One of the biggest challenges for pros and DIYers alike is accurately estimating materials for a construction project–especially for roofs as they are hard to access for taking measurements.

It doesn’t have to be so tricky if you follow these reliable methods for determining the area of a roof and tips for estimating the number of extra shingles you’ll need for waste, overlaps, and starter shingles.

Remember, it is critical to take your time and double-check your numbers when calculating the quantity of shingles, underlayment, flashings, and any other materials needed for your roofing project. Accurate estimates mean less time wasted while waiting for material deliveries during the project.

How Shingle Quantities are Measured

Roof shingles are sold by the bundle and by the square. The quantity needed to cover 100 sq. ft. of roof is a square of shingles. They are packaged in plastic or paper wrapped bundles designed to be light enough for a person to carry, so heavier shingles require more bundles per square. Typically, there are three bundles to a square; this applies to most three-tab strip shingles and some lightweight laminated shingles. Heavier three-tabbed shingles and laminated shingles may require 4 or 5 bundles to cover a square. There are 29 standard-sized shingles (12 in. by 36 in.) in each bundle when they come three bundles to the square

The first step in determining how many bundles you’ll need to order is to calculate your roof area.  You can measure a new or freshly stripped roof in two ways: the measurement method and the sheet-count method. A third method is for calculating bundles when your old roof is still in place and you’ll be laying new shingles over the existing roof.

When you have a bundle or square count for the main roof area, you will then add additional shingles to account for waste, starter shingles, and extra shingles for hip and ridge caps.

1. Measurement method

The most accurate way to calculate how many bundles of shingles you’ll need is to get up on the roof and measure each roof plane. If all the roof planes are rectangles, you simply need to multiply the length times the width of each plane to get the square footage; then you will add up the square footage of each plane. Many times, the roof may be too steep to walk on without safety equipment, so you will have to do the estimate from the ground. If this is the case, measure the length of the building at the ground level and estimate any rake-edge overhangs. Next, from a ladder, use a stiff, wide blade measuring tape to measure from the edge of the eaves to the ridge.

2. Sheet-count method

If the sheathing is still exposed you can use the sheet-count method, which is sometimes preferred more than the measurement method. This method is fast, and you can usually complete it from the ground. The caveat is you can use this method only on roofs sheathed with 4x8ft. structural panels. Each of these panels is 32 sq. ft., and you can easily count the full panels from the ground. Another way to tally them up is by estimating the relative size of ripped and crosscut sheets along the edges of the roof to the size of a full sheet. Diagonally cut sheets along hips and valleys are a bit more of a challenge to size, but typically you can assign them a relative size, like half or quarter sheet, and it will be close enough

Calculating the number of bundles, you need is simple if you are using shingles that come three bundles to a square. Each bundle covers 33.3 sq. ft. of roof area—which is close enough to the 32 sq. ft. a sheet covers, which means you can order one bundle for each sheet of roof sheathing.

If you are working with other bundle counts per square, simply divide the number of sheets of sheathing by three and you’ll have the total number of squares needed to cover the roof – this is because three sheets of sheathing equal roughly 100 sq. ft. (one square).

3. Shingle-count method

This method makes it easy to measure when the old shingles haven’t been stripped off yet or if you’ll be doing a layover (meaning shingling over existing shingles).

To start, measure the length of the eaves of each roof plane, either directly from on top of the roof or from the ground by measuring the length of the house and adding in the width of the rake overhangs, if any. Alternately, if the existing shingles are standard three-tab, determine the eaves’ length by counting the number of tabs along the ridges and eaves to determine the length in feet (note that one tab is equal to 1 ft.).

Count the existing courses of shingles from eaves to the ridge to get the length of the rakes. The exposure on each course of shingles is five inches, so multiply the number of courses by five inches and then divide by 12 to get the length of the rakes. Make sure you check that the existing shingles are not metric size and are the standard 12-in. by 36-in. shingles. To get the area in square feet just multiply the length of the eaves by the length of the rake and

Calculating the Area of a Complex Roof

Areas of complex roofs with many hips and valleys take the most time to calculate. To get started, make a rough sketch of the roof. To simplify things, break down the sketch into rectangles and right triangles and then take as many measurements of the roof as you can to match the sides of the rectangles and triangles on the sketch.

Square lines off eaves edges or ridges can be determined by using visual cues from the existing roof. These will help you measure the lengths of the sides of the rectangles and triangles. For example, the cutout slots on shingled roofs run perpendicular to the eaves, and nail rows in sheathing are very close to square as well. While it can be challenging sometimes to get accurate measurements, don’t be too concerned and round lengths to the nearest 6 in.

When your sketch is filled in with measurements, determine the size of the roof area. The area of a rectangle is length multiplied by width, while the area of a right triangle is the length of the two sides that meet at the 90-degree corner multiplied together and divided by two.

Add up the square footages of all the rectangles and triangles; this will give you the total square footage for the roof.

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