Roofing and Shingle Calculator

How Roofing Material Is Measured

Roofing is not sold by the square foot. It is sold by the roofing square, and one square equals 100 square feet of finished roof surface. A 2,400 square foot roof is 24 squares. Every supplier quote, every bundle label and every labor bid you receive will be written in squares, so the first job of any roofing estimate is converting your house dimensions into that unit.

The second job is making sure the area you convert is the actual sloped surface of the roof, not the flat shape of the building underneath it. That single mistake is responsible for most short orders on a re-roof.

Why the Building Footprint Is Not the Roof Area

If you measure a house at 40 feet by 30 feet, the footprint is 1,200 square feet. The roof covering that house is always larger, because the roof planes tilt. The steeper the roof, the more surface area is stretched over the same footprint. A 12/12 roof has more than 41 percent more surface than the box it sits on.

The fix is a pitch multiplier, sometimes called a roof slope factor. Measure the flat plan area covered by the roof, including the eave and rake overhangs, then multiply by the factor for your pitch.

Pitch Multipliers

Pitch is written as rise in inches over 12 inches of horizontal run. The multiplier is the square root of 1 plus the rise divided by 12, squared.

A Worked Example

Take a simple gable house, 40 feet long by 30 feet wide, with a 6/12 pitch and 12 inch overhangs on all four sides. The plan area including overhangs is 42 feet by 32 feet, which is 1,344 square feet.

• Plan area: 42 x 32 = 1,344 square feet
• Pitch multiplier for 6/12: 1.118
• Actual roof surface: 1,344 x 1.118 = 1,503 square feet
• Roofing squares: 1,503 divided by 100 = 15.03 squares
• Waste at 10 percent for a simple gable: 15.03 x 1.10 = 16.5 squares
• Order: 17 squares, since material is bought in whole bundles

Had the footprint been used raw, the order would have been 13.2 squares plus waste. That is close to four squares short, which on a roof deck with the old shingles already stripped is a very expensive afternoon.

Shingle Bundles

Shingles ship in bundles sized so that a whole number of bundles covers a square. For nearly all standard three tab and architectural laminate shingles, 3 bundles cover 1 square, each bundle covering roughly 33.3 square feet. Heavier designer and luxury laminates, thick slate look profiles and some premium impact rated products run 4 bundles per square at about 25 square feet each, because a full square of that weight is too heavy to handle in three packages.

The 17 square example above needs 51 bundles of a standard architectural shingle, or 68 bundles if the homeowner selects a four bundle designer profile. Always check the coverage printed on the wrapper for the exact product before ordering, and buy all of it in one lot number so the color blend matches.

Starter Strip

Starter is estimated in linear feet of eave and rake, not in squares. It is a separate product with the adhesive strip positioned near the bottom edge so the first course of shingles seals down and resists wind uplift. A bundle of manufactured starter covers roughly 100 to 120 linear feet depending on brand.

On the example roof, the eaves are two runs of 42 feet, which is 84 linear feet. The rakes each measure the 16 foot sloped run of the gable end, so four rakes at about 17.9 feet is roughly 72 linear feet. Total starter length is about 156 linear feet, so two bundles.

Ridge Cap and Hip Cap

Ridge cap is also measured in linear feet, along every ridge and every hip. Manufactured hip and ridge bundles cover about 20 to 33 linear feet each, depending on whether the profile is a high profile or a standard one. The example gable has a single 42 foot ridge, so two bundles of a 20 to 25 linear foot product covers it with a little spare.

Cutting cap from three tab shingles is still done on budget jobs, but most laminate warranties expect a matching manufactured cap, and ridge vent installations require cap over the vent anyway.

Underlayment

Underlayment is the water resistant layer between the deck and the shingles. Two common options:

• Synthetic underlayment. A standard roll is 48 inches wide by 250 feet long, covering 1,000 square feet, or about 10 squares before laps. The example roof needs 2 rolls.
• Number 15 asphalt felt. A roll is 36 inches by 144 feet, roughly 432 square feet, or about 4 squares per roll after laps. The example roof needs 4 rolls. Number 30 felt covers about 2 squares per roll.

