4 minutes
When most homeowners in Hingham, Plymouth, or Barnstable think about their roof, they picture shingles — the visible layer that gives a home its curb appeal. But the real unsung hero of your roofing system is hidden underneath: the underlayment.
Underlayment is the protective layer that sits between your roof deck (the wood sheathing) and your shingles. It’s your roof’s second line of defense against water, wind, and weather — and in New England’s mix of snow, rain, salt air, and temperature swings, it’s every bit as important as the shingles you choose.
1. What Exactly Is Roof Underlayment?
Think of underlayment as your roof’s waterproof jacket. It keeps moisture from getting to your wood decking if water sneaks past the shingles. It’s rolled out in sheets across the deck before shingles are installed, overlapping to create a continuous barrier.
In Massachusetts, this barrier plays a huge role in preventing damage from ice dams, wind-driven rain, and storm-related shingle loss.
2. The Three Main Types of Underlayment
1) Asphalt-Saturated Felt (Tar Paper)
The “old reliable” of underlayments has been used for decades.
Affordable and decent at repelling water.
Heavier grades (like 30-pound felt) resist tearing better in windy installs.
Downsides: can wrinkle when wet and is less durable than synthetics.
2) Synthetic Underlayment
Made from polypropylene or polyethylene.
Lightweight, tear-resistant, and stable in wet conditions.
UV-resistant, meaning it can be exposed longer before shingles go on — useful if a storm delays installation in towns like Scituate or Sandwich.
Increasingly the preferred choice for modern installs.
3) Rubberized Asphalt Underlayment
Premium waterproofing layer, often self-adhering (“peel-and-stick”).
Creates a watertight seal around nails.
Required by Massachusetts building code along eaves in many areas prone to ice dams (minimum of 24 inches inside the heated wall line — often more on the South Shore).
Used in valleys, around chimneys, and at roof-to-wall intersections.
3. Why Underlayment Is Especially Important in Massachusetts
We don’t get “gentle” weather here. A South Shore roof might face:
Nor’easters with 50+ mph gusts that lift shingles.
Ice dams from freeze–thaw cycles in towns like Marshfield and Norwell.
Coastal salt spray in Hull, Falmouth, or Chatham that corrodes fasteners and flashing.
Underlayment acts as your insurance policy for the roof deck when the outer layer is compromised — even temporarily.
4. Building Code Requirements in Massachusetts
The Massachusetts Residential Code requires an ice barrier underlayment along the eaves in areas where the average January temperature is 25°F or less (that’s most of the state). This barrier must extend from the edge up the roof at least 24 inches inside the heated wall line.
Many Cape Cod and South Shore roofers go beyond the minimum, running the barrier 3–6 feet up the slope, especially on low-pitch sections.
5. How Underlayment Helps with Ice Dam Prevention
Even the best shingles can’t stop water from backing up under them during an ice dam. Underlayment, especially peel-and-stick ice & water shield, seals around nails to keep meltwater from soaking into the decking.
Example: A homeowner in Duxbury had an ice dam leak in a dormer valley. After replacement, the roofer installed ice shield across the entire valley and up both slopes — no leaks the next winter despite similar storms.
6. Where Underlayment Is Critical (Beyond the Whole Deck)
Eaves & Overhangs — The first 3–6 feet from the edge is high-risk for ice dams.
Valleys — Where two roof planes meet, water volume is highest.
Chimneys & Skylights — Secondary protection for complex flashing zones.
Rake Edges in Wind Zones — Coastal gusts can drive rain sideways under shingles.
7. Coastal vs. Inland Considerations
Coastal (Hull, Scituate, Falmouth, Chatham): Wind-driven rain is a bigger risk — many roofers install full synthetic underlayment across the entire deck plus ice shield in all valleys and edges.
Inland (Hanover, Pembroke, Carver): Ice dam prevention is top priority — extended eave coverage and careful attic ventilation balance are key.
8. Underlayment and Warranties
Many shingle manufacturers require specific underlayment types and installation methods for warranty eligibility. Skimping here can void coverage — a costly mistake if you ever need a claim approved.
If you’re in Plymouth or Barnstable, ask your roofer to list underlayment type and coverage in writing before work starts.
9. Common Underlayment Mistakes
Not sealing laps correctly — Can allow water infiltration under high winds.
Skipping valleys — A guaranteed weak spot in Massachusetts weather.
Using low-grade felt on high-slope roofs in coastal areas — prone to tearing before shingles are installed.
10. How to Check Your Roof’s Underlayment (Without Tearing It Off)
Review your original installation contract — it should specify underlayment type.
Ask for installation photos — many contractors document each stage.
If re-roofing, request a “tear-off inspection” — your roofer can photograph old underlayment to see what’s there and what’s failed.
11. Final Takeaways for Massachusetts Homeowners
Your shingles are the visible armor, but the underlayment is the shield beneath — and in South Shore and Cape Cod climates, it’s indispensable. If you’re re-roofing, don’t let it be an afterthought. Discuss types, coverage areas, and code requirements with your contractor, and invest in materials that can stand up to salt, snow, and storms.
Amy Johnson
Amy runs the show at Associate.