How Many Shingles in a Bundle

Most roofs in America are covered with asphalt or fiberglass shingles. The two look extremely similar and in fact are, with the only significant difference being that fiberglass shingles use a fiberglass mat as the base, while more traditional asphalt shingles use a paper base. But that one difference makes the asphalt shingles more water resistant and fire resistant, with the ability to work well in hotter climates. However, fiberglass shingles won’t last as long in colder climates.

But it also makes them somewhat more expensive. If fiberglass shingles are installed on a home, the material costs will go up by about 20 to 30 percent. For that extra money, the homeowner receives a shingle that will last longer. Asphalt shingles are generally considered to be good for about 20 years, while fiberglass ones will last from 25 to as much as 50 years, with 30 years being typical.

Looking at the most common types of roofing available, we find:

  • Composition shingles last 12 – 20 years
  • Asphalt shingles last 15 – 30 years
  • Wood shingles last 20 – 25 years
  • Fiberglass shingles last 25 – 50 years
  • Rubber roofs last 30 – 50 years
  • Metal roofs last 50 – 75 years 

Even so, the biggest factor in how long any roof lasts is the weather that it is subject to. Any and all forms of weather have an impact on the life of a roof, including the summer sun. Hail is a bid destroyer of roofs, allowing many homeowners to get a new roof paid for by their homeowner’s insurance. But the biggest factor in roof life isn’t the weather or even the shingles; it’s ventilation in the attic.

Whatever shingles are installed on a home, they are installed over roofing felt (or paper) and plywood or OSB sheathing. The shingles protect the home from rainwater, by providing a path for the rain to flow off the roof of the home, while preventing it from getting through the shingles to the sheathing inside. But that doesn’t mean that attics are totally without moist. Humidity can be trapped in there, if there isn’t adequate ventilation. This will encourage mold and fungi to grow on the bottom side of the sheathing, weakening it. Allowed to go far enough and the roof becomes wavy, with the flexing causing cracking in the shingles.

One of the big questions that homeowners face is knowing when an asphalt roof needs to be replaced. There are several signs which can indicate this:

  • Water stains on the ceiling of the home
  • Missing or loose shingles; probably blown away by the wind 
  • Shingles are curling, cracking or blistering
  • Roof is sagging (this is different than the edges curling up)
  • Dark or dirty looking areas on the roof, indicating that the gravel is coming off
  • Gravel from the roof in the gutters
  • Visible exposed nail heads 
  • Water damage in the attic 
  • Flashing becomes damaged or is missing 

Of course, hail or hurricane damage can greatly accelerate the destruction of a roof, lowering its lifespan. Even a brand new roof can be destroyed by severe weather events, causing it to need replacement. Failure to replace the shingles on a timely basis invites further damage, increasing the cost of eventual repair and replacement.

How Much Roofing?

Most homeowners will hire a roofing contractor to replace their roof, as they are not comfortable doing the job themselves. But roofing is actually rather simple for those who don’t mind working on the roof.

The one big problem is the quantity of work. Replacing a roof requires a lot of man-hours, due to the size of the area that has to be done. A significant amount of stamina is required, due to the weight of the shingles themselves. Even so, one person can re-roof a home, if they decide to dedicate the time to do so, or they could talk a few of their buddies into coming over and helping.

Before anything, it’s necessary to calculate how much material is needed. Roofers calculate the number of “squares” of a roof there are to replace. One square equals 100 square feet. Since shingles are either sold by the bundle or by the square, this is an important number to calculate. Shingles are packaged in packs of 33.3 square feet of finished roof (not of shingle area, which will be higher due to overlap), so each square of roof requires three packages of shingles.

A standard package of shingles contains 29 – 12”x 36” shingles. That works out to a total of 87 square feet of material. But because the shingles are normally overlapped by a bit more than 50%, it is considered to have 33.3 square feet of shingle.

However, due to the slope of the roof, the number of squares of roofing can’t be calculated by dividing the home’s square footage by 100. Besides, the roof will usually overhang areas that are not included in the square footage of the home, like the garage, porches and the eaves. These make the roof’s size bigger than the home. But with a two-story home, the roof will be smaller than the home’s square footage, as the floor space will be divided between the first and second floors.

To calculate the roof’s area start by measuring the dimensions of the home. But don’t just measure the walls, measure to the edges of the eaves, as that’s what you really need to know. In the diagram below, if only the yellow rectangle, which represents the walls is measured, then it will be as if the roof were only the part represented by the pink triangle. But in reality, we have to add the blue area as well. If the home’s perimeter isn’t an exact rectangle, don’t worry, just measure it as if it was. That might cause the purchase of too many bundles, but they can always be returned.