In climates with a history of ice damming, code requires a self adhering ice barrier along the eaves, extending from the eave edge to a point at least 24 inches inside the exterior wall line measured along the slope. A standard ice barrier roll is 36 inches by 66.7 feet, which is 200 square feet or 2 squares. On low slopes between 2/12 and 4/12, asphalt shingles require a doubled underlayment application. Below 2/12, asphalt shingles are not permitted at all.

Drip Edge, Valleys and Flashing

Drip edge is metal L trim installed at eaves and rakes to shed water clear of the fascia. It comes in 10 foot 6 inch lengths and laps about 2 inches, so figure roughly one piece for every 10 feet of edge and round up. The example roof has about 156 linear feet of eave and rake, so 17 pieces.

Valley flashing is measured along each valley line. Open valleys use W profile or painted valley metal in 10 foot lengths with a 6 inch lap. Closed cut valleys use woven or cut shingles instead of exposed metal, but still want a full width self adhering membrane underneath. Every valley also needs an ice barrier strip centered on the line, typically 36 inches wide.

Do not forget step flashing where the roof meets a wall (one piece per shingle course, so roughly one piece for every 5 inches of wall length), counter flashing, pipe boots, and chimney flashing kits. These are counted piece by piece off the roof plan.

Nails

Standard practice is 4 nails per shingle in the field and 6 in high wind zones or on steep slopes. At 4 nails per shingle with 80 shingles in a square, that is about 320 nails per square, which works out to roughly 2.5 pounds per square for a 1 1/4 inch roofing nail. The 17 square example needs about 5,400 nails, or 43 pounds, so a 50 pound box covers it with margin. Six nail patterns push that to about 480 nails and 3.75 pounds per square.

Nail length has to be long enough to penetrate through the sheathing or at least 3/4 inch into it. Over a tear off with a single layer of shingles left in place, a longer nail is required.

Waste Allowance

Waste covers starter cuts, rake cuts, valley cuts, hip and ridge trimming, damaged bundles and the material used for the final short course.

• 10 percent for a straightforward gable with no valleys and few penetrations.
• 15 percent for a hip roof, or any roof with two or more valleys.
• 15 to 20 percent for cut up roofs with dormers, turrets, multiple planes and heavy penetration counts.
• Add extra on top for architectural shingles installed on a hip roof, since every hip cut throws away a large piece of laminate.

Keeping a few spare bundles for future repairs is cheap insurance, because shingle colors are discontinued and lot blends drift over time.

Tear Off Versus Overlay

An overlay, also called a re-cover or a nail over, installs new shingles directly over the existing layer. It saves the cost and mess of removal, but it hides deck rot, adds weight, shortens the life of the new shingles because the trapped heat is higher, and it voids some manufacturer warranties.

Most jurisdictions following the International Residential Code allow no more than two total layers of asphalt shingles on a roof. If there are already two layers, everything must come off before new material goes on. Removal is also required if the existing shingles are buckled, if the deck is water soaked or deteriorated, or if the existing roof is wood shake or slate. Some local amendments and high wind or seismic regions require a full tear off regardless of layer count, so confirm the requirement with the local building department before bidding a re-cover.

Dumpster Sizing for Tear Off

Debris weight is the driver, not volume. One square of stripped three tab shingles weighs roughly 250 pounds. Architectural laminate runs 350 to 450 pounds per square. Add felt, nails and any rotten decking.

• 10 yard container: about 12 to 15 squares of a single layer.
• 20 yard container: about 25 to 30 squares of a single layer.
• 30 yard container: about 40 to 45 squares of a single layer.

Double the debris estimate for a two layer tear off, and remember that most roll off contracts include a tonnage allowance with an overage charge per ton beyond it. Shingle debris is dense, so a container can hit its weight limit while still looking half empty.

Putting the Order Together

A complete order for the 40 by 30 gable example looks like this: 51 bundles of architectural shingle (17 squares), 2 rolls of synthetic underlayment, 2 bundles of starter, 2 bundles of hip and ridge cap, 17 pieces of drip edge, 2 rolls of ice barrier for the eaves in a cold climate, 1 box of 1 1/4 inch roofing nails at 50 pounds, plus pipe boots and any wall flashing. Building codes and material requirements vary by state and city, so verify slope minimums, ice barrier requirements, fastener patterns and layer limits with the local building department before ordering.