Roofing felt is easy to overlook because it disappears under shingles, tile, or metal panels. But when wind-driven rain gets under the roof covering, when a project sits exposed for a few days, or when a roof deck needs a little extra protection, the felt matters.

The choice usually comes down to two common options: #15 felt and #30 felt. They look similar on the roll, but they behave differently on the roof. One is lighter, easier to handle, and often enough for straightforward jobs. The other is thicker, tougher, and better suited for projects where durability matters more than speed or material savings.

If you are weighing #15 felt vs #30 felt, the right answer depends on the roof covering, slope, climate, exposure time, budget, and what the manufacturer or local code requires.

What roofing felt does under your roof

Roofing felt is an underlayment installed over the roof deck and under the finished roof material. It acts as a secondary layer of protection. The primary roof covering, such as asphalt shingles, tile, or metal, sheds most of the water. Felt helps protect the deck when water gets past that top layer.

A good underlayment can help with:

• Water shedding It helps direct incidental moisture down the roof and away from the deck.

  
  

• Temporary protection It covers the decking during installation before the final roofing material goes on.

  
  

• Deck separation It creates a barrier between the roof deck and roof covering.

  
  

• Minor backup protection It gives the roof an extra layer of defense during wind, rain, or damaged shingles.

  
  

Felt is not a cure for poor installation. It will not fix bad flashing, missing drip edge, incorrect fasteners, or roof coverings installed against manufacturer instructions. Think of it as backup protection, not the roof’s main waterproofing system.

Traditional asphalt-saturated felt has been used for decades. Modern synthetic underlayments are also common, especially on larger projects and in areas where strength, lighter rolls, or longer exposure ratings are useful. Still, many residential roofing projects use felt because it is familiar, widely available, and accepted by many shingle manufacturers when installed correctly.

The basic difference between 15 and 30 felt

The biggest difference is thickness and strength. #30 felt is heavier and tougher than #15 felt. It usually resists tearing better, lays flatter in some conditions, and gives more protection when crews walk on it during installation.

15 felt is lighter. That makes it easier to carry, roll out, cut, and fasten. It can be a practical choice for simple asphalt shingle jobs, especially when the roof will be covered quickly and the slope is not demanding.

30 felt costs more per square foot in most cases, but it may be worth the added cost when the roof needs extra durability.

The numbers can be confusing. Today, #15 felt does not always weigh 15 pounds per roofing square, and #30 does not always weigh exactly 30 pounds. Product standards and manufacturing have changed over time. The labels are now more like category names than exact weight promises.

The more reliable way to compare products is to read the manufacturer’s specifications. Look for the applicable standard, thickness, roll coverage, approved uses, and installation instructions.

When 15 felt makes sense

15 felt is the lighter option, and that can be a real advantage. On a modest roof with standard asphalt shingles, it often gives enough backup protection when installed correctly.

It works best when the project is straightforward and the underlayment will not be exposed for long.

A simple asphalt shingle replacement

For many asphalt shingle tear-offs, #15 felt is a common choice. If the roof deck is in good shape, the slope is moderate, and the shingles will be installed the same day or soon after, #15 felt can do its job well.

It is easier to move across the roof and easier to cut around valleys, walls, vents, skylights, and roof penetrations. That can help the job move faster, especially on smaller homes and simple roof shapes.

A budget-conscious project

Material cost is not the only cost in roofing, but it matters. #15 felt typically costs less than #30 felt. On a smaller roof, the difference may not be huge. On a larger roof, the difference can be more noticeable.

If the roof covering manufacturer allows it, the local code allows it, and the roof conditions are not demanding, #15 felt can keep the project cost down without automatically being the wrong choice.

A roof that will be covered quickly

Exposure time is one of the biggest practical factors. If felt sits uncovered in sun, wind, rain, or foot traffic, it can wrinkle, tear, dry out, or lose performance.

15 felt is less forgiving when a project gets delayed. It is best suited for jobs where the finished roof covering will go on promptly.

In sunny, hot areas like Tucson, exposed roofing materials can heat up fast. A lighter felt that sits exposed too long can become more fragile and harder to work with. If delays are likely, a heavier felt or a synthetic option may be the better call.

When 30 felt is the better choice

30 felt is the stronger option. It is thicker, more durable, and better able to handle abuse during installation. That does not make it the right choice for every roof, but it does make it a safer pick in tougher conditions.

Steeper roofs and more foot traffic

A steep roof puts more stress on underlayment during installation. Crews need safe footing, and materials are more likely to shift, wrinkle, or tear as they work.

30 felt usually handles foot traffic better. It is less likely to rip around fasteners and edges. That matters when the crew has to move across the roof repeatedly to install shingles, tile battens, flashing, or metal panels.

No felt should be treated like a walking surface, especially on steep slopes. Safety equipment and proper staging still matter. But a tougher material reduces the chance of damage before the roof covering is installed.

Heavier roof coverings

Tile and some metal roof systems may require specific underlayment types. In some assemblies, #30 felt works better as a base layer because it has more body and strength than #15 felt.

