7.4 Material Takeoffs: Squares, Bundles, Waste, Underlayment, Ice Barrier, Metals & Fasteners

From Area to Bundles

1. Net squares = true roof area ÷ 100.
2. Gross squares = net squares × (1 + waste factor).
3. Bundles = gross squares × bundles per square. Always round up, because suppliers do not break bundles.

Typical waste allowances (use experience and the roof's shape):

Hip roofs waste more because each course is cut at an angle at every hip. On gables, rake cut-offs can often start the next course.

Packaging: Most standard 3-tab and laminated shingles come 3 bundles per square. Some heavyweight designer shingles come 4 or 5 bundles per square. Always read the manufacturer's stated coverage.

Example: True area 2,482 square feet on a hip roof at 15% waste. Net = 24.82 squares. Gross = 24.82 × 1.15 = 28.54 squares. Bundles = 28.54 × 3 = 85.6, so order 86 bundles.

Linear Accessories

Drip edge example: A gable roof has two 48-foot eaves and four 16.77-foot rakes. Perimeter = 96 + 67.1 = 163.1 feet. With a 10% allowance, that is 179.4 feet. At 10 feet per stick, order 18 pieces.

Ice Barrier (MRC R905.1.2)

Michigan requires an ice barrier on shingle-type coverings statewide. It extends from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. On slopes 8:12 or steeper, it must also reach at least 36 inches along the slope from the eave edge.

Coverage needed along the slope = (horizontal overhang measured to the outside face of the wall + 24 inches) × common slope factor.

Allow for the manufacturer's side laps between courses. Add valleys, and add areas around chimneys and skylights if you plan to line them. Divide the total linear feet of each course by the usable roll length, taking end laps into account.

Underlayment

• On 4:12 and steeper, one layer lapped at least 2 inches at the head and 4 inches at the ends, with end laps offset 6 feet (Table R905.1.1(2)).
• From 2:12 to less than 4:12, two layers: a 19-inch starter strip, then 36-inch sheets lapped 19 inches. This roughly doubles the underlayment quantity for those areas. Self-adhered ASTM D1970 underlayment over the whole deck is an alternative (R905.1.1 exception).
• Deduct the area covered by ice barrier if the underlayment will not run under it, and add laps. Synthetic roll coverage varies by product, so use the manufacturer's net coverage.

Decking

Include a unit price for replacing damaged sheathing, such as per 4-by-8 sheet installed, or an allowance. On older homes with board decks, budget for overlaying spaced boards if needed. Asphalt shingles require a solidly sheathed deck (R905.2.1).

Nails

The code (R905.2.5 and R905.2.6) requires:

• corrosion-resistant roofing nails at least 12 gauge (0.105 inch) shank with a 3/8-inch head (ASTM F1667);
• penetration at least 3/4 inch into the sheathing, or through sheathing thinner than 3/4 inch;
• at least 4 nails per strip shingle (2 per individual shingle), or more if the manufacturer requires.

A square of 12-by-36-inch 3-tab strips at 5-inch exposure uses about 80 strips, so 4 nails each is 320 nails per square. A 6-nail pattern is 480. Metric-size laminated shingles cover more per piece (about 64 to 66 per square), so count by the manufacturer's pieces per square. Reroofing over an existing layer, where allowed, needs longer nails to reach full penetration.

Putting It Together: Sample Order

A gable roof has a true area of 1,610 square feet, 96 feet of eaves, 67 feet of rakes, 48 feet of ridge, 12-inch overhangs, and a 6:12 slope.