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Six Inch Roof Drain *Would two 3” roof drains be the equivalent of a singlesix inch drain?r d/2A π r2r 3”/2 1 ½”2-3” roof drains have an area of 3.14 x (1 ½” x 1 ½”) 7.065 sq. in. x 2 drains 14.13 sq. in.r 6”/2 36” roof drain has an area of 3.14 x (3 x 3) 28.26 sq.in.Clearly, 2-3” roof drains will not provide the equivalentdrainage of a single 6” roof drain.54

Could overflow scuppers be usedto provide secondary drainage?Closest distance from drain to parapet, 42.5’42.5’ x tangent of the angle x 12 10.62”If the primary drain were to be blocked, thedepth of water above the drain would be 10 5/8”before the water would reach the parapet.Maximum depth of water allowed - 5” in theHVHZ, less outside HVHZ.As a result, overflow scuppers could not be used,unless the deck was specifically designed for thisload.A plumbed secondary drain would then benecessary for this roof.55

The rain load at the maximumdepth would be 55 lbs./sq. ft.Water weighs 5.2 lbs. per square foot,per inch of accumulation.42.5’10.6”¼ in./ft. slope(1.1930)56

How are the overflow scuppersand drains to be sized?Secondary roof drain systems shall besized the same as the primarydrains(P1107.3)Overflow scuppers shall be sized toprevent the depth of water from exceedingthe roof design.Over sizing the secondary system is nolonger required.Secondary flow can be no less than theprimary. (ASCE 7-02 Section 8.2)57

Secondary Roof DrainagePiping must be separate from the primary. (P1107.2)Secondary must discharge above grade. (P1107.2)Discharge must be in a location where it would benormally observed. (P1107.2)Height placement of the invert of the secondary drainmust be above the depth of water of the primary drain atits design flow. This requirement seems to be58implied by the Code, though not explicit.

(Example #1 Cont.) A secondarydrain will be required.Since it is established that based on the roofdesign, a secondary scupper cannot be used.Sizing of the secondary drain is now the sameas for the primary drain.No over sizing of the secondary system isrequired.59

Plumbing design for Example #1Based on the projected area and the rainfallrate, Table P1106.2 establishes the requireddrain sizes.Primary drain – 6” diameter.Secondary drain – 6” diameter.Sizing of the drains, complies with the FBC.Does this plumbing design comply with thestructural rain load requirements of the Code?60

Plans Examiners responsibilitiesPlumbing will verify primary drain and secondarydrain sizes based on the rainfall rate and theprojected areas.Even if the drains and overflows are sized perthe Plumbing Code, the installation still may notcomply with the rain load requirements of theBuilding Code.Structural must verify the placement of thesecondary drainage to ensure that the depth ofwater that could accumulate, anywhere on theroof, in excess of the design.Who looks at primary and secondary scuppersizes and placement?61

Reminder of the applicable CoderequirementsNo more than 5” of water accumulationwhen not designed per 1616.1.(1617.2)Depth caused by hydraulic head neededto cause flow through the secondarydrain shall be included in determiningthe load. (1617.2) (1611.1)62

The hydraulic head of waterTable C8-1 is in ASCE 7-02Hydraulic head is the depth of water above adrain needed to cause the design flow.Hydraulic head is considered zero for a roofthat drains over the eaves.Table C8-1 establishes heads of water forvarious sized drains and scuppers.The table is based on rainfall in gallons perminute, not inches per hour.The FBC uses rainfall rates in inches per hour.The first step in determining hydraulic head isto convert inches per hour of rainfall intogallons per minute.63

Flow rate conversion constantOne inch of rainfall per hour, over onesquare foot, equals 144 in.3 of water.There are 231 in.3 in one gallon of water.First, determine the accumulation ofwater in gallons per hour, per inch ofrainfall, by dividing 144/231.Then determine gallons per minute, bydividing the result by 60.(144/231)/60 0.0104 conversionconstant.64

Converting inch/h to GPMQ 0.0104 AiA roof area served by a singledrainage system in square feet.i design rainfall intensity as specifiedin the code having jurisdiction.Q flow rate out of a single drainagesystem, in gallons per minute.0.0104 is the conversion constant.65

Example #1, roof slope is ¼in./ft. to a single interior drain.90’85’3’ High Continuous Parapet66

Example #1 hydraulic head atthe primary drainTo establish rainfall in gallons per minute.Multiply square footage, by the rainfallrate, then multiply by the conversionfactor. (Q 0.0104 Ai)7650 x 5 x 0.0104 398 gal/min.Refer to ASCE 7-02 Table C8-1.Hydraulic head for a 6” drain, that mustdrain 398 gal/min., falls between a 3” and3.5” hydraulic head.Interpolation is necessary to establish an67exact hydraulic head.

