The last time JLC treated the subject of synthetic roofing underlayments in depth (May 2006), author John Nicol acknowledged that synthetics are stronger, lighter, and faster to install than asphalt felts, but asked whether they’re worth the extra cost — estimated at about 30 percent at the time. Today, with even more synthetic underlayments on the market and their premium over felt a bit higher, the question is worth revisiting.
My own acquaintance with felt underlayment began more than 40 years ago (at the time 15# felt actually weighed 15 pounds per square), when I went up on the roof of my mother’s 1952 cape to find and fix the source of a leak in the garage. I stripped away the asphalt shingles and, voilà, there was a patch of torn, buckled felt near the valley and some rotted plywood sheathing.
I replaced the spongy sheathing, nailed down some new 30# felt, flashed the valley with aluminum coil stock, and the leak was gone. Several years later when the roof was showing its age, I put a new layer of asphalt shingles over the original roof, and the house stayed dry for the 25 remaining years that Mom lived there.
Methodology
Synthetic underlayments weren’t available back then, of course, but would I have done anything differently if they had been? That’s what I wanted to work through as I gathered information on more than 20 underlayments — both synthetic and organic — in preparation for this article. I pored over sell sheets, spec sheets, and installation instructions, and I talked with manufacturers’ technical reps along with builders and installers who work with both types of underlayment. In addition to matters of cost, ease of installation, and durability, I found we have to wrestle with another fundamental issue: Is it better for a roofing underlayment to be permeable or impermeable?
The Permeability Debate
One of the most important data points with regard to a roofing underlayment is its permeability rating — that is, its resistance to penetration by moisture in both liquid and vapor forms (see “Vapor Permeability Classifications”). Everybody agrees that impermeability to liquid water is a good thing in an underlayment; it means the roof deck won’t get wet, even if wind-driven rain or ice dams force water under the finish roofing, or the weather turns rainy once the underlayment is down but before the finish roofing has been installed.
But impermeability to water vapor might not be such a good thing — at least in certain cases (see “Whatever Happened to 30# Felt?”). Some pros argue that if a roof deck were to get wet due to a leak at a flashed penetration or a tear in a roofing layer, or because of moisture buildup in an unventilated attic, an impermeable underlayment would trap vapor. The potential result: mold, mildew, and rot in the wooden parts of the roof structure.
Most but not all synthetic underlayments are impermeable to both water and vapor. Asphalt felts are semi-permeable — they’re pretty good at shedding water, and they’re great at letting vapor pass through, particularly when they get wet.
General-Purpose Underlayments. These materials are typically recommended for use with one or more of the common roofing types. Generally they are the lightest weight, least expensive products, and carry shorter warranties.
The Case for Synthetic Underlayments
In addition to being impermeable to water, synthetic underlayments are more tear- and puncture-resistant than asphalt felts. Synthetics also resist expansion and contraction with temperature cycling, so they wrinkle and buckle less severely than felts, with less tearing around fasteners. Virtually all synthetics have a UV-resistant coating so they don’t degrade with prolonged sunlight exposure. That means that roofs dried-in with synthetic underlayments need not be finished for six to 12 months if they’ve been fastened with cap nails or cap staples, which are recommended by most manufacturers.
Synthetic underlayments claim significant advantages over felts for installers. Generally, rolls of synthetic material are wider, longer, and lighter than felts. This means easier handling and faster installation. Most claim to have slip-resistant coatings or textures for better traction under wet or dusty conditions. Synthetic materials also remain more flexible in cold weather and roll out without cracking or chatter. This characteristic can lengthen the work season for roofers in colder climates.
And after installation, synthetic underlayments promise better performance than felts. Since they provide an impervious secondary layer of moisture protection, a roof protected with synthetic underlayment isn’t likely to leak even if a few shingles crack or blow off. Unlike felts, synthetics don’t absorb moisture, so they can’t rot or dry out, nor do they support mold and mildew growth.
Premium Underlayments. These materials are generally thicker, heavier, and tend to carry longer warranties. They are also more expensive: The products listed here are almost 50% more expensive on average than the general-purpose materials. Premium underlayments ?are used most frequently with metal, tile, slate, and heavyweight asphalt shingles.
