The parts of a roof you never see decide much of how long it lasts. Underlayment is the backup waterproof layer between the deck and the covering. Ventilation is the air path that keeps the attic dry and closer to outdoor temperature. A replacement is the best chance to get both right, because the deck is exposed and every layer is being renewed at once.
Request a free quote using the quote button on this page.
What underlayment actually does
Think of the roof as two lines of defence. The covering, whether shingles, metal or tile, sheds almost all rain by overlap and slope. Underlayment handles the rest: rain driven sideways by wind, water that backs up at an eave, and small leaks that develop as a covering ages.
It also protects the deck during the job itself, keeping the home dry between tear off and covering, and it gives the covering a clean, even surface to sit on. Without it, any water past the covering reaches bare wood, where it can stain, swell and rot decking over time.
Chapter 9 of the International Residential Code, published by the International Code Council, sets requirements for underlayment, flashing and roof coverings as part of a complete roof assembly in its roof assemblies chapter. That is why a quote should name the underlayment, not treat it as an unnamed extra.
The main underlayment types
Traditional felt is asphalt saturated sheet, lapped in courses. It has a long record, but it can wrinkle when wet and tear more easily underfoot during fitting.
Synthetic underlayment is a woven or spun polymer sheet. It is light for its coverage, resists tearing, and usually lies flatter. It must still be the type approved for the covering above it, and lapping and fastening should follow its maker instructions.
Self adhering membrane, often called ice and water membrane, is a rubberised sheet that sticks directly to the deck and seals around nails. It is used selectively, in the places where water is most likely to linger, rather than across a whole roof in most homes. A sound specification often combines types: membrane at eaves, valleys and penetrations, and felt or synthetic sheet across the open field of the roof.
Where placement matters most
Eaves are first in cold country. When snow melts higher on the roof and refreezes at the colder edge, water can back up under shingles. University of Minnesota Extension explains that an ice dam is a ridge of ice at the roof edge that stops melting snow draining away, and that backed up water can leak into the home and damage walls, ceilings and insulation in its guide to ice dams. Membrane carried up the roof from the eave, past the inside wall line, is the standard defence at that edge.
Valleys carry the water of two roof faces in one channel, so they deserve membrane beneath the valley lining. Penetrations, including chimneys, plumbing vents and skylights, interrupt the covering, so membrane and flashing work together there. Low slope sections drain slowly and also merit stronger protection than the open, steeper field.
How attic ventilation works
A vented attic is meant to stay close to outdoor temperature and to dry out any moisture that reaches it. Air enters low, through soffit vents at the eaves, travels up along the underside of the deck, and leaves high, through a ridge vent or roof vents near the top. Baffles at the eaves keep insulation from blocking that entry path.
The U.S. Department of Energy consumer guide to durable attics states that keeping attics dry and well ventilated helps ensure durability and avoid moisture problems, and that pairing soffit vents with a ridge vent increases effectiveness in its durable attics guide. The same guide warns that venting bathroom or kitchen exhaust into the attic is a major moisture source and a bad practice.
Balance is the key word. Intake and exhaust should be matched, and vents of different types should not be mixed in a way that short circuits the air path, for example by combining a ridge vent with nearby gable fans that pull air from the ridge instead of from the soffits.
Why moisture builds up without it
House air carries moisture from cooking, bathing and laundry. Where ceiling gaps let that air into the attic, it meets a cold deck in winter and condenses. The Department of Energy strategy guideline for cold climate retrofits notes that proper attic ventilation is important for moving excess moisture out of the building and maintaining the durability of the roof in its retrofit strategy guideline.
Over seasons, repeated condensation can darken decking, feed surface mould in the attic and reduce insulation performance. In summer, a poorly vented attic also holds heat against the underside of the roof, which adds heat stress to the covering above. Air sealing the ceiling plane, venting exhaust fans outdoors and keeping the soffit path open are partner measures to ventilation itself. One measure alone rarely fixes a wet attic.
What to ask for in a replacement quote
Because both systems are hidden once the covering is on, get them specified before work starts. The quote should state the underlayment type for the field of the roof, the membrane locations by name, the flashing plan, and the ventilation plan with intake and exhaust described separately.