FLOW RATES IN GPM VERSUS RESULTING HYDRAULIC HEADHYDRAULIC HEAD, INCHESDrainage122.533.544.55784” dia. drain801701806” dia. drain1001902703805408” dia. drain1252303405608501,1001,1706’ wide channelscupper1850709011514016719432139324” wide �x 4” closed scupper1850709011514016717723125324”x 4” closedscupper722002803604605606687089241,0126”x 6” closed scupper1850709011514016719430334324”x 6” closedscupper722002803604605606687761,2121,372

Interpolation procedure to establishthe hydraulic head at the primary drain3 (398-380/540-380)(.5) 3.06”In order for the 6” drain to handle thenecessary flow, there will be a depth of 3.06”of water above the primary drain at its peakdesign flow.The secondary drain is typically placed about1/2” above the depth of water above theprimary at its design flow.In this way, the secondary does not activateunder peak design flow and debris is notcarried into or block the secondary.69

Establish the hydraulic headabove the secondary drainHydraulic head for a 6” drain, that must drain398 gal/min., falls between a 3” and 3.5”hydraulic head.3 (398-380/540-380)(.5) 3.06”If the primary drain could not handle the peakflow for any reason, the secondary wouldaccommodate the design flow.At the design rainfall rate, there would be adepth of water 3.06” above the secondary drain.70

FLOW RATES IN GPM VERSUS RESULTING HYDRAULIC HEADHYDRAULIC HEAD, INCHESDrainage122.533.544.55784” dia. drain801701806” dia. drain1001902703805408” dia. drain1252303405608501,1001,1706’ wide channelscupper1850709011514016719432139324” wide �x 4” closed scupper1850709011514016717723125324”x 4” closedscupper722002803604605606687089241,0126”x 6” closed scupper1850709011514016719430334324”x 6” closedscupper722002803604605606687761,2121,372

Are the structural rain loadrequirements of the Code met?Depth above the primary 3.06”Placement of the invert of the secondary 4”above the low point.Depth of water above the secondary 3.06”3.06” .44” 3.06” 6.56”This depth of water exceeds the maximum 5” forthe HVHZ rain load and the 20 psf live loadelsewhere.Even though the plumbing requirements are met,the structural requirements are not. What are theoptions?Enhance the design of the structure, more drains,72more overflows or both.

Establishing the rain loadR 5.2 (ds dh)(ASCE 7) and (Equation 16-37)dh Additional depth of water on the undeflectedroof above the inlet of secondary drainage system atits design flow (i.e., the hydraulic head), in inches(mm).ds Depth of water on the undeflected roof up tothe inlet of secondary drainage system when theprimary drainage system is blocked (i.e., the statichead), in inches (mm).R Rain load on the undeflected roof, in psf(kN/m2). When the phrase “undeflected roof” is used,deflections from loads (including dead loads) shall notbe considered when determining the amount of 73rainon the roof.

Example #1 Rain Load3.06” .44” 3.06” 6.56”R 5.2(3.5 3.06) 36.7 psfMaximum allowable in the HVHZ is26 psf. Unless the roof has beendesigned for additional rain load.Outside the HVHZ design loadscould be as low as 12 psf.74

100 year 5 minute stormWhy doesn’t the Code require a roofdrainage design that would handle themore conservative 100 year, five minutestorm?The Code represents a minimumstandard, but :In effect the Code has made provisionsfor the chance that a intense burst ofrainfall will be experienced.75

100 year 5 minute storm example.60’100’3’ High Continuous Parapet

100 year 5 minute exampleTypical Drainage Design:Roof area60’ x 100’/2 drains 3000 ft.2Rainfall rate for Miami 5”Read down the 5” rainfall column of TableP1106.2 until a projected roof area figure thatis equal to or greater than the sample roofarea is found. Then read across the Table tofind the required drain size.A 4 drain is found to be appropriate.77

TABLE P1106.2SIZE OF VERTICAL CONDUCTORS AND LEADERSDiameterofLeader(inches)HORIZONTALLY PROJECTED ROOF AREA(square feet)Rainfall rate (inches per ,60017,300 11,530 8,6506,9205,765654,00027,000 17,995 13,500 10,800 9,0008116,000 58,000 38,660 29,000 23,200 19,315