Not All Synthetic Underlayments Are Created Equal
It’s important to note that while the manufacturers of synthetics tout their most desirable general properties — water-shedding ability, strength, light weight, UV resistance, slip-resistance, and such — not all synthetic underlayments are the same. Some manufacturers make more than one synthetic underlayment, recommending each for a different application. For example, while some products are recommended for use with all common roofing types — asphalt shingles, metals, clay tile, slate, and wood shingles and shakes — others are recommended for only one or two applications, say tile and metals.
In many cases, the product brand names aren’t terribly descriptive in suggesting specific applications or indicating a product’s place in the “good-better-best” hierarchy. For that reason, I’ve tried to sort out key differences among the synthetic underlayments I found in the market, and grouped products with similar properties in each of three tables displayed throughout this article. The fourth table lists high-temperature synthetic underlayments.
Products I’ve listed as “General-Purpose” are the lightest, least-expensive materials, and carry the shortest warranties. They are typically recommended for use with one or more of the common roofing materials. “Premium” products are generally thicker, heavier, and more expensive, and carry longer warranties. While they may be recommended for use with all common roof finishes, in practice they seem to be used most frequently with metal, tile, slate, and heavyweight asphalt shingles.
Vapor-permeable underlayments. These materials are recommended for use on unventilated roofs and with finish roofing materials that are installed with an air space between the roof deck and the underside of the finish roofing, such as wood shingles and shakes, some metals, and tiles.
“Vapor-Permeable” materials are unlike the vast majority of synthetic underlayments, which are impermeable. Doug Snyder, a technical representative for VaproShield, which makes SlopeShield (perm rating: 59) recites the mantra of die-hard fans of asphalt felt: “A building’s got to breathe!” While the common wisdom holds that impermeable underlayments don’t pose a significant risk when placed in a well-ventilated roof design, Snyder asserts that attic ventilation doesn’t carry away water vapor. SlopeShield promotes drying by allowing water vapor to escape, Snyder says, and should be used in any application in which the roofing material is fastened to battens or has an air space underneath, such as tile, wood shakes and shingles, and some metals. Like the impermeable synthetics, SlopeShield will protect sheathing from condensation that typically forms beneath metal and tile roofing.
Who’s Using Synthetic Underlayments — and Why?
Paul Ecclestone, product manager for Intertape Polymer Corp., which manufactures Nova Seal underlayments and similar synthetics sold under a variety of brand names, says that sales are on an upswing. “We have seen an excellent rebound from the economic downturn and excellent support for our product lines. As contractors become more and more familiar with the benefits of our synthetic roofing underlayment, we’re seeing higher adoption rates.” Ecclestone also attributes at least part of the sales growth to the recent incidence of destructive hurricanes and other storms. “On weather-damaged homes,” he says, “emergency crews can use synthetic underlayments for temporary roofing that’ll keep the houses dry until the finish roofing can be put on.”
Lou Hale, a general contractor from western Massachusetts, says his experience supports most manufacturers’ claims about the benefits of synthetics. “When my crew and I are drying-in an addition ourselves, we mostly use synthetics because it’ll generally be a couple of weeks before our roofer puts the finish on,” Hale says. “We find that the synthetics are much easier to use [than felts] and provide much better mid- and long-term protection for the house.”
Convenience also plays a part in Hale’s decision on which brand to use. “My underlayment choice is usually based on my choice of supplier — it’s not worth paying a $20 hot-shot charge to have another supplier bring out a particular brand of underlayment,” he says. “We use Grace Tri-Flex 30 or RoofTopGuard II, depending on where the material is coming from. My preferred lumberyard keeps Tri-Flex in stock, so if they are making the drop, that’s what we use. But I have also used a good bit of the RoofTopGuard II, and it’s just as good a product.”
