How to Measure the Diagonal of a Square for House Building


One of the ways to determine if a building is square, that each corner is a perfect right angle, is to measure the distance of the diagonals. If these measurements are equal the building is square. For small buildings this measurement is easily made. For larger buildings the sag of the measuring tape can be a factor in the accuracy of the measurements. This type of measurement requires skill and tools not normally part of the do-it-yourselfer's set.

Things You'll Need

  • Stakes
  • Carpenter's string
  • 100-foot tape measure
  • Tape tension handle
  • Create corners that are as square as possible to start the project. Place two stakes about 2 feet to the left and right of each planned corner. Connect each set of stakes with a 1-by-4-inch board. Stretch carpenter’s string from each of the boards to a board at the next corner. Properly laid out, the intersections of each string is the planned corner of the building.

  • Hold the end of the tape measure at the intersection of strings at one corner. Pull the tape to the intersection of the string at the opposite corner. Fasten the tape to a tape tension handle. This device measures the tensions on the tape as it is stretched from corner to corner. Pull the tape to a 10 lbs. tension, for example and note the measurement of the diagonal.

  • Go to the other diagonal and make the same measurements. Hold the tape tension at the same tension during the measurement. If the two measurements are equal the building plan is perfectly square or rectangular. Move the point where the strings are attached to the boards to adjust the building site if the measurements are not equal. Continue making adjustments and measurements until the planned site is square.


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