Metal Roofing Underlayment: Performance, Types, and Applications – Continuing Education Course
Credit: 0.25 LU/HSW
Learning Objective: Upon successful completion of this learning program, you will come to appreciate the differences between synthetic and felt paper underlayment by learning of the advantages and disadvantages of each building material.
Please Note: After watching the video and reading the text, click the button at the bottom of this page to take the quiz.
Felt Paper Underlayment Disadvantages
LACKING IN LONGEVITY – Felt paper underlayment typically carries a 15- to 20-year warranty, depending on the manufacturer and installation details.
HIGH-TEMPERATURE PROBLEMS – Felt paper will break down and weaken over time, especially if exposed to high heat, sun exposure or frequent temperature fluctuations.
LESS EXPOSURE TIME – Some roofing felt can bubble, buckle and wrinkle within just 24 hours of UV exposure.
POSSIBLE SLIP SHEET REQUIREMENT – The addition of a slip sheet will increase the cost.
FIRE CONCERNS – If the felt paper is damaged and then exposed to fire, the paper could combust, as the paper is soaked in asphalt, made from petroleum and a highly flammable substance itself.
HEAVIER – Felt underlayment usually comes in two different sizes, either 15 or a 30 pound, with these numbers representing how much the underlayment will weigh per roofing square.
LESS RECYCLABILITY – You technically can recycle felt paper underlayment to a landfill devoted to only asphalt mixtures.
Synthetic Underlayment
What is Synthetic Underlayment?
Synthetic underlayment is a roofing accessory created by weaving or spinning together polypropylene or polyethylene and a polymer to form a protective barrier to put between the roofing material and the roof deck. Synthetic underlayment is a newer innovation that is taking over the metal roofing industry as a better, more advanced replacement for felt underlayment. Most synthetic underlayments are mechanically attached with roofing caps, while others have a glue backing.
Synthetic Underlayment Advantages
LONGEVITY – Synthetic underlayment offers superior longevity compared to felt paper, often lasting 40+ years. This extended lifespan also applies to its ability to withstand UV exposure during installation, as many synthetic underlayments can remain exposed to the sun for up to a full year without drying out or degrading. This makes synthetic underlayment especially beneficial on projects where installation may be temporarily halted or where harsher weather conditions are present. Its long exposure ratings are often crucial following catastrophic events such as hurricanes, allowing roofs to be properly dried-in and remain weathertight while awaiting full roof replacement.
MORE DURABLE – Synthetic underlayment is engineered with thermoplastic polymers that are significantly stronger to resist tearing, moving, or other damage that could occur during installation.
HIGH-TEMPERATURE TOLERANCE – Synthetic underlayment makes a great choice in regions with higher temperatures. For example, some products can withstand temperatures of up to 260 degrees (Fahrenheit), which makes it a good choice for buildings in desert, tropical, or other high-heat climates.
RETAIN LESS HEAT – Most synthetic underlayments come in a mid-to-lighter gray color, which means the material retains less heat from the sun and doesn’t get as hot to the touch. This is especially helpful for the contractors handling and installing the underlayment.
COMES IN LARGER ROLLS – Synthetic underlayment rolls tend to have more material and square foot coverage per roll because it’s often longer; meaning it’s a better “bang for your buck.” For reference:
- Peel and stick synthetic underlayment tends to come in rolls of 2 to 5 roofing squares (or 200 to 500 square feet per roll)
- Mechanically attached synthetic underlayment tends to come in rolls of 10 roofing squares (or 1,000 square feet per roll)
- Felt paper tends to come in rolls of 2 to 4 roofing squares (or 200 to 400 square feet per roll)
LIGHT-WEIGHT – Synthetic underlayment averages around 2 to 4 pounds per roofing square, compared to asphalt felt paper of about 15 to 30 pounds per square. Not only does this reduce the stress inflicted on the building’s frame/structure, but it also makes it easier to haul up onto the roof.
WARRANTIED – Most reputable synthetic underlayment manufacturers offer warranties on their products based on testing and industry standards. Some superior synthetic products even come with as high as a 50-year limited warranty.
WALKABILITY – One of the biggest concerns is having a good, slip-resistant grip while walking and working up on a roof. Luckily, many synthetic products have a slip-resistant surface built into the material.
RECYCLABLE – One of the major perks to synthetic underlayments is that polyethylene and propylene are both recyclable materials. So, when it comes time for the underlayment to be disposed of or if there are unusable leftover scraps, it can be recycled appropriately.
ADHESIVE BACKING – Most synthetic underlayments offer their product in a peel and stick / ice and water shield version. This is a great option or roofs with lower slopes or ones in harsh environments, and is the most common choice for specified commercial projects.
FIRE RESISTANCE – Many synthetics have a Class-A fire rating, meaning that when used in conjunction with a Class-A fire rated roof assembly, it offers the highest resistance to combustion or catching on fire.
