Installing new asphalt shingles on your roof? Don’t overlook the importance of using the right underlayment beneath those shingles. Underlayment provides a critical second layer of protection between your shingles and the roof deck, preventing leaks and moisture damage.
But not all underlayments are created equal when it comes to use with asphalt shingles.
What is Asphalt Shingle Underlayment?
Underlayment, also sometimes called felt paper, is a sheet material that is installed under roofing shingles to add a water-resistant barrier. Underlayments for asphalt shingle roofs are typically made from materials like:
- Asphalt-saturated organic felt or fiberglass mat
- Synthetic polymer fabrics
- Rubberized asphalt
- Polypropylene plastic
Underlayment serves several important functions:
- Creates a water barrier as additional protection in case of wind-driven rain and prevents leaks
- Allows shingles to expand and contract with temperature changes
- Helps prevent moisture damage to roof deck and framework
- Provides a smooth and consistent surface for shingle installation
Differences vs. Traditional Roofing Felt
Roofing felt is made from asphalt-saturated paper and was long considered the go-to underlayment for shingle roofs. Today, new synthetic underlayment materials provide improved tear strength, UV resistance, and longevity compared to standard roofing felts.
Pros and Cons of Asphalt Shingle Underlayment
- Prevents leaks and moisture damage
- Allows for expansion and contraction of shingles
- Smooth surface for easier shingle installation
- Added cost for underlayment materials
- Additional installation time and labor
- Some types prone to breakdown over time
Why Underlayment is Critical for Asphalt Shingles
Asphalt shingles are one of the most popular roofing materials because they are affordable, easy to install, and provide good protection. However, despite their durability, asphalt shingles are not 100% waterproof. Underlayment provides valuable extra leak protection.
In particular, underlayment is essential because it acts as a secondary water barrier in case wind-driven rain or melted snow infiltrates through the asphalt shingles. It also allows the shingles room to expand and contract with temperature changes without damaging the roof deck.
Without underlayment, water could leak through the shingles and cause moisture damage, rot, and mold growth in roof rafters and interior ceilings. Underlayment for asphalt shingles really should be considered mandatory, not optional.
Prevents Leaks from Wind-Driven Rain
While asphalt shingles are water-resistant, heavy winds can still force rainwater to seep through the small spaces between shingles. A high-quality underlayment provides a watertight second line of defense to prevent leaks.
Allows for Shingle Expansion and Contraction
Asphalt shingles expand and contract with changing temperatures. Without underlayment, this could cause the shingles to buckle or lift off the roof in extreme heat or cold. The underlayment provides a smooth protective layer that allows for this expansion and contraction.
Protects Roof Deck from Moisture Damage
Underlayment shields the roof deck (typically plywood or OSB boards) from moisture infiltration. This prevents warping, rotting, mold growth, and deterioration of the roof deck that could otherwise shorten the lifespan of your roof.
Key Features to Look For in Asphalt Shingle Underlayment
All asphalt shingle underlayment products are not the same. Here are some of the key performance features and specs to evaluate when selecting an underlayment:
The primary job of the underlayment is to add a waterproof layer, so water resistance is critical. Look for products made with waterproof materials and coated with water repellants.
Underlayment takes the full brunt of UV exposure before shingles are installed. Look for underlayment made with UV inhibitors that won’t break down quickly in sunlight.
High Tear Strength
To avoid rips and damage during installation and high winds, the underlayment should have a high tear strength rating.
The underlayment needs to allow water vapor from inside the attic to pass through to prevent moisture buildup.
Underlayment must be able to move and flex as shingles expand and contract.
Follow manufacturer guidelines, but generally between 1-2mm thick. Thinner underlayment = less protection.
Top Materials for Asphalt Shingle Underlayment
Now let’s take a look at the specific materials that make the best underlayment choices for use beneath asphalt shingles.
Asphalt-saturated felt has long been the tried-and-true choice for asphalt shingle underlayment. It typically consists of fibers (organic felt or fiberglass mat) coated in asphalt. The asphalt coating makes it naturally waterproof.
