The roof over your head requires a substantial chunk of the money you spend on your home. A roof is a major structural component of any home and should receive attention to detail. Mistakes from the outset could be costly. If in doubt, seek advice from a professional contractor.
Calculating the number of roof shingles
Calculating the roof shingles to order for a construction project can be challenging. However, you can use one of three reliable methods to determine the area of a roof. These methods include tips to estimate the number of extra shingles needed to compensate for wastage, overlaps and starter shingles.
The calculation
- Firstly, you will have to estimate the total square footage of the surface of the roof. Measure the length by the width of each plane on the roof. Include any dormers.
- Now, multiply the length x the width of the roof to arrive at the square footage of each plane.
- Finally, add the square footage of each of the planes to obtain the total square footage of the roof.
Examples
In this example, we look at a roof with one plane, such as a shed. If the length measures 120ft and the width measures 100ft, the total square footage of the roof will be 12,000sq.ft.
A gable roof has two planes. Plane 1 measures 120ft x 100ft = 12,000sq.ft. Plane 2 has the same measurements: 120′ x 100′ = 12,000sq.ft. Now add the two planes together. The total square footage of the roof is 24,000 sq. ft.
Measuring in squares
Always measure roof surfaces in squares of 100 square feet. To determine the number of squares on the gable roof discussed above, divide the total of 24,000sq.ft by 100. The result is 240, meaning that 240 squares of shingles will be needed to cover that roof.
Tip: Always add 10% to all material requirements for trim allowance.
Ordering your shingles
Roof shingles are sold either by the square or by the bundle.
A square of shingles covers 100sq.ft. of the roof. Three bundles to a square are the most common. There are 29 standard-sized shingles (12 inches x 36 inches) in each bundle. However, heavier shingles, such as three-tabbed and laminated shingles, require four to five bundles to cover 100sq.ft.
Roof calculation methods
Determiningthe roof area is the first step to ascertain how many bundles of shingles to order for the project.
There are two methods of sizing a new or a freshly-stripped roof. These are the measurement method or the sheet-count method. There is also a third method for calculating bundles when laying new shingles over old roofing.
Don’t forget that you will need to order extra shingles for wastage, hips and ridge caps.
Now, let’s take a look at the different sizing methods.
- The measurement method
The most accurate way in which to determine how many bundles of shingles are needed is to measure each roof plane. If the planes are all rectangular, multiply the length of the roof plane by the width of each plane. The square footage of each plane is then added to arrive at the total square footage of the roof. If a roof is steeply pitched and difficult on which to walk safely, you will have to do the measurements at ground level. Measure the length of the building and then estimate any rake-edge overhangs. Using a ladder, climb up and measure from the eaves edges to the ridge of the roof with a wide-blade measuring tape.
- The sheet-count method
The sheet-count method is best adopted when the sheathing is still exposed. This is a quick method and can be done at ground-level. However, you can only use this method on roofs that are sheathed with 4ft. x 8ft. structural panels. It is easy to count the panels from the ground because each structural panel is 32sq.ft. (4ft x 8ft). Using shingles of three bundles to a square makes calculations a simple matter. If each bundle of shingles that you order cover 33.3sq. ft. of the roof, you can order one bundle for each sheet of roof sheathing.
For other calculations, remember that three sheets of sheathing are approximately 100sq.ft. So, by dividing the number of sheets of sheathing by three, you will calculate the total number of squares required to cover the roof area. This may sound like a basic measurement method, but working on a 1.3sq.ft. of difference in size per sheet is nevertheless a good margin to allow for wastage.
- The shingle-count method
This method is used when laying new shingles over old roofing. It is a particularly simple calculation if the old shingles are still intact.
Firstly, measure the length of the eaves of each roof plane. This can be done physically, or from the ground by measuring the length of the house and then adding the width and any rake overhangs. Alternatively, if the old shingles are standard three-tab size, you can determine the length of the eaves by counting the number of tabs along the ridges and eaves. One tab is equal to 1ft.
To determine the length of the rakes, count the existing courses of shingles from the eaves to the ridge of the roof. The exposure on each course of shingles is 5 inches. Simply multiply the number of courses by 5 inches and then divide by 12 to get the length of the rakes. However, don’t forget to make sure that the existing shingles are a standard 12 inches by 36 inches. Now, multiply the length of the eaves by the length of the rake to ascertain the total area in square feet.
Planning for wastage
There is no exact calculation to determine the extra shingles needed for waste, but these examples should get the job done.
Most roofs require shingles to be cut at the rakes and don’t forget that wastage increases with every obstruction. These can be chimneys, hips or valleys.
For a simple roof, you can expect a waste factor of about 1 percent. A more complex roof, with open valleys, will require at least 5 percent extra shingles.
Damaged shingles also create waste and this possibility is inevitable, particularly on a steeply pitched roof when shingles are moved around and slip off the edge. Another waste-factor is caused by nails which can damage the shingles when improperly installed.