Underlayment, Drip Edge, Starter and Ridge Cap: How Much to Order

Shingles are the visible part of a roof order. Underlayment, drip edge, starter strip and ridge caps are the parts that get forgotten or guessed. Each is counted differently: underlayment by area after laps, the others by length. This guide shows the arithmetic for each, using the same example roof throughout.

The example is the calculator’s starting roof: a gable roof over a 40 × 30 ft building with 1 ft overhangs and a 6:12 pitch. It has 1,503 sq ft of surface, a 42 ft ridge, 84 ft of eaves and 71.6 ft of rakes.

Underlayment: use the coverage, not the roll size

Underlayment is rolled out in overlapping courses, so a roll covers less roof than it contains. Data sheets state both figures. TAMKO’s Synthetic Guard Plus, for example, is a 42 in × 286 ft roll that contains 1,001 sq ft and is listed with a roll coverage of 905.67 sq ft, about 9.5% less. The coverage figure is the one to divide by.

How much is lost depends on the laps the installation calls for. The model residential code, as adopted in California, describes a single layer for asphalt shingles at 4:12 and steeper, applied shingle fashion and lapped 2 inches. From 2:12 up to 4:12 it requires two layers, starting with a half-width strip at the eave, so the same roof takes about twice the underlayment. End laps are 4 inches in the two-layer method.

In the materials calculator, enter your roll’s coverage in “Underlayment / roll”. For a low-slope section that needs two layers, enter half the coverage.

Ice barrier

In areas with a history of ice forming along the eaves, the same code requires an ice barrier: two cemented layers of underlayment or a self-adhering membrane, from the eave edge to a point at least 24 inches inside the exterior wall line. The 24 inches is measured level, so the strip up the slope is wider than that. On the example roof the barrier has to cover the 1 ft overhang plus 2 ft inside the wall, 3 ft of run, which is 3.35 ft measured along the 6:12 slope. On roofs of 8:12 and steeper the code text gives 36 inches measured along the slope. Whether the requirement applies is decided by your local building department. Our calculators do not count ice barrier.

Drip edge: roof edge length and a 2-inch lap

Drip edge is metal flashing along the roof edges. The code text requires it at eaves and rake edges of shingle roofs, extending at least ¼ inch below the sheathing and at least 2 inches back onto the deck. The Town of Firestone, Colorado, summarizes the lap and fastening rules of the 2018 code for its inspections: adjacent pieces overlap at least 2 inches and are fastened no more than 12 inches apart.

It is sold in straight pieces. Our calculator assumes 10 ft pieces; if your supplier stocks another length, scale the count. With a 2-inch lap each 10 ft piece adds 9.83 ft. The count is edge length, plus allowance, divided by that figure:

Pieces = round up((eaves + rakes) × (1 + waste) ÷ 9.83 ft)

The example roof has 155.6 ft of edge. With 10% allowance that is 171.1 ft, or 18 pieces. A hip roof over the same building has only level eaves, 148.0 ft in total, and needs 17 pieces.

Eave and rake profiles can differ. If your supplier sells them as separate items, count them separately: 84 ft of eave and 71.6 ft of rake on the example.

Starter strip: lineal feet of edge

The starter course goes under the first row of shingles at the eave, and along the rakes where the shingle manufacturer’s instructions call for it. It is packaged by length. Owens Corning’s Starter Strip Plus data sheet, for example, lists 105 lineal feet per bundle.

For eaves only, the example needs 84 ft: 1 bundle at that coverage, with 21 ft to spare. Eaves and rakes together are 155.6 ft, 2 bundles. Our calculators do not include starter strip; add it from the edge lengths.

Ridge and hip caps: lineal feet over coverage per bundle

Caps cover the ridge and the hips. The quantity is the combined length, plus allowance, divided by the lineal coverage of a bundle. That coverage depends on the cap’s exposure and differs between products; Fine Homebuilding’s estimating guide gives about 35 lineal feet per bundle as usual for three-tab shingles cut into caps and for companion cap products. The calculator starts at a more cautious 25 ft so that you replace it with your product’s figure.

The hip roof needs about twice the cap bundles of the gable roof over the same building, or more. If you switch a section from gable to hip in the calculator, this line is where the change shows.

The whole accessory list for the example roof

What this list still leaves out

Fasteners, step and counter flashing at walls and chimneys, valley lining, pipe boots, vents, sealant and any deck repair are specified from the roof itself, not from its area. Ask your supplier or installer to go through them with the measurements in hand; the quote checklist lists what to bring.