The Secret Life of Roof Drains

by

Ted Sheridan

The Secret Life of Roof Drains

by

Ted Sheridan

The Unassuming Roof Drain

The Link Between...

The Link Between...

Mother Nature

Human Nature

The Link Between...

Mother Nature

Human Nature

1. And the Skies Opened…

And the Skies Opened…

And the Skies Opened…

• Description of precipitation events:

• 1. INTENSITY

And the Skies Opened…

• Description of precipitation events:

• 1. INTENSITY

• 2. DURATION

And the Skies Opened…

• Description of precipitation events:

• 1. INTENSITY

• 2. DURATION

• 3. FREQUENCY

Precipitation and Climate Change

• Ontario has had the same precipitation design

criteria for the past 50 years!

• Updated marginally in 2010

DESIGN DATA NBCC 1961 NBCC 2010

1 day, 50 year return period (Ottawa)

90 mm 92-96 mm

15 minute, 10 year return period (Ottawa)

23 mm 23-25 mm

Precipitation and Climate Change

Toronto Star, Tuesday, July 10, 2013

“Monday's rains, measured at 126 mm for the day,

set an all-time record for Toronto Pearson Airport,

breaking a record that had stood for nearly 59

years. What's more amazing is that this new record

was set in just 7 hours, whereas the previous record

was set over 22 hours of rain, when Hurricane Hazel

hit the city on October 15, 1954.”

NBCC 2010 says 92 to 108 mm!

Roof Drainage – Designing at Cross Purposes

Roof Drainage – Designing at Cross Purposes

Retention on Roof

Evacuation off Roof

Roof Drainage – Designing at Cross Purposes

Retention on Roof

• Reduce development

impact

• Reduce pipe sizes

• Reduce pipe pressures

• Irrigate rooftop plants

Roof Drainage – Designing at Cross Purposes

Evacuation off Roof

• Reduce leak severity

• Reduce membrane /

insulation deterioration

• Reduce structural loads

• Comply with warranty

• Improve rooftop safety

Roof Drainage Design 101 - Loads

• Hydraulic Load (L) = 15 minute rainfall intensity

per NBCC (mm) multiplied by effective drainage

area (m2)

• Example:

Roof Drainage Design 101 - Loads

All this Area

Roof Drainage Design 101 - Loads

½ this Area

Roof Drainage Design 101 - Loads

None of

this Area

Roof Drainage Design 101 - Loads

Horizontal

Projection of this

Area (into gutter)

Drainage Design Aids

• Sloped structural deck

• Tapered insulation / crickets / saddles

• Drain at true low point (in sump)

• Multi-level drainage (grooved XPS or drainage

composite)

• OBC only limits slope in Part 9

Retention Design Aids

• Storage capacity in system (trays or mat)

• Absorption by growing medium

• Deliberate ponding / drain undersizing

• Flow control weirs

2. Close up and Personal with ArDee

Close up and Personal with ArDee

Roofer

Plumber

ArDee - The Early Days

• Roof drains are designed and specified in new

construction by mechanical engineers.

• Then they are among the first mechanical items

shipped to site, and are usually dumped

unceremoniously on the super’s doorstep and

get mistreated until the roofer arrives to install

them with his roof system.

• The roofer or roof consultant then complains

that the roof drain doesn’t have all the required

features – and fingers start pointing.

Anatomy of a Roof Drain

Sump

Receiver

Anatomy of a Roof Drain

Drain Bowl

Anatomy of a Roof Drain

Clamping

Ring

Anatomy of a Roof Drain

Strainer

Anatomy of a Roof Drain

Flow Control

Weir

Flow Control Weirs – Design Basis

• Have a published flow-height characteristic

• Allow flow to be limited but not really controlled,

since the height of water depends on

precipitation IDF

Flow Control Weirs – Design Basis

Anatomy of a Roof Drain

Ballast Guard

Anatomy of a Roof Drain

Anti-Backflow

Device

Anatomy of a Roof Drain

Underdeck

Clamp

Roof Drainage Design 102 – Inlet Capacity

• Note that there is no sizing of our friend ArDee

the roof drain – STRANGE SINCE A LACK OF

CAPACITY HERE WILL AFFECT DOWNSTREAM

FLOW!

• Drain inlet size is generally taken from the

designed size of the leader below

The Revenge of ArDee

• Are all 76 mm drains alike in their inlet flows?

Why isn’t this knowledge part of the design

process?

• Testing conducted by American Society of

Plumbing Engineers in 2011 found huge

differences in inlet performance

The Revenge of ArDee

ArDee’s Workplace Anxieties

• Roof drains have many duties, and therefore

many ways to fail:

• 1. Failure of anti-backflow seal

• 2. Failure of membrane flashing on drain flange

• 3. Blockage at inlet by debris

• 4. Hydrostatic pressure buildup due to head

• 5. Temperature or mechanical stresses

• 6. Condensation at underside of drain bowl

ArDee’s Workplace Anxieties

ArDee’s Workplace Anxieties

ArDee’s Workplace Anxieties

ArDee’s Workplace Anxieties

3. It’s All Downhill From Here (?)

It’s All Downhill From Here (?)

Design Tenets for Stormwater Drainage Systems

• Stormwater drainage systems are not expected

to be pressurized i.e. open channel flow in pipes

• Pipe connectors are not rated for high

pressures.

• Development agreements may limit rate and

amount of stormwater discharge –green roofs,

retention ponds and storage tanks are being

considered.

Roof Drainage Design 103 – Leaders and

Drains

• The Chezy-Manning equation

Roof Drainage Design 103 – Leaders and

Drains

• Sample Chart from National Plumbing Code

Roof Drainage Design 103 – Leaders and

Drains

• Use hydraulic load previously calculated to size

‘storm building drain or sewer’ (horizontal

element) using table in Plumbing Code – must

select a design slope

• Size ‘leader’ (vertical element) using table in

Plumbing Code

• Pipe sizes never decrease moving downstream –

hydraulic loads are additive

Issues in Building Stormwater Management

• The designer assumes that downstream

capacity is adequate -- not within his control

• Added downstream loads from new land uses,

building additions, system blockages

• Maintenance is underappreciated – camera

inspection and routing out are important

• No municipal connection = headaches

• Combined storm / sanitary lines

Stormwater System Failures

• Linear system – a lack of capacity at one

location affects everything upstream

• Standard plumbing connectors have limited

resistance to hydrostatic pressure – backups

are dangerous

• ‘Thrust Block’ issues at high flows

• Unexpected results to system pressurization –

backflow in fixtures such as standpipes, floor

drains, lower roofs

Stormwater System Failures: Example 1

• Large new expensive highrise condo on banks of

a major river in Toronto (sloped property)

• Development limits on discharge from sloping

site to river – build a cistern!

• Main lobby level one floor below street (grade

sloping back to river – rear walkout)

• Water collection cistern beside parking garage

at lobby floor level . Building storm drains run

along ceiling of lobby to empty in top of cistern

Stormwater System Failures: Example 1

Stormwater System Failures: Example 1

Stormwater System Failures: Example 1

• SO WHAT HAPPENED?

• Cistern filled up and backup continued

upstream, across horizontal line above lobby,

and up into vertical leaders

• 6 of 8 leader pipe connectors failed at

vertical/horizontal turn at ceiling of lobby.

• Massive flooding of lobby and common rooms in

floor below. 7-figure construction claim.

Conclusion

Conclusion

Conclusion