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- by Dawn Hollingsworth
More and more downlights are being installed in slope ceiling conditions. If a fixture is adjustable from 0° (nadir) to 30° from vertical then it can be installed at any degree of slope within that range. However, many times the fixture of choice is not adjustable and must be adapted for use in a sloped ceiling. With sloped ceilings, an adapter must be used to return the lamp to the same orientation to the space as the lamp would be on a flat ceiling. Otherwise, the source would be so visible that it would create a visually disturbing environment.
Generally a sloped ceiling will be indicated on the plans by a ratio of rise to horizontal distance, usually measured in inches. For example, a typical architectural note might indicate a slope of 3:12, which means for every 12 inches horizontally the slope would rise 3 inches.
FIGURE 1 illustrates this relationship. Since sloped ceiling adapters are cut to common angles in 5° increments and not in ratios, we need to translate this information into an angle. We know that Line A equals 12" and that Line B must equal 3" since this information was contained in the given ratio. Now that we have the length of two sides of the right triangle, we can use the Pythagorean Theorem to find the length of the third side:
3(3) + 12(12) = X
9 + 144 = 153, where the square root of 153 = 12.37,
thus the length of the slope =12.37"
From there we can calculate the angle of the slope (q) by dividing Line A by Line C to get a cosine of the angle q:
12/12.37 = .9701
A table of trigonometric functions will tell us that .9701 cosine would equal a 14° angle.
Fourteen degrees would be closest to a 15° slope adapter so this is what should be ordered for the fixture. It can be quite cumbersome and time consuming to calculate this for every project, especially if you do not have a trigonometry table handy. The following table will help eliminate the math. These values have been rounded to the nearest 5° increment.
1:12 = 5°
2:12 = 10°
3:12 = 15°
4.5:12 = 20°
5.5:12 = 25°
7:12 = 30°
8.5:12 = 35°
10:12 = 40°
12:12 = 45°
FIGURE 2 below shows the complete installation of a slope ceiling fixture.
Copyright 2001 VISUAL TERRAIN, INC.
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