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ROOF DRAINAGE DESIGN
Early determination of how roof drainage is to be provided is extremely
important. The method chosen not only affects the structural design and
building elevations, it can also impact floor plan layouts due to chase
requirements.
"FLAT" ROOFS WITH DRAINS AND INTERIOR DOWNSPOUTS
Basic Considerations
The first factor to analyze before an internal roof drainage system is
considered, is to verify that there is a means to discharge storm water into an
underground piping system or drainage ditch. If roof drain piping can not be
routed from the building below ground, the pipe will need to discharge above
grade through the exterior walls. You can imagine how six, eight or ten inch
diameter holes would look dotting the face of your exterior walls. (Not to
mention the streams of water pouring out of them during a rain).
In the event below grade discharge is not possible, consider utilizing
perimeter roof gutters and exterior downspouts. Gutters and downspouts will
allow you to have more control of discharge points and will negate the
necessity of chase spaces within the building for interior piping.
Determine whether a parapet will be provided. Building Codes require a means of
disposing rain water from a roof in the event a primary roof drainage system
does not function. The inclusion of a parapet causes a decision to be made
concerning the method by which storm water emergency overflow will be achieved.
Evaluate the type of roof system to be specified. Various manufactures have
defined slope and drain placement requirements included within their system
warranty. The physical size, local rainfall rates and the number & size of
interior leaders you can be facilitated within the floor plans, determine the
quantity of roof drains that will be required. Also, remember that large roof
areas may be drained more economically by sloping the roof structure, thereby
limiting the depth of insulation. Acquire input from the Structural Engineer to
determine if this is an option to consider.
To Scupper or Not to Scupper
As previously mentioned, parapets introduce a need to protect the roof from the
build-up of storm water on a roof in the event of drainage system malfunction.
There are two methods of meeting this requirement, Providing the proper number
of scuppers through the parapet or Providing an additional system of roof
drains and leader piping.
Scuppers are by far the most economical of the two. Consult the applicable
Building Code for proper sizing and placement of scuppers and remember that the
bottom of scupper openings must not be more than two inches above the low point
of the roof. This will affect the location of roof drains and the arrangement
by which the roof is sloped. An exception can be made to the two inch
requirement by designing the roof structure with the capacity to support the
weight of water that is contained on the roof before the scuppers provide
relief.
In the event scuppers are not feasible, a secondary system of roof drains and
internal leaders must be designed. This secondary system creates a few
logistical problems and can be expensive. The Standard Plumbing Code as well as
the International Plumbing Code requires that the secondary system piping be
sized significantly larger than the primary system. All of the major Codes
require that the discharge of the secondary system be independent of the
primary system. This means that the secondary leaders will have to discharge
through the exterior walls above grade.
Please consider the logistic, economical and aesthetic consequences when
determining the method of emergency overflow and consult with the Plumbing
Engineer before you make a final decision.
PERIMETER GUTTERS WITH EXTERIOR DOWNSPOUTS
Obviously, the construction type and height of the building will limit the use
of perimeter gutters and downspouts. However, when they are provided, consider
the following:
Discharge of downspouts should not flow over pedestrian walkways (Especially in
areas subject to freezing temperatures)
Provide a sufficient quantity of downspouts to limit the slope of gutters and
to evenly distribute discharge
SUMMARY
Coordinate with the Plumbing Engineer during the preliminary phase of your
design and discuss the various options and considerations regarding roof
drainage. Spending the time to evaluate, design and coordinate the roof
drainage system as an initial step in your project will save time, cost and
reduce frustration for everyone involved.
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