The challenging part of this is measuring the height of the roof peak above the eaves. That usually means climbing up a ladder, with the help of a buddy to hold the lower end. If that’s not possible, then make an estimate, erring to the high side. As with the outer perimeter of the home, erring high will mean buying more shingles than needed; but once again, the unused bundles can be returned.

From that information, it is possible to calculate the total square footage of the roof using the Pythagorean Theorem. Let me see if I can simplify that.


To start with, we need a right triangle. If you remember, a right triangle is any triangle that has a right angle or one that is 90 degrees. Since your roof probably doesn’t have that, we’re going to cut the triangle in half, vertically, right at the roof peak. That will give us two mirror-image triangles of the same size, shown as “A” and “B” above.

When we pull one of those triangles out, we can give the three sides their own names, as in the diagram below. Remember, the dimensions of triangle B will be the same, with the exception that the triangle is a mirror image.

triangle from roof
Triangle from roof

What we need to know is the length of side Z, but what we have is the measurements of sides W and Y. To get the length of side X, we merely need to divide the length of side W in half. Then it’s time to apply the dreaded Pythagorean Theorem. Here it is.

X2 + Y2 = Z2

Let’s walk through that slowly for those who don’t like math. You’re going to want a calculator which has the ability to do square roots for this. For convenience sake, I’m going to apply some numbers to those letters, so that we can better see how it works. So:

  • W = 24
  • Y = 6
  • A (the length of the roof, which isn’t shown) 30
  • That means that X =12
StepAction Shown as FormulaShown as Example
Step 1Put numbers in formulaX2 + Y2 = Z2122 + 62 = ?
Step 2Square the numbers (multiply them by themselves)X * X =Y * Y =12 *12 = 1446 * 6 = 36
Step 3Add the squared numbers together Z2144 + 36 = 180
Step 4Find the square root of that, using a calculator (the calculator included in Windows OS will do it)Z13.4
Step 5Multiply the result by the length of the roof Z * A = Sq. Ft. 13.4 * 30 = 402
Step 6Multiply that by 2 to account for the other side of the roofSq. Ft. * 2 = Total roof402 * 2 = 804
Step 7Divide by 100 to get squares Total roof ÷ 100 = Squares804 ÷ 100 = 8(add an extra package) 

So in our example, we find that the “Z” side of the roof is 13.4’, which gives us a roof square footage of 804 square feet. Since we can only buy whole packages of shingles, it means buying an extra package for those four square feet. But then, it’s a good idea to have some extra on hand anyway, as there’s always something which makes it necessary to use more than expected.

Some of the things that might require using extra shingles are dealing with dormers, valleys and the roof peak. So keep that in mind when buying your material and make sure you buy enough to account for the extra material and extra scrap those situations will cause.

One final point on this measurement; if the house is L shaped, it makes sense to cuft it into two pieces and calculate them separately, adding them together. Otherwise, the area calculated for the roof will be way too big, as it will include filing in the inside of the L.

A Few Notes on Shingling the Roof

Most homeowners who shingle their own roof will start by removing the existing shingles. There’s nothing wrong with that, but it may not be necessary. Building code allows for three roofs on a house, so unless the building code in your municipality is more restrictive or there is such extensive damage to the roof or the roof sheathing as to require removing a layer of shingles, putting the new roof on over the old one can save a lot of labor.

The trick when doing this is using the existing shingles as a guide for the new ones, butting the shingles up against the existing overlap. That helps prevent unsightly humps in the roof, which can lead to leaks.

While not common, it is also possible to lay asphalt or fiberglass shingles over cedar shakes or shingles. This is tricky, as the old roof won’t be as smooth as the new one. It’s best to get some help from a professional, if this is the decision that’s made.

It isn’t necessary to lay a new layer of roofing felt over the existing shingles, when installing a second roof over the first. In fact, it’s better not to, as it makes it easier to see the edges of the existing shingles. However, some professionals will add a single row layer of waterproof shingle underlayment at the eave ends, to help prevent ice dams.

New flashing should always be installed to help prevent leaking at that point. This includes a drip edge. Make sure that the flashing used is for re-roofing, as the dimensions will be different. Flashing in valleys should also be replaced, even if it looks good. That flashing is going to have to last another 30 years.

Any roofing job is done from the bottom up. Special care must be taken at any point where the roof is pierced, as that is a prime location for a leak. This piercing is usually by some sort of vent, which will have flashing built into it. Always make sure that the flashing overlaps shingles downhill of it and is overlapped by shingles above it. As for the shingles that are on line with it, to either side, that’s up to personal discretion; but overlapping the shingles over the flashing will look better and be easier to seal.

It’s a good idea to replace vents at the same time that the roof is being replaced. All vents, regardless of the type, are affected by weather. So unless they look like new, they should probably be replaced. The additional cost of the vents is minor, in the total project budget.

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