There are two ways to estimate the total number of squares that are installed on

the roof. One way includes measuring the area of the house footprint and multiply

that by a roof slope factor. Another way includes measuring the area of each

plane on the roof and add them all together. In this video, we're going to

learn how to measure each plane of the roof to calculate the total square

footage. To do this, you have to measure the length and width of each plane on

the roof including the dormers then multiply length by width to get the

square footage of each plane finally calculate the roofs total square footage

by simply adding the square footage of each of the planes together there are

several different styles of roofs including shed roofs hip roofs

and gable roofs this is a shed roof a shed roof has one roof plain simply

measure the length of a then measure the length of B now the area of the shed

roof is a times B which is in this example twelve and a half feet times

twelve and a feet which is 156 in a quarter square foot for the total square

footage of the shed roof this is a square the area of a square is simply

base times height in this example the base is twelve and a half and the height

is twelve and a half this is a rectangle the area of a rectangle is just like a

square it is base times height and in this example the base is 15 feet and the

height is 12 and a half the area of the rectangle is therefore 15 times 12 and a

half which equals 180 seven and a half square feet

this is a gable roof a gable roof has two planes so multiply the base times

the height for each plane or rectangle to get the square footage for each plane

then add the two rectangles together to get the total square footage of the roof

so let's assume one rectangular plane of the roof is sixteen by 40 and the other

plane is sixteen by 40 the area of each plane is 16 times 40 equals 640 square

feet by adding the two rectangular planes together the total square footage

of the roof is 1280 square feet this is a hip roof now a hip roof has four

planes and they are not rectangular like the other roofs so you'll need to know

some simple geometric formulas to calculate areas of roof shapes including

trapezoids and triangles a hip roof will include a trapezoid shape to calculate

the area of a trapezoid you add the length of each base base one plus base

two and then multiply it by height then you divide that by two to get the square

footage of the one trapezoid plane in this example one of the hip roofs

trapezoid planes is 10 plus 15 times 12 equals 300 divided by 2 equals 150

remember that's just for one of the trapezoid shapes of the hip roof in this

example there are two trapezoid shapes in the illustration you can see this hip

roof has two trapezoid shapes and two triangle shapes so let's look at

calculating the area of a triangle the area of a triangle regardless of the

triangle shape is base times height times 1/2 in this example the area of

the triangle is 8 feet times 10 feet times one-half equals 40 square feet the

four planes of a hip roof include two triangles and two trapezoids

calculate the area of each shape and add them all together to get the total

square footage of the hip roof area there are several different types of

roofs and their area calculations

let's take a complex house roof as an example and calculate its total square

footage this complex roof has lengths of various geometric shapes of the roof

measured the main house roof is a large gable roof looks like the front entry

has a small gable roof the rear deck has a hip roof there and there's a gazebo

roof attached to the back of the house let's calculate the area of this complex

roof in total square footage let's start with the red octagon a4 which is just

like a bunch of triangles stuck together we know that the area of a triangle is

base times height times 1/2 so the octagon is 8 of those triangles added

together so if the base is 5 feet and the height is 6 the area of the octagon

is 8 times base times height times 1/2 and it equals 120 square feet now let's

look at the yellow shapes the area of the east gable a3 is the addition of 2

trapezoids we know that the area of a trapezoid is base one plus base 2 times

height times 1/2 so for this house the east gable a3 is 16 feet 3 inches plus

10 times 5 which is height times 1/2 but because there are two trapezoids you

double that amount to get the total square footage of the east gable roof in

yellow the west gable roof is measured by calculating the area of one of the

trapezoids and then doubling it to get the total square footage it's the same

method we used for the East gable roof next is the big blue section a gable

roof with two large rectangular Plains we know that the area of a rectangle is

base times height for this house roof we have base times height as 10 times 60 or

600 square feet there are two rectangular planes the

total square footage of the blue main house roof a 1 is double that 1200

square feet due to the overlap in areas we must subtract a portion of each

yellow gable from the main house roof total so the green dashed lines indicate

the areas that must be subtracted from the main house roof a 1 here's the

breakdown of all the shapes and their areas

you

when you add up all the shapes the total roof area is 1820 square feet now

convert the total square footage to roofing squares in roofing terminology a

square is 100 square feet now divide the total area by 100 so 1820 square feet

that's the total area of the entire house roof divided by 100 equals 18.2

squares this answer is the number of roofing squares for the roof 18.2 so in

summary we learned how to calculate the size of a roof by measuring the total

square footage of the roof and dividing that by 100 remember the area of a

square is simply base times height the area of a rectangle is just like a

square base times height the area of a triangle is base times height times 1/2

the area of a trapezoid is base one plus base two times the height times 1/2 and

a square in roofing terms is 100 square feet