Ask what happens if the crew finds blocked soffits, damaged baffles or a fan venting into the attic. Those small corrections are cheapest during a replacement, when access is open. Our asphalt shingle roof replacement page shows where these layers sit in a full system, and our roof tear off and re-deck page covers the deck inspection that comes first.
How the referral works
Request a free quote using the quote button on this page. Share your city and roof details, and get connected to a local provider who offers full roof replacement. You then deal direct with that provider for the quote, schedule and work.
FAQ
Is underlayment really needed if the shingles shed water?
Yes. Shingles are a water shedding covering, not a sealed waterproof layer. Wind driven rain, backed up water and ageing laps can all let small amounts of water past the surface. Underlayment is the backup layer that directs that water down to the eaves and protects the deck while the covering does the main work above it.
Where does ice and water membrane matter most?
It matters most where water lingers or backs up: at eaves in cold climates, in valleys, around chimneys and vents, and along low slope sections. This self sealing membrane sticks to the deck and seals around fasteners. Your provider should state on the quote exactly which areas receive it, rather than listing it as a general extra without locations.
How much ventilation does an attic need?
The right amount depends on attic size, vent type and layout, and local code rules. What matters in practice is balance: enough intake at the soffits and enough exhaust at or near the ridge, with a clear air path between them. A provider should measure the attic, check existing vents and explain the intake and exhaust plan in the quote.
Can adding more exhaust vents fix a moisture problem?
Not on its own. Exhaust without matching intake can pull air from the living space instead of from the soffits, which can bring more moist house air into the attic. Blocked soffits, bathroom fans ending in the attic and ceiling air leaks also need attention. Ventilation works as a system, so the whole air path should be checked together.
Areas
Attic conditions vary by climate, so the right membrane and vent plan in Phoenix, AZ will not read the same as one in Chicago, IL. Published area pages include Los Angeles, CA, Brooklyn, NY and Houston, TX; the rest are listed on the areas hub.
Sources
- International Code Council, International Residential Code, Chapter 9, Roof Assemblies: https://codes.iccsafe.org/content/HARMDRC2018P1/chapter-9-roof-assemblies
- University of Minnesota Extension, Dealing with and preventing ice dams: https://extension.umn.edu/garden-and-home/home-maintenance/caring-for-a-home/dealing-with-and-preventing-ice-dams
- U.S. Department of Energy, Consumer Guide to Durable Attics, via govinfo.gov: https://www.govinfo.gov/content/pkg/GOVPUB-E-PURL-gpo158987/pdf/GOVPUB-E-PURL-gpo158987.pdf
- U.S. Department of Energy, Strategy Guideline: Energy Retrofits for Low-Rise Multifamily Buildings in Cold Climates: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/strategy_guide_retrofit_multifamily_cold.pdf
Common questions
Is underlayment really needed if the shingles shed water?
Yes. Shingles are a water shedding covering, not a sealed waterproof layer. Wind driven rain, backed up water and ageing laps can all let small amounts of water past the surface. Underlayment is the backup layer that directs that water down to the eaves and protects the deck while the covering does the main work above it.
Where does ice and water membrane matter most?
It matters most where water lingers or backs up: at eaves in cold climates, in valleys, around chimneys and vents, and along low slope sections. This self sealing membrane sticks to the deck and seals around fasteners. Your provider should state on the quote exactly which areas receive it, rather than listing it as a general extra without locations.
How much ventilation does an attic need?
The right amount depends on attic size, vent type and layout, and local code rules. What matters in practice is balance: enough intake at the soffits and enough exhaust at or near the ridge, with a clear air path between them. A provider should measure the attic, check existing vents and explain the intake and exhaust plan in the quote.
Can adding more exhaust vents fix a moisture problem?
Not on its own. Exhaust without matching intake can pull air from the living space instead of from the soffits, which can bring more moist house air into the attic. Blocked soffits, bathroom fans ending in the attic and ceiling air leaks also need attention. Ventilation works as a system, so the whole air path should be checked together.