Plan Reading, Drainage, and Insulation Verification on a 160 ft by 100 ft Roof
A roof plan is drawn at 1/8 in. = 1 ft. The main roof measures 160 ft by 100 ft and slopes at 1/4 in. per foot to two primary internal drains located on the centerline. A 20 ft by 30 ft mechanical penthouse sits near the center of the roof. The parapet is 42 in. above the roof deck at the perimeter. The plan shows overflow scuppers set 2 in. above the roof at the drain low point, tapered polyiso with a minimum of 1/2 in. at the drain, and a 1/2 in. cover board. A local amendment limits each primary roof drain to 7,500 ft². The plan shows a cricket on the upslope side of the penthouse and expansion joints at 100 ft intervals. The contractor is preparing a bid and must verify the roof area, drain count, and insulation elevations.
1. Calculate the roof area in squares, including the penthouse, and state a reasonable waste allowance for bidding.
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Main roof area is 160 ft x 100 ft = 16,000 ft². Penthouse roof area is 20 ft x 30 ft = 600 ft². Total plan area is 16,600 ft². One roofing square is 100 ft², so the plan area is 166 squares. For bidding, the contractor should add waste for hips, valleys, penetrations, crickets, and edge cuts. A 10% waste allowance gives 16.6 squares, for about 183 squares. The contractor should also verify dimensions by scaling the 1/8 in. = 1 ft drawing, but should not rely on scale alone; field measurements should confirm the 160 ft and 100 ft dimensions, parapet returns, and penthouse setbacks. If the membrane is ordered in rolls, the order should be rounded up to full rolls, and the contractor should account for the expansion joints and cricket cuts separately.
2. Using the local 7,500 ft² per drain limit, determine whether two primary drains are adequate and explain how the overflow scuppers fit into the drainage design.
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The main roof area is 16,000 ft². At 7,500 ft² per primary drain, 16,000 / 7,500 = 2.13, so two drains are not enough; the plan needs at least three primary drains. The penthouse roof also needs drainage, but it may drain separately or to the main roof; that must be shown. Overflow scuppers are secondary drainage, not a substitute for primary drains. They must be located above the primary drain and must discharge where water can safely flow. The note setting the scuppers 2 in. above the roof at the drain low point is suspicious because the roof rises toward the perimeter. At the parapet, the roof surface may be much higher than at the drain, so a scupper set only 2 in. above the low point could be below the roof surface and useless. The scupper elevation should be checked against the actual roof surface at the scupper location, and the primary drains should be sized for the design rainfall.
3. Check the 42 in. parapet and the 1/4 in. per foot slope for insulation elevation conflicts, and explain what the cricket at the penthouse must do.
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The slope is 1/4 in. per foot, or 1 in. per 4 ft. From the drain to the far perimeter, the horizontal run can be about 80 ft, so the rise is 80 x 0.25 = 20 in. With a minimum of 1/2 in. of tapered insulation at the drain, the insulation at the far perimeter is about 20.5 in., plus 1/2 in. cover board, or about 21 in. above the deck. A 42 in. parapet leaves only about 21 in. above the perimeter roof surface. That may be acceptable for fall protection or edge height, but the overflow scupper must be set above the roof surface at its location, not at the drain. The scupper may need to be raised well above the 2 in. note. The cricket at the 20 ft wide penthouse must divert water around the upslope side and toward the drains; it should be built with the same slope and should extend far enough to prevent ponding at the penthouse corners. The contractor should request a roof plan detail showing cricket slopes, drain elevations, and scupper elevations before bidding.