Homeowners choose a shingle color and a warranty. Almost nobody asks about the layer underneath. But the underlayment rolled over your deck is the roof's second line of defense, and on a West Michigan roof it is worth understanding before you sign.
Quick answer: Roof underlayment is the water-resistant layer rolled over the wood deck before the shingles go on, and it is the roof's backup when wind-driven rain or a cracked shingle lets water past. The two choices are asphalt felt and synthetic. Felt is cheaper and meets code. Synthetic is far more tear resistant, sheds water instead of absorbing it, tolerates weeks of sun instead of days, and lasts 25 to 30 years. On a West Michigan roof built to last, synthetic is the stronger layer.
A roof is a system, not a pile of shingles. Under the shingles, and over the wood deck, there is a continuous water-resistant layer called underlayment. It is the part of the roof you will never see once the job is done, and it is the part that saves the deck when the shingles are not enough. Shingles shed most of the weather, but a hard, wind-driven Michigan rain will push water sideways under the tabs, a lifted shingle will let a little through, and ice at the eaves will back water uphill. The underlayment is what carries all of that down and off the deck so the wood underneath stays dry.
It sits in a specific order. The deck, or sheathing, is the plywood or OSB nailed to the rafters. Over that goes the underlayment across the whole field of the roof, with a self-adhering ice-and-water membrane at the eaves and valleys where the risk is highest. The shingles go on top. Get the order and the materials right and the roof works as one system. Skimp on the layer nobody sees and you have built a roof that depends entirely on the shingles never failing, which no roof manages for its whole life.
For most of the last century, underlayment meant tar paper. Asphalt-saturated felt, the black roll roofers still call felt or tar paper, comes in two common weights: number 15 and the heavier, tougher number 30. It is made of a paper or fiberglass mat soaked in asphalt to make it water resistant. It is inexpensive, it has protected roofs for decades, and it meets building code. For a straightforward roof on a budget, felt still does the job it always has.
Its weaknesses are the flip side of its price. Felt tears easily, and the lighter number 15 will rip in a gust of wind during installation or under a boot on a steep pitch. It absorbs water, so if a rain shower catches the deck before the shingles are on, felt wrinkles and buckles, and a wrinkled underlayment can telegraph through the shingles. And it has almost no tolerance for sun: left exposed, felt dries out and breaks down within days. On a roof that gets torn off and dried in over several days, that short exposure window matters.
Synthetic underlayment fixed most of what felt got wrong. Woven or spun from polypropylene or polyester, synthetic underlayment is a lightweight plastic sheet engineered specifically for this job. Its advantages over felt are not marketing, they are structural. It has a very high tear strength, so it does not rip in the wind or under foot the way felt does. It does not absorb water, so it sheds a rain shower during the job and will not wrinkle or swell. And it carries UV inhibitors that let it sit exposed on the deck for 60 to 180 days depending on the product, instead of felt's few days, which is real insurance when weather stalls a job. Manufacturer resources such as the Owens Corning comparison lay out the same tradeoffs.
It also lasts longer. Felt is generally good for 12 to 20 years, while synthetic is commonly rated 25 to 30 years, which is the point that matters most on a West Michigan reroof: the underlayment should last as long as the shingles it protects. A 30-year architectural shingle roof deserves an underlayment that will still be doing its job in year 28, not one that aged out a decade early. The cost difference is real but small in the context of a whole roof, and it buys a tougher, longer-lived layer.
| Factor | Asphalt felt | Synthetic |
|---|---|---|
| Tear strength | Low, number 15 rips easily | High, resists wind and foot traffic |
| Water absorption | Absorbs, wrinkles and swells if wet | Sheds water, will not wrinkle |
| UV exposure tolerance | Days before it breaks down | 60 to 180 days, product dependent |
| Typical lifespan | 12 to 20 years | 25 to 30 years |
| Weight and coverage | Heavier, fewer squares per roll | Lighter, more squares per roll |
| Cost | Lower | Higher, still a small share of the roof |
| Meets code | Yes | Yes |
Climate is why this decision is not just theory here. The National Weather Service Grand Rapids office tracks 40 to 60 freeze-thaw days in a normal winter, and that cycling is hard on every layer of a roof. Underlayment that stays intact and sheds water is what keeps meltwater and ice-dam backup off the deck through a long winter. A wrinkled, water-swollen felt or a torn seam is exactly where trouble starts.
Installation weather matters too. A West Michigan tear-off can get caught by a fast-moving rain shower or a windy afternoon before the shingles are down, and that is precisely where synthetic earns its keep: it does not tear in the wind and it does not soak up a passing shower. None of this replaces the membrane at the edges. The eaves still need a self-adhering ice barrier, which is a separate product with its own code requirement, covered in our guide to ice-and-water shield and Michigan code. Underlayment is the field. Ice-and-water is the armor at the vulnerable edges. A good roof uses both.