- Ideal level of waterproofing for asphalt shingles
- Effective for simple roofs
- Prone to tearing
- Breaks down faster than synthetic underlayments
We recommend an asphalt-saturated organic felt made with polyester or fiberglass reinforcement . Heavier weight felts, like 30 lb. or 40 lb., provide better durability and protection than standard 15 lb. felt.
Synthetic underlayments are engineered from woven or non-woven polypropylene, polyester, or nylon polymers. They are designed to be stronger, lighter, and longer-lasting than traditional felt.
- Excellent tear strength and durability
- Maintains strength when wet
- Lighter and easier to install than felt
- Can be more expensive than felt
- Not as breathable as felt
Quality brand name synthetic underlayments like GAF Tiger Paw or CertainTeed RoofRunner are wise investments under asphalt shingles. They’ll last longer and provide better protection than basic felt.
Rubberized Asphalt Underlayment
Rubberized asphalt underlayments consist of a fiberglass or polyester mat coated with a thick layer of rubberized asphalt. The rubberized asphalt is extremely waterproof.
- Superior leak protection
- Seals around nails and penetrations
- High durability and longevity
- Rigid, so can inhibit shingle expansion
- Difficult to install in cold weather
- Very expensive
Rubberized asphalt products like Grace Ice & Water Shield provide the ultimate in waterproofing. Use them for trouble spots like eaves, valleys, and chimneys.
Underlayments made from polypropylene plastic offer good durability and moisture resistance. They come in a woven, fabric-like style or rigid, non-woven panels.
- Lightweight and easy to cut and install
- Prevents water infiltration, even when damaged
- Doesn’t absorb moisture or decompose
- Can be slippery to walk on
- Not as breathable as other options
Quality polypropylene products like DuPont Tyvek Protec are great underlayment choices under asphalt shingles. Look for coated options for even better water resistance.
Proper Installation Techniques for Asphalt Shingle Underlayment
To gain the full benefit of underlayment, it’s essential to install it correctly:
Follow Manufacturer Guidelines
Always check the underlayment manufacturer’s instructions for proper installation methods.
Overlap Seams and Edges
Overlapping the underlayment sheets and edges a minimum of 2″- 4″ prevents leaks through the seams.
Secure with Roofing Nails
Use roofing nails (not staples) spaced 6″-12″ apart across the entire roof deck to hold the underlayment tight and prevent wind blow-off.
Pay Attention to Valleys, Eaves, and Penetrations
Take extra care to overlap and seal underlayment in roof valleys. Extend it over rake edges and down fascia boards. Cut tight around pipes, vents, and chimneys.
FAQ on Underlayment for Asphalt Shingles
What is the best underlayment for asphalt shingle roofs?
The best all-around underlayment choice is an asphalt-saturated organic felt made with fiberglass reinforcement. Synthetic underlayments and rubberized asphalt membranes also offer excellent waterproofing for asphalt shingle roofs.
Should you use tar paper under asphalt shingles?
Yes, #30 asphalt-saturated felt is an affordable and effective underlayment for asphalt shingles. Heavier weight felts, like 30 lb. or 40 lb., are recommended over lighter 15 lb. products for better durability.
Is synthetic underlayment better than felt?
Synthetic underlayments generally provide better tear strength, UV resistance, longevity, and wind uplift resistance compared to standard asphalt felts. However, heavier asphalt-saturated felts can still perform well under asphalt shingles.
Can you install asphalt shingles without underlayment?
It’s not recommended. Underlayment provides critical extra leak protection. Not using underlayment risks moisture damage, mold growth, and roof deck deterioration down the road.
Choosing the right underlayment for your new asphalt shingle roof ensures you’ll get the best possible performance and protects your investment in the roof. Look for asphalt-saturated felts or advanced synthetics with excellent water resistance, strength, and durability.
Install the underlayment properly, sealing seams and edges and securing with roofing nails. Underlayment adds little to the overall cost of a roof and provides invaluable leak protection and longevity. Don’t cut corners here!
You can pick out the perfect asphalt shingle underlayment with the expert recommendations in the guide.