All buildings are made up of a series of triangles, squares and rectangles even when the walls appear to be broken up with windows and doors. In order to figure the square footage of a wall you will need to multiply the height and width of the wall. When you measure a wall to determine the square footage, include the areas where there are windows and doors. The trick to figuring square footage is to break the wall up into squares, then figure the triangle areas, such as the gable end wall or a gambrel end wall. When ordering vertical siding you will need to add 20 percent more siding or, for horizontal siding, add a foot to the height of the wall to allow for waist. It is better to have too much siding than not enough.
Things You'll Need
 Measuring tape
 Calculator

Starting at the sidewall of the barn, measure the length of the exterior wall of the barn from corner to corner. Measure the height of the wall from the overhang of the roof, called the eave, down to the concrete foundation or where the siding will end. Multiply the length of the wall times the height of the wall. This will figure the square footage of the area. Do not exclude the areas where there are windows or doors. Once you have figured the square footage of the sidewall of the barn, multiply the figure by two for the other side of the barn.

Measure the length of the gable end wall of the barn from corner to corner. Measure up to the eaves on the sidewall, creating a square. Multiply the length times the height up to the eave, equaling the square footage of the wall area. Now, measure from the peak down to the eave to get the height. Multiply the height of the peak times the length of the wall, previously measured. Divide the answer by two, equaling the area of the triangle peak. Add the two square footage of the wall areas together equaling the total square footage of the gable end wall. Multiply the total square footage of the gable end wall by two to figure for the other gable end.

Measure and use the equation for a gable end wall to figure the square footage of the front of a dormer. Measure the top of the dormer where the roof of the dormer meets the wall down to the roof. Measure from the corner of the dormer to where the dormer roof meets the roof of the barn. Multiply the length times the height, then divide by two to get the square footage of the side of the dormer. Multiply the answer by two for the opposite side of the dormer.

Measure the length of the gambrel end wall of the barn from corner to corner. Measure up to the eaves on the sidewall, creating a square. Multiply the length times the height up to the eave, equaling the square footage of the lower wall area. Write the answer down on a piece of paper. Measure between the two angles on the roof to find the width between the angles and measure from the angle straight down to the eave, cutting the gambrel part of the wall into a square in the middle, two triangles on each side of the square and a triangle from the peak to the center square. Multiply the width between the angles on the roof times the height from the eaves to the angled roof. Write down the answer below the first answer. Measure the distance from the eave horizontally over to the angle of the roof, this being a triangle. Multiply this measurement by the previous measurement from the angle of the roof to the eaves. Do not measure along the roof. You do not need to divide this figure because the other end of the wall will create a triangle as well and together it would be a square. Write this answer below the others. Measure from the peak straight down to the angle part of the roof. Multiply the height of the peak to the angles on the roof by the length of the wall between the angles on the roof, previously measured. Divide this answer in two to figure for the triangle part of the roof's square footage. Write this answer under the others and add the answers to all the equations for the gambrel wall together to equal the total square footage.
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 Photo Credit barn image by michael langley from Fotolia.com