Reviewed by Mohammad KhanCo-owner, United DevelopersMHIC #111971
What is roof underlayment, and does synthetic really beat felt?
Underlayment is the water-resisting layer fastened directly to your roof deck, beneath the shingles. Synthetic underlayment — spun or woven polypropylene — has replaced asphalt-saturated felt on quality re-roofs because it does not absorb water, does not wrinkle and telegraph through the shingles after a rain, and holds a fastener far better. Self-adhering ice-and-water membrane is a third, different product: it bonds to the deck and self-seals around every nail that goes through it, and it belongs at the eaves, in every valley, around every penetration, at every wall-to-roof junction and on any low-slope section. The premium spec is membrane at the vulnerable details and synthetic across the field. Felt everywhere is the budget spec.
Why there is a layer under the shingles at all
Pull the release film off a roll of self-adhering membrane on a cold morning and it comes away with a sound like tearing silk. What is underneath is black, tacky, rubber-modified asphalt, and it will bond to bare plywood hard enough that you cannot lift it again without bringing wood fibres with it. Drive a nail through it and the compound closes around the shank. That is the whole trick, and it is the reason this material exists.
It costs a fraction of the shingles that will cover it. Nobody will ever see it again — not you, not your buyer's inspector, not the next roofer until the day he tears it off. And it is the component most likely to decide whether a bad afternoon shows up on your ceiling.
Underlayment is the only part of a roof where the money is invisible and the consequences are not.
The reason it matters comes down to one sentence of building science. Asphalt shingles are a water-shedding system, not a waterproof membrane. They work by overlap and gravity: each course covers the joints of the one below, and water runs down the surface. That works beautifully until something interrupts the flow — wind driving rain uphill under a tab, a lifted shingle, granule loss, ice building at the eave, a nail that missed, debris damming a valley.
Underlayment is the second line. It decides whether an interrupted shed becomes a damp mark on your ceiling or nothing at all. It also protects the deck during the install itself, which in the DMV can mean a roof standing open when an afternoon thunderstorm arrives over the Potomac with twenty minutes' notice.
There are three products in play, and they are not interchangeable. Everything worth knowing about this layer follows from the differences between them.
The assembly, from the deck up
A correctly built asphalt shingle roof is eight layers deep at the eave. In order:
Roof deck (sheathing)
Plywood or OSB, and on older DMV houses often 1x6 or 1x8 plank boards. Everything above it is held by nails driven into this.
Fails when: Soft, delaminated or gapped decking gives fasteners nothing to hold. See decking and sheathing.Drip edge at the eaves
An L-profile metal that carries water off the deck edge and into the gutter instead of behind it. It goes on before the underlayment at the eave.
Fails when: Omitted, or reused from the old roof. Water then wicks back onto the fascia and rots it from behind.Ice-and-water membrane
Self-adhering polymer-modified bitumen with a peel-off release film. It bonds to the deck and seals around fasteners. Eaves, valleys, penetrations, sidewalls, low-slope sections.
Fails when: Installed in the valleys only, or not run far enough up from the eave to get past the exterior wall line.Synthetic underlayment
The field of the roof. A dimensionally stable secondary water barrier that also serves as the temporary weather layer while the roof is open.
Fails when: Fastened with the wrong cap nails or plain roofing nails, or left exposed beyond the product’s UV-exposure limit.Drip edge at the rakes
Same profile, different order: at the rake (the sloped gable edge) the metal goes over the underlayment so water on top of the underlayment still exits.
Fails when: Installed under the underlayment at the rake, which routes water into the edge of the deck.Starter strip
A purpose-made first course at the eaves and the rakes, with a factory sealant line positioned to bond the first row of shingles and cover its joints.
Fails when: Substituted with cut-up shingles. See starter, hip and ridge.Field shingles
The visible roof. Fastened in the manufacturer’s nailing zone, in the pattern the wind rating was tested with.
