 inches depending on felt weight.
- Vertical laps: minimum of 4 inches.
- On low-slope roofs, increase overlaps significantly.
Step 5: Continue Up the Roof
- Each layer should cover the top edge of the one below.
- Think of it like shingles: water should always shed downward.
Step 6: Finish at the Ridge
- At the peak, felt should overlap evenly on both sides.
- Add ridge cap underlayment if needed for extra protection.
Common Mistakes with Roofing Felt Overlap
Even small errors in overlap can compromise the roof.
- Too Little Overlap: Water can seep in at the seams.
- Overlapping Wrong Direction: If seams face uphill, water runs under instead of over.
- Loose Installation: Felt must be tight; wrinkles can catch water.
- Staples Without Caps: Staples alone can tear felt, allowing leaks.
- Skipping End Laps: Vertical seams are often neglected, but they’re critical.
Roofing Felt Overlap for Different Roof Types
Asphalt Shingle Roofs
- Standard overlaps are typically enough.
- Focus on clean lines and consistent spacing.
Metal Roofs
- Require underlayment overlap but may also need synthetic felt for added heat resistance.
Tile Roofs
- Because tiles shed water differently, underlayment overlap must be larger to handle runoff.
Low-Slope Roofs
- Critical to use wider overlaps since water drains slowly.
- Sometimes double-layers of felt are used.
Roofing Felt vs. Synthetic Underlayment
While traditional felt is still widely used, synthetic underlayment has become popular.
- Felt: Affordable, effective, but heavier and can wrinkle when wet.
- Synthetic: Lighter, tear-resistant, better water-shedding, but more expensive.
Regardless of the material, overlap principles remain the same.
How Roofing Felt Overlap Protects Against Weather
- Rain: Proper overlap prevents water from working its way under shingles.
- Snow: Creates a protective barrier against melting snow.
- Wind: Overlapping reduces uplift during storms.
- Ice Dams: Provides backup when melting water tries to seep under shingles.
Building Codes and Roofing Felt Overlap
Many regions have building codes that specify minimum overlaps for roofing felt. These requirements are based on climate and roof pitch. Always check local codes before starting a project.
Professional Tips for Perfect Roofing Felt Overlap
- Chalk Lines: Use chalk lines to keep felt straight and overlaps consistent.
- Weather Check: Avoid installing on windy or rainy days.
- Cap Nails: Use cap nails or plastic caps instead of bare staples for better holding power.
- Double Coverage: On low-slope roofs, double coverage may be required for water protection.
- Work Methodically: Always maintain overlap as you progress upward.
Repairing Problems with Roofing Felt Overlap
Sometimes, overlap isn’t done properly or damage occurs.
- Minor Tears: Patch with a piece of felt overlapped at least 6 inches beyond the tear.
- Misaligned Overlaps: Remove the section and reinstall correctly.
- Water Damage: Replace damaged felt immediately to protect decking.
Costs of Installing Roofing Felt
The cost of underlayment installation depends on roof size and material:
- 15-Pound Felt: $5–10 per roll (covers about 400 square feet).
- 30-Pound Felt: $10–20 per roll (covers about 200 square feet).
- Synthetic Felt: $75–200 per roll (covers 1,000+ square feet).
Labor is the biggest factor, but proper overlap doesn’t add much time and is always worth it.
Roofing Felt Overlap and Longevity
Correctly installed felt with proper overlap can last decades beneath shingles. Without overlap, the roof becomes vulnerable to water intrusion, leading to wood rot, mold, and expensive repairs.
Historical Use of Roofing Felt
Roofing felt has been used for over a century. Early versions were simple tar-impregnated paper. Today’s products are more durable, but the principle of overlap has never changed—it’s always been about keeping water out.
Roofing Felt Overlap in DIY Projects
If you’re re-roofing a shed, garage, or small structure, proper overlap is still essential. Even small roofs can suffer leaks if seams aren’t aligned correctly.
Comparing Roofing Felt Overlap to Shingle Overlap
Just as shingles overlap to shed water, so does roofing felt. The difference is that shingles are the visible, finished layer, while felt serves as a hidden layer of defense. Both rely on overlap to perform effectively.
Frequently Asked Questions
1. Can I install roofing felt without overlap?
No. Overlap is essential to prevent water intrusion.
2. How do I know if the overlap is enough?
Follow manufacturer guidelines: usually 2 inches horizontally, 4 inches vertically.
3. What happens if felt wrinkles under shingles?
Minor wrinkles may flatten out, but large ones can cause uneven shingles. Always keep felt tight.
4. Is synthetic underlayment overlapped the same way?
Yes, though manufacturers may allow slightly different measurements.
5. Can roofing felt be exposed without shingles?
Temporarily, yes, but prolonged exposure can degrade it. Always cover with shingles as soon as possible.
Conclusion
The key to a watertight, long-lasting roof often comes down to details like roofing felt overlap. By making sure each sheet overlaps properly—whether horizontally or vertically—you’re ensuring water sheds correctly, wind doesn’t lift the underlayment, and your shingles have reliable backup protection. Overlap may seem like a small step, but it’s one of the most important parts of a roofing system.
When done right, roofing felt overlap gives peace of mind, better roof performance, and long-term protection against the elements. 440-235-3124