For clay or concrete tile, underlayment is especially important. Tile sheds water, but wind-driven rain can still get beneath it. The underlayment often becomes a major line of defense.

In hot, dry climates with intense sun and monsoon rain patterns, roof systems need to manage both heat stress and sudden storms. Tucson-area tile roofs, for example, often call for careful underlayment selection because the material under the tile may be expected to perform for many years.

Longer installation windows

Not every roofing project goes perfectly on schedule. Weather can interrupt work. Material shipments can arrive late. Repairs to decking can take longer than expected.

30 felt gives more breathing room than 15 felt. It still should not be left exposed beyond the manufacturer’s recommendations, but it is usually more resistant to tearing and wrinkling while the project is in progress.

This can matter on:

• Large custom homes

• Roofs with many valleys and penetrations

• Tile conversion projects

• Jobs with decking repairs

• Projects during unsettled weather

  
  

Rougher roof decks

The underlayment is only as good as the surface below it. Splinters, uneven sheathing, old fastener heads, gaps, and rough decking can damage thinner felt.

If the deck has rough spots, #30 felt is more forgiving. It can bridge small imperfections better than #15 felt, though it should never be used to hide serious deck problems.

Bad decking should be repaired. Soft, rotted, delaminated, or broken sheathing needs attention before any underlayment goes down.

How roof slope affects the choice

Roof slope changes how water moves. A steep roof sheds water quickly. A low-slope roof drains more slowly and gives wind-driven rain more time to work under the roof covering.

Building codes and manufacturer instructions often set minimum underlayment requirements based on slope. Low-slope asphalt shingle roofs may require special application methods, such as double coverage underlayment or an approved self-adhering membrane in certain areas.

Do not choose #15 felt just because it is cheaper if the roof slope calls for something stronger or a different installation method.

For asphalt shingles, common slope categories include:

• Standard slopes These usually allow standard underlayment installation when all other conditions are normal.

  
  

• Lower slopes These often require extra underlayment protection. Standard felt may need to be doubled, or another approved underlayment may be required.

  
  

• Very low slopes Asphalt shingles may not be appropriate at all, depending on the exact pitch and manufacturer rules.

  
  

A roof that looks only “slightly flat” from the ground may be low enough to require a different approach. Measuring slope before choosing materials prevents expensive mistakes.

How climate changes the decision

Climate matters because underlayment deals with heat, moisture, wind, and time.

In cold climates, ice dams can force water under shingles. In those areas, roofers often use ice and water shield along eaves, valleys, and other vulnerable areas. Felt alone is not enough to solve ice dam risk.

In hot climates, heat ages roofing materials. Asphalt-saturated products can become brittle over time, especially if trapped under roof coverings that hold heat. Roof ventilation, roof color, and roof covering type all play a role.

In desert regions, the roof may face:

• Strong ultraviolet exposure during installation

• High surface temperatures

• Sudden heavy rain during storm season

• Dust and wind

• Large day-to-night temperature swings

  
  

That mix can be tough on thin underlayment. For many Tucson roofing projects, especially tile roofs, choosing the minimum material may not be the best long-term move.

This does not mean #15 felt always fails in hot climates. It means the roof assembly matters. A simple shingle job completed quickly may perform fine with #15 felt if it meets all requirements. A tile roof expected to last for decades may call for a more durable underlayment system.

Cost is more than the price of the roll

It is tempting to compare #15 and #30 felt by roll price alone. That misses the bigger picture.

A stronger underlayment may cost more upfront, but the labor to install either material can be similar. If the roof covering is expensive, the underlayment is a small part of the total project cost. Saving a little on the hidden layer may not make sense if the roof is complex or exposed to harsh conditions.

Think about these cost factors:

• Material price #30 felt usually costs more than #15 felt.

  
  

• Roll coverage #30 felt rolls often cover less area than #15 rolls, so a roof may need more rolls.

  
  

• Labor Heavier rolls take more effort to move and install.

  
  

• Risk of damage Thin felt can tear more easily, which may slow the crew down or require repairs.

  
  

• Roof covering value Premium shingles, tile, or metal often justify better underlayment.

  
  

• Future access Once the roof covering is installed, replacing damaged underlayment means removing roofing materials.

  
  

The hidden nature of underlayment is exactly why the choice matters. It is much easier to make a good decision during installation than to fix a weak layer later.

What building codes and manufacturers require

The best underlayment choice is not just personal preference. Building codes, product approvals, and roof covering manufacturers all matter.

Before choosing between #15 and #30 felt, check:

1. Local building code

   Requirements can vary by location, roof slope, wind exposure, and roof covering.

   
   

2. Roof covering instructions

   Shingle, tile, and metal manufacturers may require a certain underlayment type.

   
   

1. Warranty conditions

   Using the wrong underlayment can affect warranty coverage.

   
   

2. Fire rating requirements

   Some roof assemblies need specific materials to maintain a fire rating.

   
   

1. Slope limitations

   Low slopes may need special underlayment applications.

   
   

2. Valley and penetration details

   Valleys, skylights, chimneys, vents, and wall intersections often need added protection.

   
   

If you are hiring a roofing contractor, ask what underlayment they plan to use and why. A clear answer is a good sign. Vague answers like “that is just what we always use” deserve a follow-up question.

