September 26, 2025

Underlayment Options: Felt vs Synthetic Explained

What underlayment actually does, and why it matters

Underlayment is the thin, water-shedding layer between your roof deck and the visible roofing material, whether that is asphalt shingles, metal roofing, tile roofing, cedar shake roofing, slate roofing, or flat roofing materials like TPO, EPDM, or PVC. Think of it as the backup quarterback. Your shingles take the hits from UV, wind, and rain, but when wind-driven rain, ice dams, or a missed nail line let water sneak past, the underlayment buys you time by channeling moisture downward until it can drain off. It also protects the deck during roof installation and keeps resins in wood from contacting asphalt shingles.

Skipping quality underlayment, or choosing the wrong type, can turn a routine roof repair into an emergency roof repair. It impacts roof warranty coverage, roof maintenance needs, and how well your home resists ice dams, wind damage to roof edges, and UV degradation of roofing materials. On new roof installation projects, I treat underlayment as a system decision, not a commodity line item. The right choice depends on climate, roof pitch, roofing material, budget, and your tolerance for risk.

Felt vs synthetic underlayment, in plain terms

Traditional felt, sometimes called tar paper, comes in 15-pound and 30-pound varieties. It is an asphalt-saturated felt made from cellulose or fiberglass mat. Roofers like it because it is familiar, affordable, and grippy underfoot. The 30-pound version is thicker and more tear resistant than 15-pound, which matters when crews walk the roof during installation. Felt does absorb water, so prolonged exposure during a rainy install can wrinkle or degrade it. It also has fewer months of UV resistance when left uncovered, generally days to a couple of weeks at most before performance drops.

Synthetic underlayment is typically a woven or spun polypropylene or polyethylene with coatings for water resistance and UV tolerance. It is lighter per square, stronger against tearing, and usually offers better traction. Most synthetics do not absorb water and can stay exposed longer, often 60 to 180 days depending on the brand. Fastener pull-through resistance is better, which reduces blow-offs on windy days. Many synthetics are compatible with asphalt shingles and metal roofing, though you must check manufacturer instructions when pairing with high-temperature assemblies under metal panels or under solar shingles. High-temp synthetics keep their integrity around dark roofs, low-slope areas with heat buildup, and under self-adhered ice and water shield at eaves and valleys.

How do you even know which one your roof needs?

I start with pitch, climate, and roofing material. On steeper residential roofing with asphalt shingles in mild climates, upgraded felt or entry-level synthetic both work. If we are dealing with low-slope roof sections in the 2:12 to 4:12 range, a higher-grade synthetic with robust water shedding and taped seams is safer, especially where poor drainage or wind-driven rain is common. In hot regions and on roofs with dark shingles or metal roofing, heat can bake lighter materials and make them brittle. There I prefer high-temp synthetics or a hybrid approach with ice and water shield at eaves, valleys, and penetrations, and synthetic field underlayment elsewhere. Metal roofing, tile roofing, and slate roofing often benefit from high-temp underlayment because these materials retain heat. Tile and slate can outlast underlayments by decades, so I choose the longest-lasting synthetic or a breathable option, depending on deck ventilation.

Weather during construction matters too. If the job might be staged over weeks due to roof financing options or coordination with solar installers, a synthetic with documented UV exposure tolerance gives you a safety net. For quick tear-off and install projects in stable weather, a heavy felt can be perfectly serviceable and cost-effective. Finally, consider your roof aging profile and maintenance habits. If you are on a preventive roof maintenance plan with annual roof inspection services, felt might pencil out. If access is tough and you want fewer touchpoints, synthetic’s durability reduces risk.

The real cost picture, beyond material price

On paper, felt often wins the price-per-roll comparison. But roof repair cost and roof installation cost are not only about materials. Roofing labor cost, schedule risk, and callbacks drive the real bill. Synthetics are lighter, cover more per roll, and reduce tear-outs from foot traffic or gusty afternoons. Fewer tears mean fewer return trips and less chance of wet decking, which can snowball into sagging roof concerns or mold. If a storm pops up mid-install, felt can wrinkle when wet and cause uneven shingle fields that telegraph through, especially in sunlight. Wrinkles can force section redo and add hidden cost. Synthetics resist that behavior.

For homeowners comparing roof replacement cost, I usually quantify the delta like this: on an average 2,000 to 2,400 square foot home, moving from 30-pound felt to a solid synthetic might add a few hundred dollars in material but reduce a similar amount in labor handling, rework risk, and delays. When you factor average roof cost per square foot, the difference is often a few cents per square foot to upgrade, not dollars. Over a 20 to 30 year roof lifespan, that is a cost I can defend. Insurance and warranty implications also matter. Some shingle manufacturers tighten warranty language around underlayment types and installation practices. A mismatch can weaken roof warranty coverage or make storm damage roof repair claims harder. I document underlayment brand, spec sheets, and fastener patterns in the job file to sidestep those debates.

