Roof Underlayment Guide
Roof Underlayment Guide
Underlayment is the hidden layer between your roof deck and your visible roofing material. Though you never see it once the roof is finished, underlayment is your second line of defense against water intrusion. If wind-driven rain, ice dams, or a damaged shingle allow water past the surface layer, underlayment is what stands between that water and the interior of your home. Choosing the right underlayment and installing it correctly is critical for a long-lasting, leak-free roof.
Three Types of Roof Underlayment
Asphalt-Saturated Felt (Tar Paper)
Felt underlayment has been the standard for over a century. It consists of a fiberglass or organic mat saturated with asphalt to provide water resistance. It comes in two common weights:
- #15 felt (sometimes called 15-pound felt): Thinner, less durable, cheaper. Adequate as a temporary weather barrier but tears easily and wrinkles when wet. Commonly used under standard asphalt shingle roofs in mild climates.
- #30 felt (30-pound felt): Roughly twice as thick and significantly more durable than #15. Better resistance to tearing during installation and foot traffic. Required by some codes for steeper pitches and under certain roofing materials like Wood Shingles and Shakes.
Roll size: Standard rolls cover approximately 400 square feet (36 inches wide by approximately 144 feet long for #15, 72 feet for #30).
Cost: Approximately $0.05 per square foot -- the cheapest underlayment option.
Pros: Inexpensive, widely available, familiar to all contractors, adequate for many applications.
Cons: Absorbs water (can wrinkle and buckle when wet), tears easily during installation and in wind, becomes brittle in heat, can outgas and stick to shingles in extreme heat, heavier than synthetic alternatives, cannot be left exposed for more than a few days without degradation.
Synthetic Underlayment
Synthetic underlayment is made from woven or spun polypropylene or polyethylene. It has largely replaced felt in modern roofing due to its superior performance in nearly every category.
Roll size: Typically covers 1,000 square feet per roll -- more than double the coverage of felt, which means fewer rolls to buy, fewer seams, and faster installation.
Cost: $0.10 to $0.25 per square foot -- moderately more expensive than felt, but the labor savings from faster installation often offset the material cost difference.
Pros: Lightweight (much easier to handle on a roof), extremely tear-resistant, does not absorb water or wrinkle, lays flat and stays flat, can be left exposed for weeks or months (UV-stabilized formulas), provides better traction for workers on steep roofs, wrinkle-resistant surface produces a smoother finished roof.
Cons: More expensive than felt, some cheaper brands can be slippery when wet (look for textured/grip surface versions), not all products are vapor-permeable (check specifications for your application).
Ice and Water Shield
Ice and water shield is a self-adhering rubberized asphalt membrane that sticks directly to the roof deck. Unlike felt and synthetic underlayment, which are merely water-resistant, ice and water shield is fully waterproof. Its critical advantage is that it seals around nail penetrations -- when a roofing nail is driven through it, the rubberized material grips the nail shank, preventing water from wicking down through the nail hole.
Roll size: Typically 36 inches wide by 65-75 feet long, covering approximately 195 to 225 square feet per roll.
Cost: $0.50 to $1.00 per square foot -- significantly more expensive, which is why it is used strategically in high-risk areas rather than over the entire roof (though some premium installations do use full-deck coverage).
Pros: Fully waterproof, self-sealing around fasteners, critical protection against ice dams, prevents leak damage in the most vulnerable roof areas (valleys, eaves, penetrations).
Cons: Expensive, permanently bonds to decking (makes future tear-off more difficult), cannot be repositioned once placed, requires temperatures above 40F for proper adhesion, can trap moisture if applied over wet decking.
Cost Comparison
| Type | Cost per Sq Ft | Roll Coverage | Rolls per 2,000 Sq Ft Roof | Material Cost per 2,000 Sq Ft Roof |
|---|---|---|---|---|
| #15 Felt | $0.05 | ~400 sq ft | 5 rolls | ~$100 |
| #30 Felt | $0.07 | ~200 sq ft | 10 rolls | ~$140 |
| Synthetic | $0.10 - $0.25 | ~1,000 sq ft | 2 rolls | $200 - $500 |
| Ice & Water Shield | $0.50 - $1.00 | ~200 sq ft | *strategic areas only | $150 - $400 (partial coverage) |
- Ice and water shield is typically applied only at eaves, valleys, and penetrations, covering roughly 15-25% of total roof area in cold climates.