Fails when: High nails, overdriven nails, or four fasteners where the rated pattern calls for six.Hip and ridge cap, ridge vent
Purpose-made cap over a cut ridge slot, so the assembly exhausts as well as sheds.
Fails when: A ridge vent over a slot that was never actually cut open. See attic ventilation.Felt vs synthetic vs ice-and-water membrane
| Asphalt-saturated felt (#15 / #30) | Synthetic underlayment | Ice-and-water membrane | |
|---|---|---|---|
| What it is | Organic or fibreglass mat saturated with asphalt, in rolls. The traditional product. | Woven or spun polypropylene or polyethylene, usually with a slip-resistant coating. | Polymer-modified bitumen with a self-adhesive back and a release film. |
| When it gets wet | Absorbs water, then wrinkles and buckles as it dries. Those ripples telegraph through the finished shingles. | Does not absorb. Stays flat. | Bonded to the deck, so there is nowhere for water to travel. |
| Around a fastener | Tears. The hole stays a hole. | Much higher tear strength; still a hole. | Self-seals around the shank. This is the whole point of the product. |
| Left exposed mid-job | Degrades quickly in sun; short exposure tolerance. | Rated for a stated exposure period — check the product’s own limit. | Rated for a stated exposure period; not intended as a finished surface. |
| Walking on it | Slick when damp, tears under boot heels. | Designed with traction. A real safety difference for the crew. | Grips, but it is not a walking surface. |
| Where it belongs | Budget re-roofs and outbuildings. | The field of the roof. | Eaves, valleys, penetrations, wall junctions, low-slope transitions. |
The honest summary: synthetic is better than felt in every property that matters, and the price difference on a typical house is small relative to the job. But synthetic in the field does not substitute for membrane at the details, and membrane at the details does not substitute for the field layer. A quote that names only one product has left something out.
And where felt is still the right answer
It would be easy to write felt off, and it would not be true. Asphalt-saturated felt is a hundred-year-old product that works: it is code-recognised, it is cheap, it is repairable with a knife and a hammer stapler, and on a detached garage, a shed, a porch roof being patched, or any structure where the cost of failure is a wet floor rather than a wet ceiling, it is a perfectly rational specification. What it is not is the right layer under a five-figure architectural roof you intend to keep for thirty years. The problem is never that felt exists. The problem is felt appearing on a premium quote at a premium price, on the assumption that you will not read the line.
What the label actually tells you, and what it hides
Underlayment is sold with numbers on the wrapper, and almost none of them mean what a homeowner would reasonably assume. This is the section that lets you read a product data sheet instead of a brand name.
A "#15 felt" has not weighed 15 pounds in decades
The designation comes from an old convention: pounds of material per 100 square feet, one square. It is now a nominal class, not a measurement — modern No. 15 felt typically weighs well under that, and two rolls both stamped #15 can differ in mass and tear strength. The standard that actually governs it is ASTM D226 (asphalt-saturated organic felt, Type I = No. 15, Type II = No. 30), with ASTM D4869 covering the lighter-duty underlayment grades. If a quote says "15 lb felt", it is naming a class, not a specification.
Synthetics are not covered by one ASTM sheet standard
This surprises people, and it is the most useful thing to know about them. There is no single ASTM designation that a synthetic underlayment must meet, which is why quality varies so widely between products that look identical on a pallet. What responsible manufacturers do instead is get the product evaluated to ICC-ES AC188, the acceptance criteria for alternative steep-slope underlayments, and publish an evaluation report. The number worth asking for is the ICC-ES report number. A synthetic with one has been tested for water transmission, tear resistance, fastener pull-through and accelerated weathering by a third party. A synthetic without one has been tested by its marketing department.
Ice-and-water membrane does have a standard, and the code names it
Self-adhering ice barrier is governed by ASTM D1970 — self-adhering polymer-modified bituminous sheet used as steep-roofing underlayment for ice-dam protection. The model residential code names that standard directly in its ice-barrier provision, which makes this the one underlayment product where "does it comply?" has a clean yes-or-no answer. Ask for the product name and confirm the data sheet says D1970. If it does not, whatever is being installed at your eaves is not an ice barrier in the sense the code means.
