How to Use Roofing Felt: A Complete Step-by-Step Guide
Roofing felt — also known as roofing underlayment or tar paper — is a crucial layer of protection installed beneath your roofing shingles or tiles. It acts as a secondary barrier against moisture, wind, and weather damage, helping to extend the life of your roof. Whether you’re tackling a DIY roofing project or simply want to understand the process, this guide will walk you through everything you need to know about using roofing felt correctly.
🛠️ What You’ll Need
Before you get started, gather the following tools and materials:
- Roofing felt (15 lb or 30 lb, depending on your roof type)
- Roofing nails or staples
- Hammer or staple gun
- Utility knife or roofing scissors
- Measuring tape
- Chalk line
- Ladder and roof safety equipment
- Roofing cap nails (for extra hold in windy conditions)
📋 Step 1: Choose the Right Roofing Felt
Roofing felt comes in two main weights:
- 15 lb felt – Lighter and more affordable, suitable for low-slope roofs or as a temporary cover.
- 30 lb felt – Heavier, more durable, and better suited for steep-slope roofs and areas with harsh weather conditions.
Always check local building codes and the roofing manufacturer’s recommendations before choosing your underlayment.
🔒 Step 2: Prepare the Roof Deck
Before laying roofing felt, make sure your roof deck (typically plywood or OSB boards) is:
- Clean and free from debris, dirt, and old roofing materials
- Dry — never install felt on a wet surface
- Structurally sound — replace any damaged or rotting boards
- Smooth — ensure there are no protruding nails or sharp edges
A well-prepared deck ensures the felt lays flat and provides maximum protection.
📏 Step 3: Start at the Bottom Edge (Eaves)
Always begin laying roofing felt from the bottom of the roof and work upward. This ensures each row overlaps the one below it, directing water away from the deck and preventing moisture from seeping underneath.
- Unroll the first row of felt along the eave (bottom edge) of the roof.
- Allow a slight overhang of about 1–2 cm past the drip edge.
- Keep the felt straight by using a chalk line as a guide.
📌 Step 4: Fasten the Felt Securely
Once the first row is in position, secure the felt to the roof deck:
- Use roofing nails or staples spaced approximately 20–30 cm apart along the top edge.
- Drive nails through the felt and into the deck — do not over-drive as this can tear the felt.
- In windy conditions, use roofing cap nails (also called plastic cap nails) for a more secure hold and to prevent tearing.
🔄 Step 5: Overlap Each Row Correctly
Proper overlapping is one of the most important steps in installing roofing felt. Incorrect overlaps can allow water to infiltrate beneath the material.
- Each successive row should overlap the one below by at least 10–15 cm (4–6 inches).
- At the ridge (peak of the roof), overlap felt from both sides by at least 15 cm.
- At vertical joints (where two pieces meet side by side), overlap by at least 10 cm.
- Most roofing felt rolls have printed guidelines to help you maintain consistent overlaps.
✂️ Step 6: Cut and Fit Around Roof Features
Roofs often have features like chimneys, skylights, vents, and valleys that require careful cutting and fitting of the felt:
- Use a utility knife to make precise cuts around protrusions.
- In valleys (where two roof slopes meet), lay a double layer of felt for extra water protection.
- Around chimneys and skylights, cut the felt so it fits snugly and seal edges with roofing adhesive or flashing tape.
🏠 Step 7: Work Your Way to the Ridge
Continue unrolling, cutting, and fastening rows of felt working upward toward the roof’s ridge. As you go:
- Maintain consistent overlaps at each row.
- Keep rows as straight as possible using a chalk line.
- Avoid wrinkles or bubbles — smooth out the felt as you nail it down.
- Ensure all edges are properly fastened to prevent wind from lifting the felt.
✅ Step 8: Inspect Your Work
Once all the felt is laid, do a thorough inspection before installing the final roofing material (shingles, tiles, etc.):
- Check for any tears, punctures, or missed areas.
- Ensure all overlaps are correct and consistent.
- Verify all edges and corners are fully secured.
- Repair any damaged sections with a patch of felt and roofing adhesive.
⚠️ Safety Tips to Remember
Working on a roof can be hazardous. Always prioritise safety:
- Use a sturdy, properly positioned ladder.
- Wear non-slip footwear with good grip.
- Use a safety harness when working on steep pitches.
- Never work on a wet or icy roof.
- Work with a partner when possible.
- Be aware of power lines near your roof.
💡 Pro Tips for Best Results
- Install felt on a dry day — wet felt is slippery and will not adhere or fasten properly.
- Don’t leave felt exposed for long — UV rays can degrade roofing felt quickly. Install your final roofing material as soon as possible after laying the felt.
- Use a chalk line to keep rows straight and ensure even overlaps throughout the installation.
- Double-layer in vulnerable areas such as eaves, valleys, and around penetrations for extra protection.
Final Thoughts
Roofing felt is an essential component of any well-built roof. When installed correctly, it provides a vital layer of weather protection that helps safeguard your home for years to come. By following these steps carefully and prioritising safety throughout the process, you can successfully install roofing felt as part of your roofing project.
If you’re ever unsure about any aspect of your roofing project, don’t hesitate to consult a professional roofer — getting it right the first time can save significant time, money, and stress in the long run.
