MIFAB’s MODULAR ROOF DRAIN SYSTEM
Roof drains can be a big part of the average bid/spec job. They are very important in ensuring
that the storm drain system can handle even the worst anticipated rainfall and can conduct that
from the roof of the building in an acceptable time frame. For this reason, engineers are looking
for assurance that the product specified will perform as required. They are eager to hear of any
products that give this assurance by virtue of quality and superior design. The MIFAB modular
roof drain system is full of the features they are looking for. In the next while, I will show you an
overview of these features, and take you through the roof drain sizing example on pages viii and
ix of the Roof Drain intro in the catalog.
The MIFAB Roof drains are different from all the other specification drainage products in our
product line because instead of connecting to the sanitary system, (like showers, sinks and
waterclosets), they are storm drains and discharge directly into the environment. Usually this is
through above ground spillways or by below grade storm sewer mains. Because they carry only
rainwater, the discharge from roof drains takes a different path that all the other drains in the
building. For this reason, there are special design considerations that apply only to roof drains.
For one, since there are no fixtures in the storm drain system, it can run at full capacity, since
there is no worry of flooding through traps or fixtures. This is called running “fully saturated” by
engineers, and this is important because ultimately the volume of water that can be carried away
by the system will determine the size and placement of the roof drains based on the maximum
capacity of the vertical pipes (leaders). Chart A on page viii relates the area of the roof in square
feet to the rates of flow that leaders of various diameter can accommodate based on maximum
anticipated hourly rainfall in inches. To insure the maximum rate of flow, MIFAB’s R1200
series roof drains feature ductile iron domes (suffix –M) with 136 square inches of free area. Our
standard dome is 8” high; meaning it is less likely to be stopped up by wind blown debris. Our
competitors standard dome height is 4-1/2 inches. The sump of our drain body is deeper and
narrower, meaning greater volume with a smaller deck opening. A narrower body also has a
smaller floor (which is sloped), allowing a smaller area for sediment to build up.
Roof drains have to be available for a number of different combinations of roof deck material,
insulation and waterproofing membranes. There are a vast array of materials and installation
techniques used in modern commercial construction and careful consideration was made during
the development of MIFAB’s roof drain product line to accommodate as many as possible. In
general, there are two main types of roof systems, conventional and IRMA (inverted roof
membrane assembly). A conventional roof is one where the membrane is the topmost
component. It is usually made from layers of tar impregnated felt (in some cases, specified 12
layers thick!) and then topped off with a coating of pea gravel so the maintenance staff don’t sink
into it if they have to service the A/C unit on a hot summer afternoon. An IRMA system is where
the waterproofing membrane is the bottom most component. The idea is that the roof membrane
will need no maintenance if the membrane is protected from the elements by a thick layer of
ballast and insulation.
Most drains specified will require an under deck clamp. In fact, unless a roof drain is “poured in
place”, i.e. set in the concrete of the roof deck, it should have an under deck clamp. MIFAB
scores a big advantage with our deck clamp design because we use the same component (A1 0r
A2-C3) for both the flashing clamp (the part that secures the dome to the drain body) and the
under deck clamp.
If a conventional roof system features insulation above the deck with the main waterproofing
membrane above that, the roof drain will require an extension to bring the drain body flange up to
the level where the flashing clamp can be secured. Again, the MIFAB system has a huge
advantage here, as the same component that is used as the flashing clamp and the under deck
clamp also is used as an adjustable extension. In the conventional systems offered by our
competitors, like the Z-100-EA-R-C, (see appendix A) the design requires 2 gaskets with a
threaded collar that means a much greater potential of leakage. The MIFAB system uses an A1/2C3 clamp to push the drain body up to the membrane level. The result is only one union at the
roof drain outlet, (no extra gaskets or collars). This is best illustrated on page I of the roof drain
section of the catalog.
The design of an IRMA system means that the roof drain will require a ballast guard. Large
gravel is used to weigh down the insulation protecting the membrane. Rainwater must be able to
drain through this gravel and into the roof drain. A problem can arise if the holes in the ballast
guard are not sized correctly. If they are two small they can become clogged with sediment.
MIFAB’s ballast guards on the R1200-G and R1200-JC are sized with 1/2” holes to pass through
any fine sediments that may be mixed in with the ballast before they can cake up around the
ballast guard and greatly reduce the flow of the roof drain. MIFAB also has a huge advantage in
the field of IRMA roof drain design with the R1200-HC, the only drain with an adjustable cast
iron ballast guard. This is an easy product to get specified in markets where IRMA roof drain
design is popular with architects.
There are some popular options to be aware of, these are discussed in detail on pages ii to v of the
roof drain intro. It is also good to know the basic principles of how the size and placement of roof
drains is determined. This is done in a number of steps, the first of which is the determination of
the maximum expected rainfall in inches per hour. This value is derived from statistical data
maintained by the government weather service. Once this figure is known, a volume calculation is
made (expressed in gallons per minute) of how much water must be moved off the roof under the
worst case circumstances, i.e. a storm that would occur statistically once in a hundred years. This
will determine the size of the leaders necessary to conduct the rainwater from the roof. The
number and placement of the actual roof drains is determined by apply some basic rules and
making allowances for any structural considerations specific to the building such as placement of
mechanical equipment on the roof, projecting walls, and interior columns and supports. Please
refer to the introduction to the Roof Drain Section of the New Catalog for a more detailed
description of all the topics we touched upon today.