The fastener is part of the specification
Synthetic underlayment is held down with cap fasteners — a nail or screw through a one-inch plastic or metal disc that spreads the load and closes the hole. Plain roofing nails tear through under wind; staples are worse, and are prohibited by most synthetic manufacturers outright. Every synthetic has a published fastening pattern, tighter at the laps and tighter again in high-wind zones, and installing outside that pattern voids the product's own warranty before the shingles even go on. This is a real, checkable detail: cap fasteners are visible on the roof before the shingles cover them, and they photograph well.
The UV clock is the product's, not the schedule's. Every synthetic and every membrane carries a maximum exposure period — the number of days it can sit uncovered before the material begins to degrade. Some are rated for a few weeks, the best for several months, felt for days. If your roof is going to stand open across a weather delay, the question is not "will it be fine", it is "what is this product rated for, and how many days has it been up there".
The five places ice-and-water membrane is not optional
These are the locations where water either stops moving or is pushed uphill. Every one of them needs a bonded, self-sealing layer rather than a shed-and-lap layer.
- The eaves. This is where ice dams form: heat escaping into the attic melts snow up-slope, the meltwater refreezes at the cold overhang, and the resulting ridge of ice backs standing water up under the shingles. The model residential code Maryland builds from requires an ice barrier in areas with a history of ice forming along eaves, running from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. On a deep overhang that is more than one course of membrane.
- Every valley. Two planes of roof dump into one channel, so a valley carries several times the water of the field and it carries debris with it. Membrane runs the full length, under whatever valley treatment is used.
- Every penetration. Plumbing vents, exhaust terminations, satellite mounts, solar stanchions, skylight curbs. Flashing is the primary defence; the membrane is what makes the hole a non-event if the flashing is ever disturbed.
- Wall-to-roof junctions. Where a roof plane runs into a wall — dormer cheeks, a garage roof meeting the house, a chimney chase — water is being turned, not shed. Membrane up the deck and up the wall behind the step flashing.
- Low-slope sections. Porch roofs, additions and dormer tops that fall below the slope asphalt shingles are rated for. Below 2:12 asphalt shingles do not belong there at all; between 2:12 and 4:12 the underlayment requirement doubles.
Slope changes the rules
On a slope of 4:12 or steeper, a single layer of underlayment in the field is the standard requirement. Between 2:12 and just under 4:12 the requirement is two layers, lapped — a low slope moves water more slowly, so it has more time to find a way in. Below 2:12, asphalt shingles are not an approved covering; that section needs a low-slope membrane system instead. If your house has a porch or addition roof that looks nearly flat, ask specifically what is going on it.
The forgotten alternative in the code
The ice-barrier provision does not actually say "buy self-adhering membrane". It permits either a sheet complying with ASTM D1970, or two layers of ordinary underlayment cemented together — a genuine, code-legal ice barrier that almost nobody builds any more, because bedding two courses of felt in asphalt cement across a whole eave is slow, filthy work and the roll product does the same job in a tenth of the time. It is worth knowing about for one reason: if a contractor tells you the eave is protected but there is no membrane on the quote, there is exactly one other thing it could legitimately be, and it will be obvious from the line items whether that is what they mean.
The measurement that catches most shortfalls: the requirement is 24 inches inside the exterior wall line, measured horizontally — not 24 inches up the roof from the gutter. On a house with a 24-inch overhang and any real pitch, the membrane has to reach roughly four feet up the slope to satisfy it, which is two courses of a 36-inch roll, not one. One course to the wall line is the single most common way an eave is left short while the quote still says "ice and water at eaves".
How to read the underlayment line on a quote you already have
You do not need a roofer to do this and it does not require access to your roof. It requires the written scope, five minutes, and knowing what the words are supposed to be.
