Today, if you ask a room full of roofing contractors to recommend the best roofing felt, each contractor would likely recommend a felt weight rather than a brand name. Why? Because weight matters more than brand.
Traditionally, you could choose between 15-pound and 30-pound felt, though you now also have the option of a synthetic underlayment, also known as synthetic felt.
However, assuming that you’re only interested in the traditional options, which is the better between 15 and 30 roofing felt paper?
The difference between 15 and 30 lb felt is that #30 lb is thicker, stronger, and less likely to tear than 15 lb. So while #15 is a good option for most roofing jobs, #30 provides more protection and performance for steep roofs and flat roofs.
Understanding Roof Underlayment Structures
Roof underlayment is roofing material between the roof decking and your shingles. Once the decking is in place, you first install the underlayment then the shingles.
Some contractors don’t use it, claiming that the underlayment is ruined anyway when you nail the roof. Some also argue that it doesn’t help with waterproofing.
However, even if both points are true, there’s no doubt that it’s more beneficial to use an underlayment than work without one.
But, the two points aren’t entirely true in the first place. For instance, how can you complain about the underlayment getting ruined when the nails also go through the roof?
Isn’t the roof damaged too? And, if so, why do we tolerate nailed roofs? It’s because the roof is critical despite the nails running through it. The same applies to the underlayment.
Secondly, steep roofs are designed to be water-shedding. This means additional water-shedding protection is advantageous to the roofing system.
Why is Underlayment Important?
The following are five important advantages of roofing felt to consider before you think about omitting this critical roofing component;
- Underlayment offers a second protective layer if water penetrates the shingles.
- It protects the decking and inner structure of the home if the shingles are removed, such as if the roof is blown off.
- It can increase the home’s rating and fire rating.
- Underlayment provides slip protection for workers during shingles installation.
- It prevents lumber resin in roof decks from damaging shingles.
- It prevents the underneath roof sheathing from showing through the shingles.
Felt Paper vs. Synthetic Underlayment
Before comparing felt #15 and #30, the following are differences between felt and synthetic underlayment, so don’t confuse the two.
Perm Rating
Typically, felt underlayment has a higher permeability rating than synthetic underlayment. This means that moisture more easily escapes through the roof through felt underlayment.
Additionally, it means whereas ventilation is crucial for synthetic underlayment, it’s only considered “wise” for felt underlayment.
Rolls
Roofing felt comes in three-foot-long rolls that typically cover 400 square feet. By contrast, synthetic rolls are usually about four feet long and cover 1,000 square feet. Remember that the larger the roll, the more space it covers.
Safety
When they were first developed, synthetic underlayment materials weren’t impressively safe. They could become very slippery when wet, creating a high risk of falling. Newer synthetic underlayment materials are far less slippery and safer – but not as safe as felt underlayment.
Weather Conditions
Weather conditions affect both synthetic and roofing felt. Even though felt paper is waterproof, synthetic underlayment takes windy and cold conditions better if fastened properly.
Unfortunately, roofing felt tends to stiffen and curl when temperatures drop too low. Synthetic underlayment also captures less heat due to color. The grey color reflects heat better. On the other hand, felt is black, thus can become extremely hot.
Weight
Synthetic underlayment is lighter than felt underlayment. Whereas ten square feet of felt underlayment weighs about 130 pounds, an equivalent amount of synthetic underlayment weighs only 23 pounds. This means it’s easier to get synthetic underlayment onto the roof.
Price
Synthetic underlayment is more expensive, especially heavier synthetics. The main reason is that synthetics have higher performance qualities.
For instance, heavy synthetic underlayment withstands longer exposure times and affords roof installers better walkability. However, thinner synthetics are comparable to high-quality 15-pound felt and are even cheaper.
Roofing Felt Paper Types
Roofing felt papers have evolved over the years, such that you can now find several felt paper types in the market. However, the two main types of roofing felt papers are asphalt and rubberized asphalt.
Also don’t confuse roofing felt paper and tar paper. The two are almost the same and used the same way but have distinctive features.
Asphalt Felt
Asphalt felt papers are the original form of roofing felt. They are made from felt paper that’s dipped or otherwise saturated in asphalt, forming an incredibly durable felt paper. Original asphalt felt paper is also highly water-resistant.
Rubberized Asphalt
Rubberized asphalt sheets are a much more modern version of traditional asphalt underlayment paper. They come with an adhesive layer for swift application and typically have polymer coatings. In addition, rubberized asphalt felt papers are more waterproof than traditional sheets.
Understanding Asphalt Felt Weight
The figures probably refer to the weight if you see numbers attached to asphalt felt paper.
The weight of the paper is critical because it controls how much felt is needed for safe roof installment. Additionally, the felt number tells you how much protection the felt provides.
Beware, however, that some of the numbers on the felt paper may indicate the width. Felt underlayment is available in many widths. Smaller widths are necessary for delicate work, while larger widths are advantageous because they leave fewer seams behind.