Could overflow scuppers be usedto provide secondary drainage?No.Crickets would be installed and typicallyat a slope of ½ : 12 if the roof is slopedat ¼ : 12.Distance of drain to parapet is 25’.So the cricket height at the parapetwould be 12.5”.79

How are the drains to be sized?Secondary roof drain systems shall besized the same as the primary drains(P1107.3).Over sizing the secondary system is nolonger a Code requirement.80

Plumbing design for 100 year, 5minute storm exampleBased on the projected area and the rainfallrate, Table P1106.2 establishes the requireddrain sizes.Primary drains – 4” diameter.Secondary drains – 4” diameter.Sizing of the drains, complies with the FBC.Does this plumbing design comply with thestructural rain load requirements of the Code?81

Establish the hydraulic headTo establish rainfall in gallons per minute.Multiply square footage, by the rainfall rate,then multiply by the conversion factor. (Q 0.0104 Ai)3000 per drain x 5 x 0.0104 156 gal/min.Refer to ASCE 7-98(02) Table C8-1.Hydraulic head for a 4” drain, that must drain156 gal/min., falls between a 1” and 2”hydraulic head.Interpolation indicates 1.85”, set invert ofsecondary at 2.5”.82

FLOW RATES IN GPM VERSUS RESULTING HYDRAULIC HEADHYDRAULIC HEAD, INCHESDrainage122.533.544.55784” dia. drain801701806” dia. drain1001902703805408” dia. drain1252303405608501,1001,1706’ wide channelscupper1850709011514016719432139324” wide �x 4” closed scupper1850709011514016717723125324”x 4” closedscupper722002803604605606687089241,0126”x 6” closed scupper1850709011514016719430334324”x 6” closedscupper722002803604605606687761,2121,372

Are the structural rain loadrequirements of the Code met?Yes.The primary drains have a hydraulichead of 1.85”.Likewise, the secondary drains have ahydraulic head of 1.85”1.85” 1.85” .65” separation 4.35”maximum depth of water at the designrainfall rate complies with the HVHZstructural requirements.84

How does this system handle the100 year 5 minute storm?It is considered that the primary andsecondary drains would work incombination to handle the shortduration intense rainfall of 10” that the100 year 5 minute storm wouldproduce.85

Explanation3000 per drain x 10 x 0.0104 312 gal/min.On our example roof the primary andsecondary drains on each section of the roofwould accommodate the 312 gallons perminute that would be produced for thatsection.Each drain is handling 156 gpm with a 1.85”head. 156 156 is about equal to 312 gpm.A depth of less than 5” is maintained.86

Example #2, Monoslope, is to bedrained by two primary scuppers100’3’ HighContinuousParapet50’¼” ¼” Slope5000 ft.287

Size the two primary scuppersConvert rainfall to gallons per minute.5000/2 scuppers x 5” rainfall x 0.0104 130 gpmFor sizing of scuppers see Table 1106.7.This Table shows projected areas drained.ASCE 7-02 Table C8-1 shows gallon perminute per depth of hydraulic head.Over sizing scuppers is prudent because ofthe low cost of installation involved.Its more difficult for a large scupper to beobstructed.88

Sizing ScuppersIn the past a designer might start toexperiment with scupper sizes by taking aguess and then performing the calculations.In the 2004 edition of the FBC Plumbing thereis a new Table 1106.7.This table provides projected roof areas in sq.ft. for various sizes of scuppers with anassortment of hydraulic heads.It can be seen that it is the width of thescupper that primarily establishes the flow.89

Scupper Rules of ThumbLength of the scupper should be 4times the hydraulic head.In closed scuppers, the clear heightabove the head should be 2”.It is prudent to design scuppers forlesser duration more intense rainfallrates.90

Table 1106.7The table is prepared based on Table C8-1 ofASCE.This table is established using an assumedrainfall rate of 5” per hour.Instructions are given to calculate projectedroof areas for rainfall rates other than 5” perhour.This table is accurate for primary as well assecondary scuppers.It provides a good starting point for scuppersizing.91

TABLE 1106.7SIZING SCUPPERS FOR A 5-INCHPER HOUR RATE OF RAINFALLHEADININCHESHORIZONTALLY PROJECTED ROOF AREA (SQUARE FEET)LENGTH OF WEIR IN 942692358953847179897410769For SI: 1 inch 25.4 mm.Note: To adjust this table for other than a 5-inch design rain fall rate multiply the square footage on the tableby 5 then divide by the local design rain fall rate.Example: For 4 inches of design rainfall rate, a 4-inch long scupper with a 1- inch head would accommodate287 square feet. (230 5) 4 287.

