Best Underlayment for Metal Roofs: Keep the Outdoors Out
Are you one of the growing number of homeowners who have decided to install a metal roof?
Whether for a new build or a replacement? If you’re going to the expense of an upgraded material, don’t undermine it. Puns aside, you want to be sure you choose the best underlayment for metal roofs. Because in all seriousness, making the wrong choice really could undermine your metal roof.
In line with that, in this article, I’ll help you choose the best underlayment for your needs. Because the best choice means you can protect your roof against moisture penetrating and in turn causing a buildup of condensation in your attic.
Even though metal roofing has a significantly longer life span than a shingled roof, it’s still in danger of deterioration due to entrapped moisture.
Read on to find all your metal roof underlayment answers.
But first . . .
Do You Need to Use Underlayment under a Metal Roof?
You’ve probably read that metal roofs don’t need underlayment. But since you’re a smart homeowner, you’re looking for a second opinion.
Second opinions are always wise since the more information you have, the better position you’re in to make an informed decision. And ultimately, it is your decision.
If you’ve had a shingle roof redone in the past, you know there’s no question about underlayment. It’s absolutely necessary. In this case, underlayment provides an extra layer of protection against water penetration. Obviously, a metal roof doesn’t need an extra layer to provide a waterproof surface—or to help maintain its structural integrity.
Having said that, a metal roof still benefits from that extra layer.
Have you heard the saying water finds a way? It’s very true. It will find cracks and crevices. It will find joints that aren’t sound. If there is any way for it to get through, it will get through.
If you have the right underlayment, you don’t have to worry about that sort of infiltration. Or the buildup of condensation in your attic that I mentioned above.
Finally, when choosing an underlayment, be sure to choose one that matches the lifespan of your roof. If you’re having a 40-year roof installed, you want an underlayment that will last as long.
So yes. In my opinion, at least, you should use underlayment under a metal roof. Because ultimately it will protect your roof system—and your home.
Should I Put Plywood Under a Metal Roof?
Seriously, there are times when you should and times when it’s simply not necessary.
If you’re putting a roof on your house that should last 40 or more years, it’s a good idea to plan that far ahead. Unless, of course, you plan to sell soon, and any future work is someone else’s problem. Did you know real estate agents say that metal roofs recoup about 94% of your costs at resale? So maybe that is your goal.
However, if you see yourself in your home for that long, taking advantage of your new metal roof, you need to think about future roofing needs. What do I mean by that?
40 years from now when the time comes to replace your roof, you’ll also need to replace or repair all the insulation that’s attached to the metal—because you don’t have a plywood underlayment. So consider the additional cost of adding new insulation and perhaps plywood at whatever crazy cost it will be then compared to what it is now.
The other reason to use plywood is for its soundproofing abilities. It’s an additional layer of noise protection against rain, hail, sleet, and whatever else can hit your roof.
Now for the types of underlayment to use under a metal roof.
Types of Underlayment for Metal Roofs
There are three different types to choose from.
- Felt Underlayment
- Self-Adhering Membrane
- Synthetic Sheet
I’ve already stated my opinion above—that you should use underlayment under a metal roof. And the reasons for it. Here is one more reason.
Don’t you typically take the most care of the things that cost you the most? Would you pay $100K for a car and not insure it? Replace the recommended tires with the cheapest aftermarket tires you could find—that were the wrong size? Of course not.
Using the same line of reasoning, a metal roof is a major investment. Choosing the best support system for it—in this case, the best underlayment—is just a smart thing to do.
This is the most common type of underlayment, and it is also referred to as asphalt-soaked felt, felt paper, and finally, tar paper.
This type of underlayment is made of natural fibers, including wood cellulose, or synthetic fibers, including fiberglass or polyester base. This base is then saturated in asphalt, which creates a water-resistant material.
The recommendation is to use 30# felt underlayment–or a double layer of 15#–under metal roof panels. And if you have a steep-sloped metal roof, this is the best underlayment for your situation.
Felt is typically the cheapest choice, but it does have some drawbacks, most notably that it doesn’t do well in climates that tend to reach high temperatures for extended periods. It also has a shorter life span, which means it will need to be replaced before your roof does.
Felt Underlay Pros:
- It’s a very inexpensive option
- Water-resistant, which means your roofing structure has better protection against water penetration
- Since it’s common, it’s easy to find, wherever you live
Felt Underlay Cons:
- Shorter life span
- It’s not recommended for low slope roofs
- It doesn’t fare well in high-temperature climates
- It must be installed quickly as it can’t be exposed to the elements for any length of time
- It can tear in high winds
- Asphalt is petroleum-based, making it highly flammable
The Verdict. If you have a steep-sloped roof, live in a temperate climate, and you’re looking for an inexpensive option, this is likely your best choice.
Self-Adhering Membrane Underlayment
This type of underlay is adhered directly to the roof deck. You simply peel and stick. The adhesive is typically made from rubberized asphalt or butyl rubber, and it is mounted to a carrier sheet made of polyethylene.
Unlike felt underlayment, this is a good choice if you live somewhere with higher temperatures for extended periods. Due to this, you may also see it referred to and sold as High Temp Underlayment instead of a self-adhering membrane.
This type of underlayment is a good choice for conditions where a better air barrier is required. And if you have a roof with significant valleys and dormers, you’ll have better protection against ice dams in those spots plus the eaves.
Self-Adhering Membrane Pros:
- Good choice for high-temperature climates
- Better air barrier
- Protects against ice dams
Self-Adhering Membrane Cons:
- Difficult to install
- Can be more expensive than other options
- Not recommended for climates with extended periods of low temperatures
The Verdict: This is a good choice if you live somewhere hot and have installers familiar with the product.
Synthetic Sheet Underlayment
This is a new technology and becoming a very popular choice in the metal roofing industry. The manufacturing process is to weave or spin either polyethylene or polypropylene and a polymer together.
Since this is a newer innovation, its creators were able to come up with something that answered most of the challenges—cons—found in pre-existing underlayment. Part of that is due to the thermoplastic polymers used in the manufacturing process. These polymers make the underlayment more resistant to tearing and other damage. According to Owens Corning, it’s up to 12 times stronger when compared to felt underlayment. It’s also able to withstand high temperatures.
Synthetic Sheet Underlayment Pros:
- Lightweight and easy to install
- Long lifespan
- More durable and resistant to tearing
- Able to withstand high temperatures
Synthetic Sheet Underlayment Cons:
- More expensive
- Its low permeability will mean you need to build in proper ventilation
The Verdict: If you live in a hot climate and plan to live in your home for the lifespan of your roof, this is the best option.
A metal roof is a major investment and a wise one. And using the best underlayment for it—or simply using one at all—is just good thinking.
Using an underlayment will add a layer of protection to your roof, and ultimately your home. So use the information above and take into consideration things like your climate, the slope of your roof, and the amount you’re willing to spend. All of those things will play a part in making the best decision for you and your home.