Roofing Calculator

How Many Squares Is My Roof?

A gable roof 50 × 30 ft in plan, overhangs included, at a 6/12 pitch has 1,677 square feet of roof surface: 16.77 squares, or 18.45 with 10% waste. That takes 56 bundles of architectural shingles, and with underlayment, ice barrier, starter, ridge cap, drip edge and nails the materials come to $3,298.

Typical roofs, measured in plan with the overhangs, with 10% waste:

A square is 100 square feet of roof. Floor area is a poor guide: a two-story house has half the roof of a ranch with the same floor area, and a steep roof has more surface than a low one over the same footprint.

Roofing Materials List

Where each material goes on the default roof:

The full list for the default roof, and what each item covers:

What the default roof's materials cost, item by item:

Shingles are nearly three-quarters of the bill. The edge materials change little with pitch, so on a steeper roof the shingle share grows.

How to Calculate Roofing Materials

Find the roof area from the plan and the pitch, add waste, and divide by what a bundle covers:

Then measure the edges, which set the starter, drip edge, ice barrier and ridge cap:

1. 1Eaves: the two long sides of a gable, 2 × 50 = 100 ft.
2. 2Rakes: four sloped edges, each half the width times the slope factor: 4 × 15 × 1.118 = 67.1 ft.
3. 3Ridge: the length of the roof on a gable, 50 ft; on a hip roof, length minus width plus the four hips.
4. 4Starter and drip edge run along eaves and rakes, 167 ft here; ridge cap along ridges and hips.

Measure the roof in plan from the outside of the overhangs, or the house walls plus the eave and rake overhangs. If you don't know the pitch, find it from the rise and run. Roof area for a 1,500 sq ft plan at common pitches:

How to Measure a Roof Without Climbing It

You need the roof's plan, the outline you would see from straight above, and its pitch. Every plane of the same pitch has the same slope factor, so the plan area times that factor is the roof area no matter how many planes there are.

• From the ground: measure the house walls with a long tape, then add the eave overhang on both sides and the rake overhang on both ends. A 28 × 44 ft house with 12 in overhangs is 30 × 46 ft in plan.
• From the plans: the roof plan gives the outline and the pitch. Use the dimensions to the fascia, not to the walls.
• From an aerial photo or map: the outline you trace is the plan area. Multiply it by the slope factor; the image alone can't show how steep the roof is.
• Porches and additions: measure each roof of a different pitch on its own and add the areas.

Measured from the walls alone, the 28 × 44 ft house above would come up 148 square feet of plan short, about 1.6 squares at 5/12.

Bundles of Shingles per Square

From each maker's data. The calculator counts 33.33 square feet a bundle. Products rated at 98.4 square feet for three bundles cover 1.6% less, about one bundle in 60, so raise the waste a point or two for them. Designer shingles weigh more as well: CertainTeed's Presidential Shake is 350 lb a square against 211 to 225 lb for Landmark. For designer shingles and weights, use our shingle calculator.

Underlayment and Ice Barrier

Underlayment covers the whole roof. A synthetic roll covers about 1,000 square feet, a #15 felt roll 432 and a #30 roll 216, so the default roof takes 2 synthetic rolls or 9 rolls of #30 felt. IKO suggests about 10% extra for laps and waste, which the calculator adds with the shingle waste.

Below 4/12, shingles need two layers of underlayment or a self-adhered membrane (IRC R905.2.2). Owens Corning laps each course over the one below by half its width plus 2 inches, so two layers of 48 in synthetic take 2.18 times the roof area. The calculator switches to two layers by itself under 4/12 and flags pitches below 2/12, where shingles may not go at all.

How Much Ice and Water Barrier

Where the code's climate table shows a history of ice dams, the barrier runs from the eave to at least 24 inches inside the exterior wall (IRC R905.1.2). Rolls are 3 ft wide, so count the courses from the overhang and the pitch:

Owens Corning notes that most roofs need two courses. On the default roof, two courses raise the ice barrier from 2 rolls to 3 and the materials from $3,298 to $3,427. On roofs of 8/12 and steeper, the barrier must also reach at least 36 inches up the slope.

Starter, Ridge Cap, Drip Edge and Nails

Starter and ridge cap are sold by the linear foot of edge they cover, and it varies by product:

• Starter goes along every eave and rake; Owens Corning overhangs it 1/4 to 3/4 inch past the drip edge.
• Drip edge is required at eaves and rakes of shingle roofs (IRC R905.2.8.5). Pieces lap at least 2 inches and are nailed at most 12 inches apart; underlayment goes over the drip edge at the eaves and under it at the rakes.
• Nails: at least 4 per strip shingle (IRC R905.2.6), and 6 with 4 spots of roofing cement above 21/12 by Owens Corning and CertainTeed. The default roof takes 4,723 nails, inside one box of 7,200.

Gable vs. Hip Roof

A hip roof over the same 50 × 30 ft plan at 6/12 has the same 1,677 square feet and the same 56 bundles. The edges change: 160 ft of eaves instead of 100 and no rakes, a 20 ft ridge plus four hips of 22.5 ft each. That takes 3 rolls of ice barrier and 5 bundles of hip and ridge cap, and the materials rise to $3,616. Size the hip and jack rafters for the same roof.

Roofs with valleys, dormers or several pitches are best split into rectangles of one pitch each. Add their areas, and lean toward 15% waste for the extra cutting.

Ordering and Delivery

Shingles are sold by the whole bundle, and the calculator rounds every item up. They are heavy: CertainTeed lists its Landmark architectural shingle at 211 to 225 lb a square, so the 56 bundles for the default roof weigh about 3,900 to 4,200 lb. Check your vehicle's payload before you haul them yourself.

Old roofing comes off by the same squares. The code allows one new layer over one old layer of shingles if the old roof is sound, but not over two or more layers, a water-soaked roof, or slate and tile (IRC 2021 R908.3.1.1, 2024 R908.4), and IKO notes that some shingle warranties require the old layer to come off.

Common Mistakes

• Floor area instead of roof area. The 28 × 44 ft ranch above has 1,232 sq ft of floor, but its roof, 30 × 46 ft in plan at 5/12, is 1,495 sq ft.
• Three bundles for every square. True for most architectural and 3-tab shingles, not for designer lines that take four to six.
• One layer on a low roof. Below 4/12, the underlayment doubles.
• Starter on the eaves only. IKO counts the total length of all eave and rake edges; on the default roof that is 167 ft, not 100.
• Ridge cap without the hips. On a hip roof it covers 110 ft of hips and ridge, against 50 ft of ridge on the gable.

What Roofing Materials Cost

The calculator starts with $42.97 a bundle for architectural shingles and $38.97 for 3-tab, $119 for a 1,000 sq ft synthetic roll, $129 for a 200 sq ft roll of ice barrier, $54.97 for a bundle of starter, $62.97 for a bundle of ridge cap, $6.48 for a 10 ft drip edge and $49.98 for a box of 7,200 coil nails. Click any price to change it.

With 3-tab shingles and #30 felt, the default roof comes to $3,151 in materials. A standing seam or ribbed metal roof is figured differently, in panels cut to length: count the metal roof panels. For labor, tear-off and disposal, price the full replacement.