Example #2Each primary scupper must drain 2500 SF ofprojected roof area.In the HVHZ, typically we have 5” of waterdepth that can be accommodated by the deck.A .5” separation between primary and secondaryscuppers would allow compliance with P1107.2.Refer to Table 1106.7 and find a hydraulic headof 2” and read across until a column is foundcontaining a projected roof area of at least 2500SF.93

TABLE 1106.7SIZING SCUPPERS FOR A 5-INCHPER HOUR RATE OF RAINFALLHEADININCHESHORIZONTALLY PROJECTED ROOF AREA (SQUARE FEET)LENGTH OF WEIR IN 942692358953847179897410769For SI: 1 inch 25.4 mm.Note: To adjust this table for other than a 5-inch design rain fall rate multiply the square footage on the tableby 5 then divide by the local design rain fall rate.Example: For 4 inches of design rainfall rate, a 4-inch long scupper with a 1- inch head would accommodate287 square feet. (230 5) 4 287.

Example #2In this case the primary scupperselected is 16” wide and will develop ahydraulic head of 2”.The secondary scupper will also be 16”wide and will be placed with its invertlocated 3” above the low point of thedeck.In this case the 5” maximum depth ofwater is not exceeded at the designrainfall rate.95

Design flow rate 130 gpm16”4”16”4”2” hydraulic head at the primary scupperInvert of the secondary placed 3” above the deck.(1” clearance above depth of water at primary)2” hydraulic head develops at the secondary, if the primarywere to be obstructed.96A maximum depth of 5” would not be exceeded.

Interpolation of Table 1106.7How can flow through a scupper size otherthan those shown in Table 1106.7 bedetermined?An 18” wide scupperUse flow figures for a20” wide scupper andmultiply those flowsby 18/20.18/20(1153) 1037.7 SF97

TABLE 1106.7SIZING SCUPPERS FOR A 5-INCHPER HOUR RATE OF RAINFALLHEADININCHESHORIZONTALLY PROJECTED ROOF AREA (SQUARE FEET)LENGTH OF WEIR IN 942692358953847179897410769For SI: 1 inch 25.4 mm.Note: To adjust this table for other than a 5-inch design rain fall rate multiply the square footage on the tableby 5 then divide by the local design rain fall rate.Example: For 4 inches of design rainfall rate, a 4-inch long scupper with a 1- inch head would accommodate287 square feet. (230 5) 4 287.

Example #3Roof Dimensions 100’ x50’Primary Drainage System TwoRoof DrainsSecondary Drainage System TwoClosed Scuppers

Size the primary drainage systemfor Example #3Roof area100’ x 50’ 5000 ft.25000/2 roof drains 2500 SF per drainRainfall rate for Miami 5”Read down the 5” rainfall column of TableP1106.2 until a projected roof area figure thatis equal to or greater than the sample roofarea is found. Then read across the Table tofind the required drain size.A 4” drain is found to be appropriate.100

TABLE P1106.2SIZE OF VERTICAL CONDUCTORS AND LEADERSDiameterofLeader(inches)HORIZONTALLY PROJECTED ROOF AREA(square feet)Rainfall rate (inches per ,60017,300 11,530 8,6506,9205,765654,00027,000 17,995 13,500 10,800 9,0008116,000 58,000 38,660 29,000 23,200 19,315

Establish the hydraulic head atthe primary systemTo establish rainfall in gallons per minute,multiply square footage, by the rainfall rate,then multiply by the conversion factor. (Q 0.0104 Ai)2500 x 5 x 0.0104 130 gal/min.Refer to ASCE 7-02 Table C8-1.Hydraulic head for a 4” drain, that must drain130 gal/min., falls between a 1” and 2”hydraulic head.Interpolation is necessary to establish anexact hydraulic head.102

FLOW RATES IN GPM

When roof drains are required. (1514.4.2) When other means of drainage of overflow water is not provided. (1503.4.3) When roof perimeter construction extends above the roof, where water would be entrapped if the primary drains allow build-up for any reason. (P1107.1) When parapets or curbs are constructed, water build-up in excess of that .