Roofing

The layers

From the deck up, a Wisconsin shingle roof has ¹:

• Underlayment over the whole deck: #15 asphalt felt or an equivalent type I material (synthetic underlayments meeting ASTM D 4869 or D 226 qualify).
• Ice dam protection at the eaves (below).
• Shingles, installed per the manufacturer, with at least 4 fasteners per strip shingle (2 for interlocking), at least 2 inches of head lap, corrosion-resistant nails, and a self-seal strip that bonds even at 32°F. Each bundle must be labeled with the ASTM standard it meets.
• Flashing at chimneys, valleys, and every roof penetration.

Roofs must be built to drain ¹, and a drip edge at the eaves and rakes carries water past the fascia.

Ice dams

An ice dam forms when snow on the upper roof melts, the water runs down to the eave, which hangs out past the warm wall and stays cold, and refreezes into a ridge of ice. More meltwater ponds behind the dam and backs up under the shingles, which only shed water running downhill.

So Wisconsin requires ice dam protection on shingled or shake roofs over heated space or an attached garage with a slope of 4 in 12 or less: sheet metal or a membrane labeled to ASTM D1970 (ice and water shield), running at least 30 inches up the slope from the roof edge and at least 12 inches past the inside face of the exterior wall, whichever reaches farther ¹. On a wide overhang, the 12-inches-past-the-wall rule governs: a 24-inch overhang plus a 6-inch wall on a 4:12 roof needs about 45 inches of membrane measured up the slope, so two courses of a 36-inch roll.

Many roofers run membrane in the valleys and around chimneys and skylights too, on any slope.

Stopping ice dams at the source

The membrane keeps a dam from leaking; it doesn’t stop the dam. Ice dams come from heat leaking into the attic, through gaps around light fixtures, chimneys, plumbing stacks, top plates, and attic hatches, and through thin insulation. Wisconsin’s energy code attacks both ²:

• Air sealing: attic hatches, recessed lights, plumbing and wiring penetrations, and top plates sealed (the next lessons).
• Insulation: R-49 ceilings, with baffles so it doesn’t block the eave vents.
• Ventilation: vented attics need net free vent area of at least 1/300 of the ceiling area when at least half of it is high on the roof, or 1/150 when 75 percent or more is up high, with the rest low at the eaves, so cold outside air keeps the roof deck cold ².

Valleys and chimneys

• Open valleys: at least 16 inches of No. 28 gauge corrosion-resistant metal, or 50-pound roll roofing, over #15 underlayment, sections lapped 4 inches ¹.
• Closed (woven) valleys: 20 inches of 50-pound roll roofing over underlayment, or a D1970 membrane ¹.
• Chimneys: flashed and counter-flashed at least 6 inches up; a cricket (saddle) on the uphill side of any chimney more than 30 inches wide, itself flashed and counter-flashed ¹. Step flashing goes at sidewalls, with a kickout where the roof edge meets a wall.

Reroofing

New roofing may not go over old when ¹:

• the existing roofing is water-soaked or deteriorated;
• it’s wood shake, slate, clay, cement, or asbestos-cement tile; or
• there are already two or more layers.

A reroof on a code-covered house is an alteration and must meet the current roofing rules, ice dam protection included ¹. Tearing off lets you see and replace rotted deck boards and install the membrane properly.

What to take from this

Underlay the whole deck, run ice and water membrane 30 inches up and 12 inches past the inside of the wall on roofs at 4:12 or less over heated space, nail shingles 4 per strip with 2 inches of head lap, flash valleys (16-inch metal open, 20-inch roll roofing or membrane closed) and chimneys (6 inches up, cricket over 30 inches wide), and never add a third layer. Stop ice dams at the source with air sealing, R-49 insulation, and attic ventilation.