If you're standing in the middle of a home improvement aisle or looking over a contractor's quote, you've probably seen the debate over roof felt paper 15 vs 30 and wondered if the thicker stuff is actually worth the extra cash. It's one of those "behind the scenes" decisions that doesn't feel as exciting as picking out shingle colors, but honestly, it's a lot more important for the long-term health of your house.
Choosing between 15-pound and 30-pound felt is basically a choice about how much of a safety net you want under your shingles. Both do the same basic job—acting as a secondary water barrier—but they behave very differently when you're actually up on the roof or when a nasty storm rolls through.
What's the real difference anyway?
Back in the day, these numbers actually meant something literal. 15-pound felt used to weigh exactly 15 pounds per "square" (which is 100 square feet in roofing lingo). These days, things have changed a bit. Most modern "15-pound" felt actually weighs closer to 8 or 12 pounds, and "30-pound" felt is usually around 15 to 20 pounds.
Even though the names are a bit of a marketing holdover, the ratio remains the same: 30-pound felt is roughly twice as thick and heavy as the 15-pound version. Because it's beefier, it's significantly tougher. If you try to tear a piece of 15-pound felt with your hands, it'll rip like a thick piece of construction paper. If you try that with 30-pound felt, you're going to have a much harder time.
Why 15 lb felt might be "just fine" for some
Don't get me wrong, 15-pound felt isn't "bad." It's been the standard for decades, and millions of homes are currently sitting under it without any issues. If you're on a tight budget or working on a structure that isn't particularly high-stakes—like a shed, a detached garage, or a small playhouse—15-pound felt is a perfectly reasonable choice.
It's also a lot easier to handle. Because it's thinner, the rolls are lighter, and you get more coverage per roll. If you're a DIYer doing a small repair by yourself, you'll appreciate not having to lug a 45-pound roll of 30-lb felt up a ladder. It lays down flat quite easily and is generally cheaper. If your roof has a standard pitch and you're planning on getting the shingles on the same day the felt goes down, 15-pound paper does its job well enough.
The case for upgrading to 30 lb felt
Most professional roofers I know will tell you to go with the 30-pound stuff for a primary residence. The main reason? Durability during the installation process.
When you're walking around on a roof, you're putting a lot of stress on that underlayment. 15-pound felt is notorious for tearing under a roofer's boots, especially on a hot day when the asphalt gets soft. Once it tears, you've got a hole in your secondary water barrier. 30-pound felt is much more "walkable." It stays put, resists scuffing, and doesn't rip the second someone pivots their foot.
Another big advantage is how it handles moisture before the shingles are installed. If a surprise rainstorm hits while the roof is mid-project, 30-pound felt is much less likely to "bubble" or wrinkle. 15-pound felt is thinner and absorbs moisture more quickly, which can cause it to expand and create ridges. If those ridges don't flatten out before the shingles go on, you might end up with an uneven-looking roof.
How steepness and weather change the game
The pitch of your roof—basically how steep it is—should play a big role in your decision. If you have a very steep roof, safety becomes your number one priority. 30-pound felt provides a slightly better grip for shoes, and because it's harder to tear, there's less risk of a piece giving way under your weight.
Weather is the other big factor. If you live in an area that gets hammered by heavy winds or crazy summer storms, that extra layer of thickness is basically an insurance policy. If a few shingles blow off during a hurricane or a severe thunderstorm, 30-pound felt can hold the line against water much longer than the thin stuff. It's that extra bit of "oops" protection that can save you from a massive ceiling repair bill later on.
Installation quirks: Wrinkles and weight
I mentioned the weight earlier, but it's worth repeating. If you're doing this yourself, be prepared for a workout if you choose 30-pound felt. A roll of 30-lb underlayment covers about 200 square feet, while a roll of 15-lb covers about 400 square feet. This means you'll be hauling twice as many rolls up to the roof, and each roll is heavier.
However, the thickness of 30-lb paper makes it a bit more forgiving when it comes to laying it straight. It doesn't flop around in the wind as much as the thin stuff. The downside is that if you do get a wrinkle in 30-pound felt, it's a lot harder to smooth out because the material is so stiff. You usually have to cut it and overlap it to get it back to laying flat.
What about synthetic underlayment?
We can't really talk about roof felt paper 15 vs 30 without acknowledging that synthetic underlayment is taking over the market. Made from woven polymers, synthetics are way lighter and tougher than even the best 30-pound felt.
So, why would anyone still use felt? Cost and tradition. Traditional asphalt-saturated felt (the black stuff) is usually cheaper than high-end synthetics. Also, some old-school roofers prefer the way felt "breathes" compared to some older synthetic designs. But if you're looking at 30-pound felt and realizing the price is getting close to a mid-range synthetic, it's definitely worth looking at the synthetic option. It won't rot, and it doesn't absorb water at all.
Making the final call
At the end of the day, if you're roofing your forever home, the extra twenty or thirty bucks per roll for 30-pound felt is a drop in the bucket compared to the total cost of the roof. It gives you a thicker barrier, better protection against the elements, and a safer surface for whoever is doing the actual work.
If you're just patching up a garden shed or you're in a climate where it literally never rains, 15-pound felt will save you some money and a bit of a backache. It's not a "fail" to use 15-pound; it's just the basic version of the product.
Just remember: the underlayment is the last line of defense for your plywood decking. Shingles are the armor, but the felt is the undershirt that keeps the moisture from actually touching the wood. If you can afford the "heavy-duty" undershirt, your roof will probably thank you for it in ten or fifteen years when a shingle eventually cracks or blows away.
Don't overthink it too much, though. If your contractor is adamant about one over the other, it's usually because of the specific pitch of your roof or the local building codes. In some areas, 30-pound is actually required by law for certain roof slopes. Check your local codes, look at your budget, and if you're still stuck, just go with the 30. You'll never regret having a roof that's "too strong," but you might regret one that's too thin.