How to Determine the Angle of the Slope in a Garage Floor

Lay a builder's level on the garage floor, running in the direction of the slope fall. This is usually towards the front entrance of the garage.

Gently lift the end of the level that is nearest to the entrance. Elevate it until the bubble in the glass vial rests between the two black lines at the center of the vial, indicating that the level is precisely horizontal. The gap below the level is a visual indication of the drop in the floor over the length of the builder's level.

Measure the vertical distance from the floor to the underside of the elevated level. Record the distance. For example, if the level is now 2 inches above the floor, record "2."

Move the level slightly closer or further away from the entrance and repeat Step 1 through to Step 3 a further two times. Record the vertical distance on each occasion. Add the three values and divide by three to obtain the average distance. For example, if the readings were 2 inches, 2 1/4 inches and 1 3/4 inches, add them together to obtain 6 inches. Divide 6 inches by 3. The result, 2 inches, is the average value.

Measure the length of the builder's level. Record the value in inches.

Substitute the length of the level and the distance above the ground into this equation. The tangent of the angle of slope = distance above the ground / length of the level. So, for example, with a height of 2 inches and a 3-foot level, the equation becomes: Tangent of the angle of slope = 2 / 36, or 0.056.

Determine the arc tangent of the result from Step 6. The result is the angle of slope, expressed in degrees. For example, the arc tangent of 0.056 is 3.21, so the slope is 3.21 degrees.