How to do a roofing takeoff (x/12 pitch, shingles, membrane, ridge)

On a roof plan, the roof is drawn from above. Its true area is larger: the steeper the pitch, the bigger the difference. The whole roofing takeoff starts there. Once you know the true area, the rest (shingles, underlayment, membrane, ridge) is a matter of the right lengths and waste. Here's the method, with a complete example.

What you need before you start

• Roof area in plan, overhangs included, measured on the roof plan (or on the floor plan, with the overhangs added).
• The pitch, shown on the elevations or the section as "6/12": 6" of rise for 12" of horizontal run.
• The perimeter of the roof (eaves and rakes), and the ridge length.
• Your waste factor: often around 10% for a simple roof, more for a roof with many valleys and hips.

1. The pitch factor

The true area of a roof plane is its plan area multiplied by a factor that depends only on the pitch:

This rule works for a gable roof and for a hip roof alike, as long as every plane has the same pitch. If pitches differ (e.g. a 4/12 shed roof on an 8/12 house), measure each part separately with its own pitch.

2. Asphalt shingles

Shingles are sold in bundles, and roof area is often counted in squares (one square = 100 sq. ft). Many shingles come at 3 bundles per square; some products need more. Check the wrapper of the product you've chosen.

3. Underlayment

Underlayment (synthetic or felt) covers the whole true area:

Then convert to rolls using the coverage stated by the manufacturer.

4. The edges: drip edge, starter strip, membrane

These materials follow the edges of the roof.

• Drip edge: on the perimeter, + waste.
• Starter strip (first course of shingles): on the perimeter, + waste.
• Ice and water membrane: perimeter × membrane width, + waste. The width to install depends on the drawings, the code that applies to your site and the manufacturer's instructions.

A common variation: install the membrane only along the eaves (and in valleys), and the starter strip as the manufacturer specifies. In that case, use the eave length instead of the full perimeter. Both approaches are defensible; what matters is following what your drawings and your crews actually do.

One more detail: on a gable roof, the rake (the sloped edge) is longer than its length in plan. The pitch factor applies to that length too if you want the exact figure.

5. Ridge

Ridge cap shingles are sold in bundles that cover a certain number of linear feet, which varies from product to product: divide by the coverage on the wrapper. Hips on a hip roof are often covered with the same product; add their length to the ridge.

Worked example: a gable roof

Assumptions (common values, to replace with those on your drawings):

• plan area with overhangs: 32 ft × 44 ft = 1,408 sq. ft;
• 6/12 pitch (factor 1.118);
• perimeter: 2 × (32 + 44) = 152 ft;
• ridge: 44 ft;
• 3 ft wide membrane;
• shingles at 3 bundles per square;
• waste: 10%.

With the "membrane at the eaves only" variation (2 × 44 = 88 ft): 88 × 3 = 264, × 1.10 = 290.4 sq. ft.

And for the sheathing (4x8 plywood or OSB) of the same roof: 1,574 ÷ 32 = 49.2, × 1.10 = 54.1, so 55 sheets.

Key points:

• Apply the pitch factor once, to the plan area. If you already measured the sloped area (on an elevation, for example), don't multiply it a second time.
• Round at the end, on the total bundles, not plane by plane.
• Keep the same way of counting edges from one project to the next: full perimeter or eaves only.

Common mistakes

• Taking the house footprint without the overhangs.
• Forgetting the pitch factor, or applying it twice.
• Using a single pitch for a roof that has several.
• Missing the small roofs: porch, dormers, sheds.
• Counting shingles in squares but ordering without checking the product's bundles per square.

In KOR

In KOR, you trace the roof area in plan and pick the pitch from a list (2/12 to 18/12, or any other value): the pitch factor is applied automatically. The "Shingle roofing" starter template gives the underlayment, shingle bundles, drip edge, starter strip, membrane (perimeter × width) and ridge cap from the ridge length. The "Roof sheathing" template gives the plywood or OSB sheets with the same pitch. Every rule is editable, for example to count the membrane at the eaves only, and the quantities go straight into the estimate.

Summary

1. Measure the plan area with overhangs, and note the x/12 pitch.
2. True area = plan area × √(1 + (x ÷ 12)²).
3. Shingles: true area ÷ 100 × bundles per square, + waste, rounded up.
4. Drip edge, starter strip and membrane: based on the edges to cover, + waste.
5. Ridge: length, + waste, then in bundles according to the product.