Fewer seams mean reduced possibilities for weak points to develop, thus better protection against leaks.
Felt #15 vs Felt #30
Roofing felt is weight is measured in pounds per square, where one square is equal to 100 feet of roofing material. The most commonly used felts are #15 and #30.
What is Felt Paper #15?
Felt paper #15 refers to light asphalt felt paper, typically used in standard roofing projects with steep slopes. It’s called #15 (or 15-pound) because #15 felt paper weighed 15 pounds back in the day.
15-pound felt paper is typically used in standard residential roofing projects where (1) the roof slope is 3:12 or higher and (2) the asphalt shingles are 240 pounds per square foot or lighter. Note that you need two layers of #15 for 3:12 roofs.
What is Felt Paper #30?
Felt paper #30 refers to heavy asphalt typically used in gentler slopes. It’s called #30 because #30 felt paper weighed 30 pounds back in the day.
30-pound roofing felt is typically used in asphalt roofing projects where (1) the roof slope is less than 3:12 and (2) the asphalt shingles are heavier than 240 pounds per square foot.
What Is The Difference Between Roof Felt Paper #15 And #30?
The difference between 15 and 30 lb felt is that #30 lb is thicker, stronger, and less likely to tear than 15 lb. So while #15 is a good option for most roofing jobs, #30 provides more protection and performance for steep roofs and flat roofs.
What’s the Main Difference?
The weight is the main difference between #15 and #30 felt papers. Historically, #15 felt paper weighed 15 pounds, while #30 felt paper weighed 30 pounds.
The weights have changed significantly since then. For instance, #15 felt papers are now about 8.0 pounds, while #30 are around 16 pounds. However, the original names (#15 and #30) have remained.
Tear Resistance
Both #15 felt and #30 felt can tear. However, #30 felt paper is far more resistant to tear than #15 felt paper. This makes #30 felt paper much better at roof protection.
You may also want to know that if your roofing shingles come off during a storm, 30-pound felt is far less likely to tear along with the shingle. In other words, you can count on it in case the shingles tear off.
One important thing to note is that you shouldn’t leave underlayment exposed for a long period of time before putting shingles.
Roof Slope
Generally, for shingles weighing 240 pounds per square foot or less, use #15 felt paper. If the roof slope is greater than 4:12, which provides more water drainage from the roof, use a single layer of #15 felt paper.
It’s more economical and will protect your roof just fine. However, if the roof slope is between 3:12 and 4:12, use two layers of #15 felt paper for more protection.
Only use #30 felt paper if the roof slope is less than 3:12 or the roof shingles are heavier than 240 pounds per square foot.
Location and Weather
Weather also plays a crucial role in roof felt selection. Generally, you need heavier felt for locations with extreme weather.
For instance, you need heavier felt if you live in an area that receives significant snowfall each year. You also need stronger felt for locations with stronger winds that can drive water between shingles.
Wear
Owing to its heavier weight and the thicker design, #30 felt paper wears slower than #15 felt paper. So, you can expect #30 felt to last longer than #15 felt paper.
However, it is also down to weather conditions, the quality of the other roofing materials, including shingles and decking, and the quality of the roofing job. Poorly installed felt is damaged more easily.
Important Considerations
Remember that your local building codes may mandate the use of certain roof felt. For instance, some states require the installation of #15 or heavier felt. So, you may breach the local laws if you install the roof without felt underlayment.
Similarly, roofing felt is required for insurance purposes for the Underwriter’s Laboratory fire rating to apply to roofing.
Roof Felt Paper 15 vs 30 FAQs
What is #15 felt used for?
15-pound felt paper is an economic underlayment choice ideal for standard roofs without very steep pitches. It sits between the decking and the roof shingles to provide protection and serve as a moisture barrier.
Why do they call it 30-pound felt paper?
They call felt paper 30-pound or 15-pound because roofing felt paper was historically named for weight. Thus, #30 felt weighed 30 pounds, and #15 felt paper weighed 15 pounds.
Is 15-pound felt waterproof?
Yes. All felt papers, including #15 felt, are made from fiberglass or polyester and soaked in a waterproofing agent. However, heavier felt papers are more waterproof than #15.
What happens if felt gets wet?
Roofing felt can break down after prolonged exposure to intense sunlight or moisture. The wet felt can even rip when shingles are installed. So, you must take very good care of it.
How long does roofing felt last?
Asphalt felt, the most common type of roof underlayment, lasts between 20 and 30 years. Unfortunately, the lifespan can be cut by half in extreme weather conditions.
15 vs. 30 Felt Paper: Which one should you Choose?
The difference between 15 and 30 lb felt is thickness, strength, and durability. #30 lb is thicker, stronger, and less likely to tear than 15 lb, making it ideal for steeper sloping roofs. #15 lb is lighter, less resistant to tear, and is often used on standard roofs with gentle slopes.
There you go! Now you understand the need for roofing underlayment, the different types of underlayment, what felt numbers mean, and most importantly, the main differences between #15 felt and #30 felt.