WRINKLE FREE – It’s crucial for the underlayment to lay flat on the roof deck so the metal panels aren’t exposed to any stresses that could cause the development of oil canning. Most synthetic underlayments do not wrinkle when exposed to moisture, sunlight, or other elements.
Synthetic Underlayment Considerations
LOW PERMEABILITY RATING – While it may be a good thing for a roof underlayment to have a low permeability rating, meaning that less water and air can pass through it, it also can cause issues if not installed correctly. With synthetic underlayment, moisture from within the building cannot escape from the inside outwards, which means that proper attic and building ventilation is imperative when installing synthetics. Most peel and stick underlayments have less than a 0.5 permeability rating, which makes it a better barrier product than mechanically attached synthetic underlayment (which has a higher perm rating). Be sure to check your products specific technical information.
LOWER SLOPES – For low-slope standing seam metal roof applications, strictly peel and stick underlayment is a better choice.
INSTALLATION TEMPERATURE – Peel and stick underlayments have a minimum temperature that the surface can be for the product to be properly installed. Most products require you to use a primer when installing the product in temperatures below 40 degrees. Be sure to check the specific product information.
NOT DESIGNED FOR ALL SUBSTRATES – Not all peel and stick underlayments can be adhered to every roof substrate. Check with the recommendations on the product you’re using.
Metal Roofing Underlayment Performance Comparative
| Product Features | Titanium® FR | GAF VersaShield Fire-Resistant | Polystick XFR |
|---|---|---|---|
| High Temp / Fire Resistant | ✔ | Fire Resistant Fiberglass | ✔ |
| UL Class A Fire Rated Assembly | ✔ | ✔ | ✔ |
| High Temp Rating | 240°F | Max Service Temp 240°F | 265°F |
| Self-adhered Underlayment | ✔ | N/A | ✔ |
| Material | Composite with HDPE Film | Fiberglass-Reinforced Inorganic Mat | Polymer Modified Bitumen |
| Thickness | 80 mil | ~35–40 mil | 80 mil |
| Walkability | Sure-Foot® Dimensional Mesh | ✔ | ✔ |
| UV Exposure Duration | 180 Days | 60 Days | 180 Days |
| Lifetime Warranty | ✔ | N/A | Up to 30-Year with 2-Ply XFR |
| Roll Specifications | Titanium® FR | GAF VersaShield Fire-Resistant | Polystick XFR |
| Length | 50′ | 100′ | 49’3″ |
| Width | 36″ | 42″ | 39 3/8″ |
| Weight | 80 lbs | 53 lbs | 75 lbs |
| Size | 1.5 SQ | 3.5 SQ | 160 SF |
| Testing Stds / Code Approvals | Titanium® FR | GAF VersaShield Fire-Resistant | Polystick XFR |
| ASTM E108 Class A Fire Resistance | ✔ | ✔ | ✔ |
| FM 4470 Class 1 FR Over Steel Deck | N/A | ✔ | N/A |
| Permeability ASTM E96 | ✔ | ✔ | ✔ |
| Air Permeability ASTM 2178 | ✔ | ✔ | N/A |
| Tear Strength ASTM D1970 | ✔ | ✔ Tear Resistance D5147 | ✔ Tear Resistance D5147 |
| Tensile Strength ASTM D1970 | ✔ | ✔ Tensile Strength D5147 | N/A |
| Weight Per Square ASTM D5261 | ✔ | N/A | N/A |
| Nail Sealability D1970 | ✔ | N/A | ✔ |
| Nominal Thickness ASTM D5147 | ✔ | N/A | N/A |
| Thermal Stability ASTM D1970 | ✔ | N/A | ✔ |
| Low Temp Flexibility ASTM 1970 | ✔ | N/A | ✔ |
| Adhesion to Plywood ASTM D1970 | ✔ | N/A | ✔ |
| ICC-ESR-2053 | N/A | Anti-Condensation Membrane | N/A |
| ASTM D6757 | N/A | Test Std for Synthetic Underlayment | N/A |
| Code Compliance / Research Report | CCRR-1024 | N/A | N/A |
| Florida Product Approval | ✔ | ✔ | N/A |
| Miami-Dade County Product Approval | ✔ | ✔ | N/A |
| Texas Department of Insurance | ✔ | N/A | N/A |
| Wildland-Urban Interface (WUI) | ✔ | N/A | Listed by CA State Fire Marshal |
Final Thoughts
For help deciding between these two options, ask yourself these questions:
- How long do I expect the roof underlayment to last? How about the actual roof?
- Does my clients’ budget allow for a higher end underlayment?
- Will my clients’ underlayment be exposed to long periods of sun or heat?
- Is fire resistance or having a Class-A fire rating important in my region?
- What are the engineering requirements I need to adhere to for my project?
