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
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 is the layer that 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 that is open when an afternoon thunderstorm arrives.
There are three products in play, and they are not interchangeable.
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.
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 that both Maryland and Virginia build 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.
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 or Virginia 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 or Virginia code require an ice barrier at the eaves?
Both states build from the same 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.
Where to go next
Underlayment is one component. The two most common reasons a correctly underlaid roof still fails early are what is under it and what is above the insulation.
Attic 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.
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Shingles are only as strong as what holds their nails. What rot looks like, and why the per-sheet price belongs in the contract.
Read the standard →ServiceRoof replacement in MD and VA
GAF Factory-Certified installation, most roofs completed in a single day, written line-item scope before work starts.
See the service →ToolInstant satellite roof quote
Measures your actual roof from above and prices it at current DMV rates. No salesperson required.
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