How to check the underlayment specification on a roofing quote
Run this on every quote you collect, including ours. A contractor who answers all five in writing has told you something real about how they work.
- Find a product name, not a category Look for a manufacturer and product name for the field underlayment. "Install underlayment" and "synthetic underlayment" are categories that the cheapest roll on the truck also satisfies. A named product can be looked up in ninety seconds, and its data sheet will tell you the exposure rating and whether it has an ICC-ES evaluation report.
- Check the eave, not just the valleys Search the scope for the word eave. Ice-and-water membrane listed in valleys only is the most common shortfall on an otherwise good quote, because the eave is where ice dams actually form and it is the location the code addresses. If the eave is not named, the eave is not priced.
- Do the 24-inch arithmetic The ice barrier has to reach at least 24 inches inside your exterior wall line. Measure your overhang from the wall to the edge of the fascia. If it is more than about 12 inches, a single 36-inch course cannot get there on a pitched roof, so the quote needs two courses at the eaves. Ask how many courses are priced.
- Ask for linear feet Request the linear feet of eave and the linear feet of valley used to price the membrane. These are numbers, they come off the same measurement the rest of the quote is built from, and they let you check the membrane quantity against your own roof rather than trusting it.
- Look for new drip edge at both edges Drip edge should appear twice: at the eaves and at the rakes, both new. If it says reuse existing, or names only one of the two, the correct lap order at the edge is not being built, and that is where water gets behind the fascia.
If a contractor will not put any of these five in writing, you have your answer, and it did not cost you anything to get. If they will, you have just made every quote in the stack comparable on the one component nobody can inspect afterwards.
What we specify, and why
Every UD roof replacement scope names the underlayment by product, not by category
You should be able to read a roofing contract and know exactly what is going on your deck. Ours lists these separately, with quantities, so there is nothing to interpret later.
- Synthetic underlayment across the field, named by product, fastened per the manufacturer’s pattern
- Ice-and-water membrane at the eaves, run past the exterior wall line as the code requires
- Ice-and-water membrane in every valley, full length
- Membrane at every penetration and wall junction, listed by count
- New drip edge at eaves and rakes — never reused — lapped in the correct order at each
- Purpose-made starter at eaves and rakes, not cut field shingles
- Linear feet of eave and valley stated, so you can check the membrane quantity against the roof
- Photographs of the deck and the membrane before the shingles cover them
How to tell whether a contractor is cutting this corner
You will never see the underlayment again after install day, which is exactly why it gets downgraded. Everything below can be checked on paper, before you sign, on any quote from any company.
A category, not a product. Felt and synthetic both satisfy that sentence, and so does the cheapest roll on the truck.
Ask instead: name the product and the weight or class, in the written scope.On a premium re-roof this is the tell. It is the lightest common underlayment, it absorbs water, and it wrinkles.
Ask instead: what synthetic underlayment costs on this roof, as a line item.Valleys and nothing else. The eave — where ice dams actually form and where the code asks for a barrier — has been left out.
Ask instead: how many linear feet of eave there are, and how many rows of membrane cover it.Drip edge is bent metal at the most weather-exposed edge of the roof, usually the same age as the shingles being torn off. Reusing it also makes the correct lap order impossible.
Ask instead: for new drip edge at eaves AND rakes, listed separately.An open-ended phrase in a fixed-price contract. It converts into a change order the moment the roof is open.
Ask instead: for quantities and unit prices in writing before work starts.Possibly true, and irrelevant. The question is what is going on your house, in writing, with a quantity next to it.
Ask instead: for the written scope. If it will not be put in writing, that is the answer.Underlayment questions DMV homeowners ask
Is synthetic underlayment worth paying for instead of felt?
On a re-roof you intend to keep for decades, yes. Synthetic underlayment does not absorb water, so it does not wrinkle and telegraph ripples through the finished shingles the way felt does after it gets wet. It has substantially higher tear strength around fasteners, it stays dimensionally stable, it tolerates a longer exposure period while the roof is open, and it gives the crew real traction to work on. The cost difference on a typical Maryland house is small next to the total job, and it is the layer you can never inspect again once the shingles are on.
