Cost Saving Application Examples: Using a Siphonic Roof Drain System
A traditional roof drain on a roof
covers a tributary area of 7,840
square feet of roof surface at a
rainfall intensity of 3.25 inches per
hour. According to IPC 2000,
Chapter 11, Table 1106.2 the
required roof drain outlet size and
connected drain pipe size would
be 5 inches. A five inch drain with
polyethylene dome and the first ten feet of 5 inch pipe with
1 inch insulation covering would cost about $840.00.An existing siphonic system design covering the same roof area and rainfall intensity includes a 3 inch siphonic roof drain. A three inch siphonic drain and the first ten feet of 3 inch pipe with with 1 inch insulation covering would cost about $610.00.
*That’s a savings of $233.00 per drain and ten feet of branch
piping or almost 28 percent less than a traditional design.
A horizontal
segment of a
traditional roof
drain system
is designed to
drain 15,685
square feet of
roof surface
at a rainfall intensity of 3.25 inches per hour. At a pitch of
1/8” per foot (about 1 percent), the required pipe diameter
is 10 inches (IPC 2000, Table 1106.3). At this design flow,
the pipe would be 52 percent full and have a velocity of
4.0 feet per second. Cast iron pipe with hangers every 5
feet on center and a 1 inch fiberglass insulation cover and
couplings every 10 feet would cost about $1,057.00 per ten
foot section. If installed below grade, the cost is about the
same if insulation cover is replaced with the cost of trenching
and backfill.An existing siphonic system design covering the same roof area and rainfall intensity has a calculated pipe manifold of 6 inches. Even without pitch, the design velocity is 6.1 feet per second and 100 percent full. Cast iron pipe this size with hangers every 5 feet on center and a 1 inch fiberglass 3" outlet and 10 ft. of pipe insulation cover and couplings every 10 feet would cost about $515.00 per ten foot section.
*That’s a savings of $542.00 per ten foot section in this part
of the system or about 51% less than a traditional design.
A vertical stack of a traditional roof drain system is designed
to drain 63,940 square
feet of roof surface at a rainfall
intensity of 3.25 inches per
hour. According to IPC 2000, the required pipe diameter is 12 inches. At this
flow, the pipe would be about 25 percent full and would achieve a terminal
velocity of 24 feet per second. Cast iron pipe for this stack size would
cost about $1,488.00 per 10 foot section.An existing siphonic system design covering the same roof area and rainfall intensity has a calculated stack diameter of 8 inches. The design velocity is 14 feet per second and the pipe is 100 percent full. Cast iron pipe for this stack size costs about $627.00 per 10 foot section.
*That’s a savings of $861.00 per ten foot section for this
part of the system or almost 58 percent less than a
traditional design.
*Based on R. S. Means Construction Cost Data 2005. These cost calculations
are examples from real world applications; your cost may be different.
These are given for illustration only.