Choosing underlayment without getting burned by marketing

Labels like breathable, high-temp, and ice barrier mean different things depending on standards. High-temp synthetics should list service temperatures near 250 to 260 degrees Fahrenheit for metal roofing or dark assemblies. Breathable membranes allow limited vapor diffusion, useful in assemblies with tight insulation and limited ventilation, but they still must shed liquid water. Check for third-party standards like ASTM D226 for felt and ASTM D4869 or D8257 for synthetic underlayments, along with published UV exposure limits. If the manufacturer Roofing Contractor in Macomb County claims six months of exposure, read the footnotes. I treat those numbers as upper bounds, not a challenge.

On multi-family roofing and commercial roofing, wind uplift ratings and fastener spacing tables are key. Long eave lines and open exposures catch gusts. I specify cap nails or plastic cap staples at the spacing the manufacturer requires for the wind zone. Skimping there leads to punctures and penetrations that turn into leaks later. Around chimneys, skylight leaks, and flashing damage risks, I combine peel-and-stick ice and water shield with synthetic field underlayment. This hybrid gives robust sealing at the high-risk areas without paying premium for full coverage where it is not necessary.

DIY underlayment decisions, and where to draw the line

Homeowners with good ladder skills can replace a few blown-off shingles, but underlayment work crosses into tricky territory. Even small laps, like reversing the overlap direction at a valley, can funnel water inside. Fastener choices matter too. Smooth-shank roofing nails without caps can tear out along wind lines. Misplaced staples create thousands of tiny water highways. I have seen well-meaning DIYers lay felt on a damp morning, watch it wrinkle by noon, then trap that ripple under asphalt shingles. A month later they call for a roof leak, which is now a more expensive roof repair than if we had handled the underlayment correctly the first time.

That said, there are safe homeowner tasks. Clearing clogged gutters, trimming branches to prevent tree damage to roof edges, and scheduling seasonal roof cleaning to prevent moss and algae growth on roofs extend the life of both shingles and underlayment. If you must handle a temporary tarp after hail damage or hurricane roof damage in Florida, do the minimum to keep water out, then bring in a pro. Insurance prefers documented, code-compliant repairs. Underlayment errors during DIY can jeopardize claims and warranties.

How to prevent underlayment problems before they start

Most underlayment failures stem from one of four things: exposure beyond rated limits during installation, wrong fasteners or spacing, poor lap details at eaves and valleys, and inadequate ventilation that elevates attic temperatures. You can prevent each with a steady process. Plan your roof replacement around a stable forecast or use a synthetic with longer UV tolerance if staging is unavoidable. Use the fastener schedule on the label, not what a buddy swears by. Tape seams where the product allows it, and always shingle lap downhill. At eaves in cold climates prone to ice dams, install peel-and-stick ice barrier from the edge to at least 24 inches inside the warm wall line. Pair the assembly with soffit and ridge ventilation to keep attic temperatures closer to outdoor conditions, which reduces freeze-thaw roof damage and ice dam pressure.

If your home has complex valleys, multiple penetrations, or a mix of materials like asphalt shingles vs metal roofing on different planes, step up to a higher-spec synthetic and spend time on flashing details. The underlayment is not a substitute for metal flashing, but it forgives small misses and helps shed the incidental water that always sneaks in around fasteners. A short, documented preventive roof maintenance plan with yearly roof inspection services will catch granule loss, curling or buckling shingles, cracked shingles, and flashing damage before they expose the underlayment for too long. Small issues fixed early are the cheapest roof financing options you will ever find.

Side-by-side differences that actually matter

Criteria Felt (15/30 lb) Synthetic Tear resistance Moderate, higher for 30 lb High, resists foot traffic and wind Water absorption Can absorb water and wrinkle Minimal absorption, stays flatter UV exposure window Days to a couple of weeks 60 to 180 days typical, check label Weight per square Heavier rolls, fewer sq per roll Lighter, more coverage per roll Heat tolerance Lower, can dry out under hot roofs High-temp options for metal and dark roofs Cost Lower material cost Slightly higher, often offset by labor

When felt still makes sense

Despite all the synthetic hype, I still spec 30-pound felt on straightforward residential roofing when budget is tight and the project will be torn off and re-shingled in the same week. It offers good traction, a familiar nailing feel, and plays nicely under asphalt shingles. For small roof repair work, felt lets me patch and move quickly without opening a full synthetic roll. In regions with low UV intensity and stable weather, felt’s exposure limits are less of a concern. Just do not stretch it on complex roofs, long staging timelines, or high-heat assemblies.