Hale finds that the wider, longer, lighter rolls really do make a difference in the speed of installation. This speed advantage seems to depend, however, on fastening with staples rather than the nails or cap nails most manufacturers recommend. “We frequently use staple hammers or plain old roofing guns. While that may not be a manufacturer-approved method, I can tell you it will hold through a New England summer thunderstorm just fine, and it’s at least twice as fast as using a pneumatic cap stapler. You can literally tack a synthetic roll down on one end, have a guy roll out 30 to 40 feet while another guy just walks along the roof and nails it down — even on windy days.”
For Hale, tear resistance is also a big advantage, especially on reroofs, where the surface might not be perfectly flat and smooth. With #30 felt, one missed nail during tear off can lead to a hole and a potential leak.
“We recently put this to the test on an 1840s cape we’re remodeling,” Hale says. “We needed to temporarily dry-in the house over widely spaced sheathing boards that were peppered with jagged nails. We were able to walk up and down that 8-pitch roof without causing a single hole or tear. Thirty-pound felt would’ve fallen apart under our feet.”
When I ask Hale if the labor savings with a synthetic compensate for the higher cost, he says, “The short answer is no, the labor savings are significant but don’t equal the increased material cost. However, the peace of mind that comes from all the other benefits, combined with the labor savings, more than justifies the premium for me.”
Randy Bush of Great Falls, Mont., uses synthetic underlayment because he installs mainly steel roofs. “One reason for using synthetic is I don’t like the black footprints you get from felt when stepping on it, then onto the metal,” he says. Another reason is that an impermeable synthetic more effectively protects the roof deck from dripping condensate that tends to form on the underside of metal roofing.
Joe Adams, a Houston builder whose homes have either copper or “comp” (premium asphalt shingle) roofs, tells me, “My roofer prefers a synthetic underlayment — Berger Pro-Master UDL-Plus. He supplements this with Grace Ice & Water Shield [a modified bituminous, self-adhering flashing material] at the valleys and roof-wall intersections.”
Allan Edwards, another Houston builder of high-end custom homes, uses synthetic underlayment in an unexpected way. “A majority of my homes are clay tile or slate; a few are metal. We use Grace Ice & Water Shield as the initial underlayment, then apply Titanium UDL-30 Plus over it. It’s bulletproof.”
When I ask if this isn’t overkill, Edwards sends me to his roofer, Robert Coreale, owner of Tile Roofs of Texas, for an explanation. Robert says, “For most roofers, Ice & Water or Titanium only would be okay, but here’s why we go that extra step: Ice & Water should not be exposed to UV for extended periods of time, so you need a layer to protect it. You could use #30 felt or any other type of underlayment for this ply, but we use Titanium because it is very durable and easy to walk on — it provides very good traction. The synthetic layer also protects the Ice & Water from construction traffic. Stucco and brick layers are not easy on the peel-and-stick.”
Better or Just Different?
With all of the underlayment products I’ve studied here, and all the testimony I’ve reviewed from the pros that use them, I’m convinced that there are enough good alternatives to address virtually any roofing job nicely. I’m also convinced that you’ve got to understand the issues and goals of any given job to choose an underlayment wisely. One guy wants to deliver a workmanlike job at the lowest price possible and still make a profit; another wants a bulletproof 100-year roof and has a customer willing to pay for it.
I’m convinced that synthetic underlayments are more durable and faster to install than felt, but at publication, #15 felt goes for only about 41/2 cents a square foot; #30, about 9 cents. If you’re going to cover the felt right away and you can flash vulnerable spots of the roof adequately, why not keep the cost down? On the other hand, even though synthetic materials are more costly, faster installation means lower labor cost, as does the resistance to collateral damage from foot traffic during construction.
About the only claim many manufacturers make about synthetic underlayments that’s suspect in my mind is that they’re safer underfoot. More than one guy told me a story about expecting to stride up a slope on slip-resistant underlayment only to end up on his butt. The comment in my notes reads, “They’re not less slippery; they’re just different.”
That comment applies to synthetics in many ways. While I probably wouldn’t have done anything different when reroofing my mother’s house 40 years ago even if I had known about synthetics, I would do it all differently today.
Michael Chotiner is a former contractor who has written extensively on building and home-improvement in print and on the Web.
