How to Calculate the Roof Area

Posted by:  Jeff Ahbe

When you are about to start your Denver roofing project, you will need to calculate total roofing materials needed for the job.  To get this amount, you need to estimate the total surface area of your roof so you know how much to order.  When calculating your surface area, the easiest way to do this is actually measuring your roof.  When doing so, round up to the nearest foot to make your calculations easy and to ensure you order enough! 

It is easy to calculate when you think of your roof as a flat rectangular surface.  If measuring a gable roof, find the area of a rectangle, multiply the length times the width.  Measure along the ridge or eaves to find the length and from the ridge to the eaves to find your width.  If you multiply the two numbers together you'll get the square footage of one side of your roof.

Complicated Roof Measurements

If you have a section of your roofing system that is more complicated that the basic gable roof, it is easier to divide your roof into triangles.  To find the square footage of a triangle, multiply the height by the width and divide by two.  Once you figure the area of your triangles, simply add them up to get your roofs area.

If your roof is inaccessible or hard to maneuver you will need to make your measurements from the ground.  Measure the roof as if its edges were projected on the ground and then correct for the amount of slope in the roof by using the area/rake conversion chart to the right.  Make sure to calculate the roof overhangs, don't just measure wall to wall of your home. It is easier to draw the roof outline on paper and add the ridge, dormers, chimneys or porch to your figures.

Once you have a proper outline, take the following steps to correctly calculate your roofs area:

• Once you have your drawing, its time to figure out your area.  Divide the roof into rectangles and triangles as we did above. Subtract areas for chimneys and skylights.
• Determine the slope of your roof sections and remember each area of your roof can have different slopes.
• Find the conversion factor on the area/rake conversion chart.
• Multiply the total number of square feet by the conversion factor.
• Whether you measure directly or figure from a conversion chart, always add 10 percent for waste.
• Once you have your total, divide that number by 100 to determine the total number of squares.  This number is used to determine how much of your roofing materials to order.