Where synthetic truly earns its keep

Synthetic stands out in hot, roofing services sunny climates, on long eaves and open exposures, and anywhere scheduling is unpredictable. It is my default underlayment for metal roofing, tile roofing, and slate roofing, especially the high-temp variants that hold up under heat. On commercial roofing and industrial roofing solutions with extended phases and inspections, synthetics reduce rework. They also pair well with modern building practices like tighter envelopes and more insulation where moisture control matters. If you are adding solar shingles or a rack-mounted array, synthetics handle the extra penetrations and installer foot traffic better. In coastal zones with frequent wind events, the tear resistance helps prevent emergency roof repair calls the night before the crew returns with shingles.

Budgeting and the small print

Underlayment is a small line item in the overall roof replacement cost, which is why it is tempting to downplay it. But it influences the outcome of roof leaks, hail damage, and wind-driven rain in a big way. When you gather quotes, ask which product and fastener pattern the roofer will use, their plan for valleys, and how they will transition at rakes and eaves. If you are comparing bids for new roof installation, look at whether ice barrier is included at eaves and in dead valleys that trap water. The roofing labor cost to Roofing Contractor in Farmington Hills install better details is marginal compared to the cost of living with a chronic leak. For financing, many roof financing options do not break out underlayment specifically, but you can still negotiate spec changes. Upgrading underlayment is a smart add if you have to trim elsewhere, especially if you are not choosing top-tier shingles.

FAQs: straight answers on underlayment, warranties, and real-world choices

These are the questions homeowners ask me most once we start talking felt and synthetic, warranty fine print, and how it all plays with asphalt shingles, metal roofing, and the rest of the system.

How can I tell if a roofer is exaggerating the need for a premium synthetic?

Ask for the specific product name, UV exposure limit, and high-temp rating if applicable. Have them show where your roof conditions justify it, like low-slope sections, long staging, or high heat from dark roofing. If they cannot tie it to pitch, climate, or schedule, it may be upsell fluff. If they can point to valleys, complex penetrations, or a history of ice dams, the upgrade likely earns its keep.

Is felt underlayment a guaranteed problem under asphalt shingles?

No. Good 30-pound felt, installed dry, properly lapped, and covered quickly, performs well on many homes. Most felt failures come from prolonged exposure, wrinkles, or poor fastening. If the crew controls the schedule and weather, felt is still a viable choice that can extend roof lifespan when paired with solid ventilation.

Why do quotes vary so much for the same roof?

Different underlayments, ice barrier coverage, and fastener plans drive cost. One roofer might include full valleys in ice and water shield, a high-temp synthetic, and cap nails with tight spacing. Another may use 15-pound felt, limited ice barrier, and staples. Material gaps of a few hundred dollars, plus labor time saved or added, can swing bids by 5 to 15 percent.

Will my shingle warranty be void if I choose felt instead of synthetic?

Most major shingle manufacturers accept both felt and many synthetics, but each has a list of approved products and installation rules. Problems arise when the underlayment is outside the approved list or installed incorrectly. Ask your contractor to provide the product data sheet and confirm manufacturer acceptance in writing.

Is breathable synthetic necessary for metal roofing?

Not always. Breathable membranes help in assemblies where vapor drive is a concern and attic ventilation is limited. Many metal roofs perform well with non-breathable high-temp synthetics plus proper ventilation. The right answer depends on insulation location, deck type, and your climate. A quick attic assessment and dew point check beats guessing.

Why underlayment choice is rarely just a detail

A roof is a system, and underlayment sits at the quiet center of it. It affects how your roof behaves in storms, how forgiving the assembly is during installation, and how tolerant it is of the small mistakes that happen on any job site. Whether you pick felt for a fast, economical shingle job or invest in a high-temp synthetic for a complex, heat-prone roof, make the choice intentionally. Match it to pitch, climate, roofing material, and schedule. Pair it with sound flashing, real ventilation, and a preventive roof maintenance plan that includes annual roof inspection services. That is how you turn a roof installation into a long, calm relationship with your home instead of a series of emergency roof repair calls after every squall. When you are comparing roof repair cost or planning roof replacement, give this quiet layer the respect it deserves, and it will pay you back in fewer leaks, longer material life, and a smoother path to extending roof lifespan.

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