Code sets a floor, not a standard of excellence. The International Residential Code, Chapter 9, requires underlayment on asphalt shingle roofs and points to ASTM standards for it: felt underlayment complies under ASTM D226 or D4869, and synthetic complies under ASTM D6757. Both are legitimate, code-approved products. On lower slopes, between 2:12 and 4:12, the code calls for two layers of underlayment, while a standard single layer is allowed at 4:12 and steeper. And in Michigan's cold climate, a self-adhering ice-barrier membrane is required at the eaves, extending up the roof past the interior wall line.
The takeaway is simple. Felt and synthetic both satisfy code, so choosing synthetic is not about passing inspection. It is about buying a tougher, longer-lasting layer above the minimum. The homes we reroof to a manufacturer system warranty use synthetic as a matter of course, because those warranties are built around a complete, approved system, a subject we get into in our guide to asphalt roof warranty terms.
On a Heritage Hill home, the layer under the roofing is not an afterthought either. A slate or cedar roof on a century-old Grand Rapids house is a long-life system, and the underlayment beneath it should be chosen to match. High-temperature synthetic and self-adhered membranes are the modern choices there, integrated so the historic roof covering above them can do what it was built to do for generations. It is the same discipline we bring to a full tear-off on any home: strip to the clean deck, lay the right underlayment for the roof going on, and never bury an aged layer under a new one.
Underlayment is the layer that decides whether a roof is a system or just a field of shingles hoping to hold. Felt is affordable, code-approved, and has protected roofs for a century, but it tears, it swells when wet, it cannot take sun, and it ages out in 12 to 20 years. Synthetic is tougher on every count, sheds water, tolerates weeks of exposure, and lasts 25 to 30 years, which is why it has become the standard on a quality West Michigan reroof. Neither replaces the ice-and-water membrane the edges require. Ask what is going under your shingles before you sign, because the answer tells you whether the roof was built to last or just to look finished.
Our residential roofing crews spec the underlayment to the roof and the home, and our cost guide shows where it fits in a full roof budget. We have kept West Michigan roofs watertight, layer by layer, since 1994.
Roof underlayment is the water-resistant layer rolled over the wood deck before the shingles go on. It is the roof's second line of defense. Shingles shed most of the water, but wind-driven rain, ice, and the odd cracked shingle let some through, and the underlayment carries that water down and off the deck so the wood stays dry. It also protects the deck during installation before the shingles are on.
For most West Michigan roofs, yes. Synthetic underlayment is far more tear resistant, does not absorb water so it will not wrinkle if rain hits it during the job, tolerates weeks of sun exposure instead of days, and lasts longer. Felt is cheaper and still meets code, and it has protected roofs for decades. But on a quality reroof meant to last, synthetic is the stronger, more forgiving layer.
Yes. Asphalt-saturated felt that complies with the ASTM standards in the International Residential Code is code-approved underlayment, and it is still used every day. Synthetic underlayment complies under its own ASTM standard. Code sets the minimum, including a separate ice-barrier membrane at the eaves in Michigan's cold climate. Choosing synthetic over felt is a quality upgrade above that minimum, not a code requirement.
Felt underlayment generally lasts around 12 to 20 years, while synthetic underlayment is commonly rated for 25 to 30 years, which lines up better with the life of a quality architectural shingle roof in West Michigan. In practice the underlayment should last as long as the shingles above it, which is one reason synthetic is favored on a roof built to go the distance.
No, they are different products for different jobs. Underlayment is the field layer that covers the whole deck. Ice-and-water shield is a self-adhering membrane installed at the eaves, valleys, and penetrations, where ice dams and standing water hit hardest, and Michigan code requires it at the eaves. A proper roof uses both: ice-and-water at the vulnerable edges and underlayment across the rest of the deck.
Yes. On a full tear-off the old underlayment comes off with the shingles down to the bare deck, and fresh underlayment goes on the clean wood. You cannot leave aged felt under a new roof and expect it to last, and most manufacturer system warranties require their approved underlayment installed new. Reusing old underlayment is not how a warrantied roof is built.
Roof Repair of Grand Rapids has been installing, repairing, and preserving roof systems on West Michigan homes since 1994. We build roofs as systems, from the deck up: the right underlayment for the roof going on, a self-adhered ice barrier at the eaves our winters demand, and flashings and shingles that match. On a downtown ranch or a Heritage Hill slate, we strip to clean sheathing and never bury an aged layer under a new roof. Climate figures reference the National Weather Service Grand Rapids office, and code references follow the International Residential Code, Chapter 9.