8.4 Underlayment: Standards, Application by Slope & Attachment (R905.1.1)

What Underlayment Does

Underlayment is a second line of defense under a water-shedding roof covering. It sheds water that gets past the shingles, protects the deck during construction, and helps meet the fire and wind ratings of the covering system.

Standards (Table R905.1.1(1))

Underlayments required to meet ASTM D226, D1970, D4869, or D6757 must bear a label showing compliance and, if applicable, the type (R905.1.1). ASTM D226 Type I is the traditional "No. 15" felt, and Type II is "No. 30." Synthetic underlayments are not ASTM D226 felts. They are accepted based on the manufacturer's evaluation report and instructions, so keep that documentation and follow its fastening and exposure limits.

Application for Asphalt Shingles (Table R905.1.1(2))

Slopes From 2:12 Up to 4:12: Double Coverage

• Start with a 19-inch-wide strip of underlayment parallel to the eave.
• Lay 36-inch sheets starting at the eave, each overlapping the one below by 19 inches, so every point on the deck has two layers.
• Distortions in the underlayment must not interfere with the shingles' ability to seal.

With 36-inch sheets lapped 19 inches, each new course advances 17 inches up the roof.

Slopes of 4:12 and Steeper: Single Layer

• One layer applied shingle fashion, parallel to and starting from the eave.
• Head laps of 2 inches. End laps of 4 inches, with end laps offset by 6 feet from course to course.
• Distortions must not keep the shingles from sealing.

At 140 mph and above, all laps must be at least 4 inches. This does not apply to typical Michigan sites.

Alternatives (R905.1.1 exceptions)

1. Self-adhering polymer-modified bitumen underlayment (ASTM D1970) over the whole deck, installed per both the underlayment and roof covering manufacturers' instructions for the deck, ventilation, and climate.
2. A 4-inch strip of D1970 membrane over all joints in the roof deck, with an approved underlayment for sub-140 mph conditions applied over the entire roof.

Full self-adhered coverage is common on low-slope shingle areas (2:12 to 4:12) and in areas prone to ice damming. Where a roof is fully covered with a vapor-impermeable membrane, check the manufacturer's requirements for attic ventilation or the unvented assembly rules.

Attachment (Table R905.1.1(3))

For typical Michigan sites, the code's minimum is simply "fastened sufficiently to hold in place." Synthetic manufacturers usually require cap fasteners at specified spacing and limit UV exposure time. Their instructions govern warranty coverage, and R904.1 and R905.1 require following the manufacturer's instructions. Bare staples can tear through lightweight felts and synthetics in wind, which is why most instructions call for caps.

Field Practice

1. Sweep the deck and pull protruding fasteners before starting.
2. Install drip edge at the eaves first, then the ice barrier and underlayment over it. Along the rakes, the drip edge goes over the underlayment (R905.2.8.5).
3. Snap lines to keep courses straight and laps consistent.
4. Lap into valleys and over hips and ridges as the manufacturer directs. The valley lining goes on first (Section 9.3).
5. Turn up at walls a few inches under the wall flashing and water-resistive barrier so water drains onto the roof.
6. Felt wrinkles: organic felt can wrinkle when wet. Distortions that keep shingles from sealing are not allowed, so let wet felt dry and flatten or replace it before shingling.
7. Exposure: cover the underlayment with the roof covering within the manufacturer's allowed exposure time.
8. Walkability: frost, dew, and dust make underlayment slippery. Fall protection rules still apply (Chapter 6).

Worked Example

A porch roof is 2.5:12, with 24 feet of eave and 10 feet of slope length. Using 36-inch felt: after the 19-inch starter strip, full sheets advance 17 inches each. Ten feet is 120 inches. The first full sheet covers the starter and the lowest 36 inches, and each additional sheet adds 17 inches of coverage. Five full sheets reach 36 + 4 × 17 = 104 inches, and a sixth reaches 121 inches. The job needs the starter strip plus six full courses across the 24-foot eave, and you would order extra for laps. A cleaner choice at this slope is full D1970 coverage, which the R905.1.1 exception allows.