Roof Underlayment Reference

Why this matters

Roof underlayment is the secondary water barrier between shingles + decking. Wrong choice OR install = leaks + damaged decking + customer claim. The roofer who specifies + installs underlayment correctly protects the customer for decades. This is the field card.

Underlayment types

Felt (asphalt-saturated paper): traditional; #15 (light), #30 (medium), #43 (heavy).

Synthetic underlayment: polypropylene OR polyester; lighter + stronger + UV-resistant.

Ice + water shield (peel + stick): bituminous self-adhesive; eaves + valleys + penetrations.

Self-adhering modified bitumen: full-roof self-adhesive; commercial + premium residential.

Felt underlayment

#15 felt:

• Lightest weight (15 lb / 100 sq ft)
• Lowest cost
• 30-year shingle install often paired

#30 felt:

• 30 lb / 100 sq ft
• Better durability
• Higher-end install

#43 felt:

• Heavy duty
• Commercial use; rare residential

Coverage: 4 squares per roll typical.

Install: lapped horizontally; 6 - 8" overlap.

Limits: degrades quickly in UV (exposed); typical 30 days exposure max before shingle install.

Synthetic underlayment

Benefits over felt:

• Lighter (2 - 3 lb / 100 sq ft vs 15 - 43 lb)
• Stronger (rip-resistant)
• UV-resistant (some 90 - 180 days exposure)
• Better walk-ability
• Won't crack in cold OR melt in heat
• Slip-resistant surface (some)

Common brands: GAF Tiger Paw, Owens Corning DECKDefender, CertainTeed Diamond Deck, IKO Stormtite.

Coverage: 10 squares per roll typical.

Code compliance: meets IRC R905 (asphalt shingle install).

Ice + water shield

Self-adhesive bituminous membrane: peel-and-stick.

Premium grades have polyethylene OR polyolefin face: walk-able + UV-resistant during install.

Common brands: GAF Storm-Guard, Grace Ice + Water Shield, IKO Stormshield, CertainTeed WinterGuard.

Required by code in cold climates (IRC R905.1.2):

• Eaves: 24" inside warm wall line
• Valleys: full coverage
• Around penetrations (chimneys, plumbing, vents)
• Skylights
• Roof transitions

Optional but recommended: entire roof in extreme cold climates.

Install temp: most require > 40°F OR specific cold-weather adhesive.

Self-adhering modified bitumen

Full-roof peel + stick:

• Better water resistance than typical underlayment
• Higher cost
• Premium homes + complex roofs
• Common brands: same as ice + water shield manufacturers

Underlayment install procedure

1. Decking inspection + repair: ensure solid + dry plywood OR OSB.

2. Snap chalk line: 32" up from eave (typical first course).

3. Install ice + water shield at eave: peel backing + adhere; press firm.

4. Install underlayment over deck:

• Bottom course first
• Lap horizontally 6 - 8" (felt) OR 4 - 6" (synthetic)
• Lap vertically 12 - 24" at ends

5. Around penetrations: ice + water shield collars; cut around.

6. Valleys: continuous ice + water shield down center.

7. Hips + ridges: standard underlayment; cap covered by ridge shingle.

8. Cap nails: plastic OR metal cap nails (1" - 1-1/4") at felt seams; per OEM.

9. UV-exposed time limit: don't leave exposed past spec OR install becomes invalid.

Cap nails vs staples

Plastic cap nails: required by IRC R905.2.7.1.

Metal cap nails: alternative; less common.

Staples (without caps): NOT code-compliant in most jurisdictions; doesn't hold underlayment in wind.

Underlayment for different shingle types

Asphalt shingle: #30 felt OR synthetic (most common).

Wood shake / shingle: felt #30 OR synthetic.

Metal roof (standing seam): synthetic OR ice + water shield (slipperier in install).

Tile (clay / concrete): heavier underlayment per OEM (Type IV felt OR specific tile underlayment).

Slate: heavy felt; sometimes copper / lead foil under critical areas.

Flat / low-slope roof (TPO, EPDM, modified bitumen): typically NO separate underlayment; membrane itself is the system.

Slope considerations

< 2/12 slope (low-slope): NOT asphalt shingle; use modified bitumen OR TPO OR EPDM.

2/12 - 4/12 slope: asphalt shingle; double-layer underlayment for extra protection.

4/12+ slope: standard single-layer underlayment.

Steep slope (> 8/12): standard install; harder to work.

Climate considerations

Cold + snow: ice + water shield at eaves; potentially more coverage.

Coastal / high wind: synthetic underlayment (better wind resistance); extra fastening.

Hot + sun: synthetic preferred (UV resistance OR felt protected by quick install).

Heavy rain region: extended ice + water shield + synthetic underlayment.

Common underlayment mistakes

Stapling underlayment without cap nails (wind blows; code violation). Insufficient overlap (water penetrates between courses). Leaving exposed past UV limit (degrades). Missing ice + water shield in cold climate eaves (ice damming damage). Wrong slope for shingle install (low slope = leak risk). Skipping valley ice + water shield. Damaging during install (walking with cleated boots). Cold weather adhesion issues (ice + water shield won't adhere < 40°F without prep).

Customer education

"Underlayment matters": shingles are the visible top; underlayment is the backup waterproofing.

Cold climate ice dams: ice + water shield protects.

Insurance often requires synthetic OR ice + water in claim repairs.

Lifetime warranty shingles OFTEN require synthetic underlayment for full warranty.

Manufacturer warranty considerations

Many shingle manufacturers: synthetic underlayment required for "lifetime" OR maximum warranty.

Mix-and-match brands: some require system-wide single-brand for warranty.

Inspector requirements: photo documentation during install.

Common warranty issues

Felt installed when synthetic required: warranty void.

Felt left exposed too long: warranty void OR replacement.

Ice + water shield missed at code-required locations: warranty + code issue.

Truck stock

Synthetic underlayment (Tiger Paw OR equivalent), ice + water shield rolls, felt #30 (lower-cost option for small jobs), cap nails (1" plastic), roofing knife, chalk line, stapler with cap option, ladder + safety gear.

When to upgrade underlayment

Reroof on aged decking: synthetic + ice + water shield protects investment.

Snow / ice climate: extra ice + water shield coverage.

Premium shingle install: match underlayment to shingle quality.

Customer wants longevity: discuss upgrade options.

Complex roof (multiple valleys, dormers): more ice + water shield.

References

• IRC R905 (Asphalt Shingle Roofing)
• IRC R905.1.2 (Ice barrier in cold climates)
• ASTM D226 (asphalt-saturated organic felt)
• Manufacturer technical data (GAF, Owens Corning, CertainTeed, IKO)
• NRCA (National Roofing Contractors Association)
• Manuall internal: Flashing + Weatherproofing, Common Building Materials