Things You Will Need
The quadrants have to be exactly square to each other, which is a problem in older homes where the room itself is unlikely to be truly square. Don’t rely on the wall positions to set up your grid, but rather square it yourself at the center.
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Measure one side of the floor, find the middle, and mark it with a pencil. Do the same along the opposite side of the floor.
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Snap a chalk line across the floor, from one mark to the other.
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Measure and mark the middle for the two remaining sides of the floor. Lay the snap line from one mark to the other so it intersects the first line in the center of the room. Don’t snap the line.
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Lay a carpenter’s square at one of the four corners of the intersection created by the chalk line that you laid first, and the string. If the line and the string are truly perpendicular, then each one will run right alongside one edge of the carpenter’s square.
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Adjust the string, if necessary, so it is completely square against the chalk line. Once the string is square to the line, snap the string. Tile starting at the intersection of the two lines.