Where does ice-and-water shield actually need to go?
At the eaves, in every valley, around every penetration such as plumbing vents and skylight curbs, at every wall-to-roof junction including dormer cheeks and chimney chases, and on any low-slope section. Those are the places where water either stops moving or gets pushed uphill, so a bonded self-sealing membrane is doing a job that an overlapped shed layer cannot. A quote that puts membrane in the valleys only has left out the eave, which is where ice dams form.
Does Maryland code require an ice barrier at the eaves?
Maryland builds from the model residential code, which requires an ice barrier in areas that have a history of ice forming along the eaves, extending from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. Which code edition applies, and how your county interprets the ice-history trigger, is set locally, so confirm it with your jurisdiction or ask your contractor to show you the requirement they are building to. On a deep overhang, reaching 24 inches past the wall line takes more than one course of membrane.
Can underlayment be left exposed if the roof is not finished the same day?
For a limited period, yes, but the limit belongs to the product and not to the schedule. Synthetic underlayments and ice-and-water membranes each carry a stated maximum UV exposure period, and past it the material starts to degrade. Felt has the shortest tolerance of the three. If your roof will be open overnight or across a weather delay, ask which product is protecting it and what exposure period that product is rated for.
If underlayment is a water barrier, why do roofs still leak?
Because underlayment is a secondary layer, not a waterproof envelope. It is penetrated by thousands of fasteners, it is lapped rather than sealed except where membrane is used, and it stops at every edge and opening. Most leaks happen where the roof plane ends — walls, chimneys, valleys, pipes and skylights — which is why flashing, not underlayment, is the primary defence at those points. Underlayment is what buys you time and limits the damage when something upstream fails.
Is there an actual standard for synthetic underlayment, or is it all marketing?
There is no single ASTM sheet standard a synthetic underlayment must meet, which is exactly why quality varies so much between rolls that look identical on a pallet. The meaningful third-party check is an ICC-ES evaluation report issued against acceptance criteria AC188, which tests water transmission, tear resistance, fastener pull-through and accelerated weathering. So the question to ask is not which brand, it is whether the product has an ICC-ES report number you can look up. Felt, by contrast, is governed by ASTM D226 and D4869, and self-adhering ice barrier by ASTM D1970 — that last one is named directly in the code.
How far up the roof does ice-and-water shield have to go?
Far enough to reach a point at least 24 inches inside the exterior wall line, measured horizontally — not 24 inches up from the gutter, which is the assumption that leaves eaves short. Add your overhang depth to that 24 inches and then account for the pitch, and on a house with a two-foot overhang the membrane needs to run roughly four feet up the slope. A 36-inch roll cannot do that in one course, so the eave needs two. This is the arithmetic worth doing before you sign, because a quote can say ice and water at eaves and still price only one course.
Where to go next
Underlayment is one component. Two of its jobs are big enough to have their own guides — the ice barrier at the eave, and the metal that finishes the edge it stops at.
Ice-and-water shield: where it must go
The eave is where the code makes a demand, the physics makes a mess, and the quote makes an omission. The 24-inch rule, measured properly.
Read the standard →Edge metalDrip edge: the cheapest part of the roof
Eave over, rake under — get the lap order backwards and you have engineered a fascia rot detail into a brand new roof.
Read the standard →VentilationAttic ventilation: intake, exhaust and balance
Get the balance wrong and the ridge pulls air out of your house instead of the soffits. Shingles cook, decks sweat, eaves ice up.
Read the standard →ServiceRoof replacement in Maryland
GAF Certified installation, most roofs completed in a single day, written line-item scope before work starts.
See the service →The serviceRoof replacement
GAF architectural shingle or standing-seam metal, full tear-off, corrected ventilation, and a written line-item price first.
See the service →