Ray's Roofing Supply How to Estimate Materials

Ray's Roofing Supply How to Estimate Materials

Calculating the Shingle Quantity

Roof shingles are sold by both the bundle and by the square. A square of shingles is the quantity needed to cover 100 sq. ft. of roof. Shingles are packaged in paper or plastic wrapped bundles designed to be light enough for a person to carry. Heavier shingles require more bundles per square. Three bundles to a square is most common, which applies to most three-tab strip shingles and some lightweight laminated shingles. Heavier three-tabbed shingles and laminated shingles require four, five, six, or even seven bundles to cover a square. When shingles come three bundles to the square, there are 29 standard-sized shingles (12 in. by 36 in.) in each bundle.

Figuring out the roof area is the first step to determine how many bundles you’ll need to order. There are two ways to size up a new or freshly stripped roof: the measurement method and the sheet-count method. There’s a third method for calculating bundles when you’ll be laying new shingles over old or if the old roof is still in place.

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

Measurement method

The most accurate way to determine how many bundles of shingles you’ll need is to hop up on a roof and measure each roof plane. If all the roof planes are rectangles, all you have to do is multiply the length times the width of each plane to get the square footage and then add up the square footage of each plane.

Often the roof is too steep to walk on without safety equipment, so you need to do the estimate from the ground. In that case, measure the length of the building at the ground level and estimate any rake-edge overhangs. Then, from a ladder, use a stiff, wide-blade measuring tape to measure from the eaves edge to the ridge.

Estimating a Roof from the Ground

Sheet-count method

When the sheathing or plywood is exposed, it is possible to use the sheet-count method. It’s fast and you can usually complete it from the ground. Of course, you can use this method only on roofs sheathed with 4-ft. by 8-ft. structural panels.

Each structural panel is 32 sq. ft. and you can easily count the full panels from the ground. You can also tally them up by gauging 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 little more difficult to size, but you can usually assign them a relative size such as half or quarter of a full sheet and be close enough (see the accompanying drawing).

If the shingles you are using come three bundles to a square, calculating the number of bundles you’ll need is simple. Each bundle covers 33.3 sq. ft. of roof area—close enough to the 32 sq. ft. a sheet covers. So just order one bundle for each sheet of roof sheathing.

For other bundle counts per square, divide the number of sheets of sheathing by three and you’ll have the total number of squares needed to cover the roof. This works because three sheets of sheathing equal roughly 100 sq. ft. (one square).

This may sound like a crude measurement method, but you have to be realistic about how accurate you really need to be. The 1.3-sq.-ft. difference per sheet is a good margin to allow for waste along gable or hip edges.

Sizing up Roofs using the Sheet Count Method

Shingle-count method

This method makes it easy to size up most roofs if the old shingles haven’t been stripped off yet or if you’ll be doing a layover, that is, shingling over existing shingles.

First, measure the length of the eaves of each roof plane, either directly 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, you can get the eaves’ length by counting the number of tabs along the ridges and eaves to determine the length in feet (one tab is equal to 1 ft.).

Now, to get the length of the rakes, count the existing courses of shingles from eaves to ridge. The exposure on each course of shingles is 5 in. so you can multiply the number of courses by 5 in. and then divide by 12 to get the length of the rakes. Just be sure to check that the existing shingles are standard 12-in. by 36-in. shingles and not metric size. Multiply the length of the eaves by the length of the rake and you have the area in square feet.

Additional shingles

If you order shingles based solely on the area of the roof, you won’t have enough. You’ll need shingles for starter courses along the eaves and sometimes at rakes, and you’ll need shingles to cap hips and ridges. Cutting shingles also generates waste. In some places, including at the rakes, against walls and chimneys, and at valleys, you’ll need to cut shingles. Some of the cutoffs will be big enough to use as starter shingles or in other areas where cut shingles are needed, but smaller cutoffs will be waste.

Waste factor

The only roof that will generate no waste from cutting is that rare simple gable whose roof length is divisible by the 3-ft. length of a shingle. Other simple gable roofs will require cut shingles at the rakes. From there, the waste factor increases with every obstruction, such as a chimney, and with every hip or valley.

For a simple roof, you should generally figure 1 percent as a waste factor. On a complex roof with open valleys, add 5 percent and sometimes more. There’s no calculation you can use to determine the extra shingles you’ll need for waste. With experience estimating jobs, you’ll get a feel for how many extra shingles to order.

Waste is also generated when shingles are damaged, which is inevitable when you’re moving shingles around a steep roof slope and some will slide off. You may be able to salvage part of the damaged shingle, but don’t count on it. You can also waste shingles when you nail them improperly and have to remove them. You may drive nails too low in the exposure or fasten one off a control line.

If you have a crew that tends to be sloppy and damage shingles, you’ll probably need a couple of extra bundles per job. If your crew is conscientious, one extra bundle should be enough for most jobs.

Extra valley material

Valleys are either woven or cut; both methods require about the same number of shingles. On a woven valley, each course of shingles is extended beyond the valley crease and at least 12 in. onto the adjacent roof plane. You’ll use 2 ft. of extra shingle material for each pair of woven courses. In the case of cut valleys, the shingles from the first roof plane that’s shingled also extend onto the adjacent roof plane at least 12 in. just like woven valleys. Shingles from the next roof plane are cut just up from the valley line, and the cutoff pieces usually aren’t big enough to use elsewhere on the roof. Order two extra shingles per linear foot of valley to account for the overlaps and cutoffs. On a 16-ft. valley, you would need 32 shingles or roughly one extra bundle.

When you’re using laminated shingles to roof a house, you can save a little money by ordering cheaper three-tab shingles for the starters or better yet, use shingles left over from a previous job. Since laminated shingles have no cutouts, all you really need the starter strip to do is seal the shingles down and shed any water that passes through the joints between shingles.

Rake-edge starter shingles

You don’t need to install starters along the rake, but some roofers do it to create a neater-looking edge where shingles extend past rakes. To install, you can cut the exposure off as you would for eaves starters, or, if you are using three-tab shingles, you can just install the shingles along the rake. Don’t use full laminated shingles for rake starters; the exposure portion is too thick. Determine the number of shingles to order for starters by measuring the length of the eaves and rakes, then round up to the nearest foot and divide by three (the length of a shingle).

Hip and ridge cap shingles

Hips and ridges are covered with cap shingles, which are 1-ft. shingle squares that wrap over the hip or ridge. Like regular shingles, they overlap for a 5-in. exposure.

On roofs shingled with three-tab or no-cutout shingles, you simply cut standard shingles into caps. You can cap about 35 lin. ft. of ridge or hips with each bundle of three-tab shingles that come three bundles to the square. You can also salvage waste shingle pieces and portions of damaged shingles for use as caps.

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