Installation quality matters as much as felt weight

30 felt installed poorly can perform worse than 15 felt installed correctly. Underlayment needs proper overlap, fastening, and integration with flashing.

Look for these basics:

• Horizontal courses should shed water down the roof.

• Upper courses should overlap lower courses.

• Side laps should meet the required minimum.

• Fasteners should hold the felt securely without tearing it.

• Wrinkles should be avoided because they can telegraph through shingles or trap water.

• Valleys should receive added attention.

• Drip edge and underlayment should be sequenced correctly at eaves and rakes.

• Torn sections should be repaired before roof covering is installed.

  
  

Fastener choice also matters. Staples may be allowed in some cases, but cap nails or cap fasteners often provide better holding power and tear resistance. The required fastener depends on the product, the roof system, and local rules.

Wind can turn underlayment into a problem fast. If felt is installed loosely or left overnight without enough fastening, gusts can wrinkle or rip it. Heavier felt helps, but proper fastening does the real work.

Common project scenarios

Here is a practical way to think through the choice.

These are general guidelines, not a substitute for checking the exact product instructions. The roof covering, slope, deck condition, and local code decide the final answer.

When synthetic underlayment enters the conversation

Synthetic underlayment is not the focus here, but it often comes up when comparing felt options. Many synthetic products are lighter than felt, strong for their weight, resistant to tearing, and easier to walk on when designed with a textured surface.

Some synthetic products also have longer exposure ratings than asphalt felt. That can help when projects take longer or when weather causes delays.

Still, synthetic is not automatically better for every job. It must be approved for the roof covering and installed with the right fasteners. Some roof systems, especially tile assemblies, may call for specific layers or self-adhered products rather than a basic synthetic sheet.

When comparing felt underlayment with synthetic options, focus on the whole roof system. The best material is the one that fits the roof covering, climate, slope, code, and budget.

How to choose between 15 and 30 felt

A simple decision process can keep the choice clear.

Start with the roof covering

Asphalt shingles on a simple roof may not need the heavier option. Tile, metal, premium shingles, or complex roof assemblies often justify #30 felt or another upgraded underlayment.

Check the slope

The lower the slope, the more careful the underlayment plan needs to be. Do not guess. Measure the pitch and compare it with manufacturer requirements.

Think about climate

Hot sun, heavy seasonal rain, high wind, ice, or long installation windows all push the decision toward stronger protection.

Look at the roof deck

Clean, smooth, solid decking gives either felt a better chance to perform. Rough or repaired decking favors a tougher underlayment.

Consider project timing

If the crew can dry in the roof and install the covering quickly, #15 felt may be fine for an approved shingle project. If scheduling is uncertain, #30 felt gives more room for error.

Compare total value

For a small utility building, #15 felt can be a practical choice. For a home with tile or high-end roof materials, #30 felt or a better system is often a smarter investment.

So which one is right for your project

Choose #15 felt when the roof is simple, the finished covering allows it, the slope is standard, the deck is solid, the installation will move quickly, and cost control is a priority.

Choose #30 felt when the roof is steep, complex, exposed to tougher weather, covered with tile or premium materials, likely to see more foot traffic during installation, or expected to sit uncovered longer than planned.

For many homeowners, the safest question is not “What is the cheapest felt I can use?” A better question is “What underlayment fits the roof system I am installing?”

That shift leads to better decisions.

If you are planning a roof repair or replacement and want a clearer sense of materials, timing, and what to ask a contractor, see this homeowner roofing guide.

FAQ

Is 30 felt twice as good as 15 felt?

No. #30 felt is thicker and stronger, but that does not mean it is twice as good for every roof. It is better for tougher conditions, heavier roofing, and longer exposure. #15 felt can still be suitable for approved, straightforward shingle projects.

Can I use 15 felt under asphalt shingles?

Yes, in many standard asphalt shingle applications, #15 felt is commonly used. The roof slope, local code, and shingle manufacturer’s instructions must allow it.

Is 30 felt waterproof?

No. #30 felt is water-resistant, not fully waterproof. It helps shed incidental water, but it should not replace proper roof covering, flashing, ice and water protection where required, or correct installation.

Does 30 felt last longer than 15 felt?

It can, especially under tougher conditions, because it is thicker and more tear-resistant. The full roof system, ventilation, heat exposure, installation quality, and roof covering all affect service life.

Should I use synthetic underlayment instead of felt?

Synthetic underlayment can be a good choice when strength, lighter handling, or longer exposure ratings matter. It still needs to be approved for the roof system and installed according to instructions.

The practical takeaway

15 felt is lighter, less expensive, and often suitable for simple asphalt shingle jobs. 30 felt is heavier, stronger, and better for roofs that face more stress during installation or service.

If the roof is simple and the rules allow it, #15 felt may be the right choice. If the roof is steep, complex, tiled, exposed to harsh weather, or worth extra protection, #30 felt is usually the better bet.

The underlayment is hidden once the roof is finished, but it should not be an afterthought. Choose the layer that fits the roof you are building, not just the roll that costs less today.