RAINTROL® Roof Drains Technical Data :: Specifiying and Sizing

SPECIFYING AND SIZING

A convenient worksheet (Form No. 2052) is available for sizing and determining RAINTROL® requirements.

Specifying can be done quickly and easily.

1. Determine roof area to be drained. Each area that is bounded by expansion joints, ridges and any enclosure is considered a separate roof area.

2. Divide the roof area by the area rating from the Table of Area Ratings (Table 1) to obtain the total number of weir openings.

3. Determine the number of roof drains. This is determined by the engineer and/or roof layout, using the above design consideration as a guide.

4. Divide the number of drains into the number of weir openings to obtain the number of weir openings per drain. It is not necessary that all drains have the same number of weir openings. As an example, a roof may require eight weir openings, but only six drains. In this case, four drains could have one weir opening and two drains would have two weir openings.

NOTE: There is a minimum of one weir opening per drain.

Table 1, from which the area rating is selected, also lists the corresponding flow rate and drain down time. With this data, the engineer can select the proper leader and storm sewer to accommodate the flow (Table 3). Scupper or overflow protection must be set at the depth corresponding to the flow rate (Table 1 and Table 2). This would limit the potential build-up, flow rate and roof loading. The weir height is the maximum potential build-up. If the scuppers are set at a higher level, the potential build-up would be greater. Leaders and storm sewers would have to be sized for the higher flow rates which correspond to the greater build-up. Also, a greater load might be placed on the roof. Refer to Table 3 for allowable flow rates. Select leaders and storm sewers, which will accommodate the maximum potential flow.

Local codes may be the determining criteria and deviation must be approved. Examples showing the sizing method and savings click here.

TABLES

Table 1 is the area rating table for one weir and contains the principal data. It is arranged in alphabetical order by states and cities. The data is divided according to roof type. Example: Flat, 2" 4" or 6" rise. Then four return periods are listed under each roof type. Each block shows three values. The top figure is the area rating, the lower left is the maximum flow rate for the particular area, and the lower right figure gives the corresponding drain down time. The drain down time is based on draining from the maximum depth to a depth of one half inch, which is the practical minimum. (Refer to Fig. 3 below).

For values not shown in Table 1, straight line interpolation will give acceptable figures. Using this table will provide practical solutions. For necessary data not listed, the factory should be contacted. The limits on which Table 1 is based are allowable build-up and maximum area.

The build-up limit is 3" for flat roofs, 4" for 2" rise, 5" for 4" rise and 6" for 6" rise. The area ratings are the square foot areas that will produce the above build-ups. However, if the area rating would exceed 25,000 sq. ft., the area rating was limited to 25,000 and the corresponding maximum flow rate and drain down time recorded. The corresponding build-up can be obtained from Table 2. Interpolate between values shown when intermediate values are desired.

Table 2 lists flow rates for various heads in 1 inch increments.

Table 3 lists the allowable flow rates for various pipe sizes. Rates are given for vertical leaders, and horizontal storm drains installed at three different slopes. These values are consistent with the National Plumbing Code, and values obtained using Mannings formula.