Installation Order
The correct installation sequence is critical. Each layer must overlap the one below it so water always flows over the next layer, never under it. Getting this order wrong is one of the most common DIY mistakes.
Step 1: Ice and Water Shield (Where Required)
Ice and water shield goes down first, directly onto clean, dry roof decking.
At eaves: Apply starting at the eave edge, extending at least 24 inches past the interior wall line below. In most cold-climate areas, this means 3 to 6 feet up from the eave edge. Building codes in cold-climate states and provinces typically mandate this coverage.
In valleys: Apply a full-width strip (36 inches) centered in the valley, extending the entire length. This is the single most important leak-prevention measure in any valley.
Around penetrations: Apply around all roof penetrations -- vent pipes, chimneys, skylights, and anything else that passes through the roof deck.
Step 2: Drip Edge at Eaves
The drip edge at the eaves goes over the ice and water shield (or directly on the deck if no ice and water shield is used) and under the felt or synthetic underlayment. This allows water running down the underlayment to flow over the drip edge and into the gutter rather than behind it.
Step 3: Felt or Synthetic Underlayment
Starting at the eave (overlapping the drip edge and any ice and water shield by at least 2 inches), roll out the underlayment horizontally across the roof. Each subsequent course overlaps the one below by at least 4 inches (6 inches for felt in high-wind areas). Fasten with cap nails or staples (cap nails are preferred and code-required in many jurisdictions for synthetic).
Continue courses up the roof to the ridge. At the ridge, the underlayment from each side should overlap at the peak.
Step 4: Drip Edge at Rakes
This is the step many people get wrong. At the rakes (the sloped edges of a gable roof), the drip edge goes over the underlayment -- the opposite of the eave installation. This prevents wind-driven rain from getting under the underlayment at the roof's edges.
The memory aid: Eave = drip edge under underlayment. Rake = drip edge over underlayment.
Step 5: Roofing Material
With underlayment and drip edge properly installed, the roof is weather-tight and ready for the final roofing material -- Asphalt Shingles, Metal Roofing, or whatever you have selected.
When Ice and Water Shield Is Required by Code
Building codes vary by jurisdiction, but ice and water shield is commonly required in these situations:
- Cold climate zones: Most areas where the average January temperature is 25F (-4C) or below require ice and water shield at eaves. This covers most of the northern United States and virtually all of Canada.
- Low-slope roofs: Roofs with a pitch between 2:12 and 4:12 often require ice and water shield over the entire deck because low slopes are more vulnerable to wind-driven rain and ice backup.
- Valleys: Many codes require ice and water shield in all roof valleys regardless of climate zone.
- Around penetrations: Increasingly required around all roof penetrations, especially in updated building codes.
- Eave coverage: When required, must extend from the eave edge to at least 24 inches past the interior wall line below -- this ensures the ice and water shield covers the area most vulnerable to ice dam leaks.
Always check with your local building department. See Roofing Building Codes and Permits for guidance on navigating code requirements.
Choosing the Right Underlayment
For a standard asphalt shingle roof in a moderate climate: Synthetic underlayment over the field, ice and water shield at eaves and valleys. This combination provides the best balance of performance and cost.
For a budget-conscious project: #30 felt is acceptable for most applications. Avoid #15 felt if possible -- the small cost savings is not worth the reduced durability and tear resistance.
For a premium or long-life roof (Metal Roofing, Slate Roofing, Tile Roofing): High-quality synthetic underlayment is recommended because it will not deteriorate under the long-lasting roofing material. Some metal and tile roofs require specific underlayment types -- always check manufacturer requirements to maintain your warranty.
For flat or very low-slope roofs: Traditional underlayment is not used. EPDM Roofing, TPO Roofing, PVC Roofing, and Modified Bitumen Roofing systems have their own specific membrane and adhesive systems.
