MODULE #06: Common Roof Assemblies

Learning Outcomes

"By the end of the module, Learners will be able to:  

• Identify 3 different types of commonly used roofs in commercial construction
• Identify advantages and disadvantages for PMR and Green Roofs
• State the required components for PMR and Green Roof roof assemblies and the functions they perform.
• know the difference between Extensive and Intensive green roofs

Contents

1. The Protected Membrane Roof

2. Green Roofs

1.  The Protected Membrane Roof

ADVANTAGES:

membrane is protected from damage resulting from the effects of sun, weather, wear, impact

membrane stays at a relatively constant temperature reducing freeze / thaw cycling

moisture membrane triples as a vapour retarder and air barrier

DISADVANTAGES:

may float if drainage and ballasting is not planned

susceptible to standing water build-up if not sloped and drained properly

FUNCTIONAL COMPONENTS:

1. STRUCTURAL DECK: 3 variations: site concrete, precast concrete, GWB on steel decking on OWSJ

2. AIR BARRIER, VAPOUR BARRIER,  WATER BARRIER: Hydrotech's monolithic rubberized asphaltic membrane 6125

3. THERMAL BARRIER: 100mm styrofoam roofmate DC extruded polystyrene roof insulation (DC - drainage channels)

4. FIRE PROTECTION: 2 storey Group D  fire rating not required if sprinklered

5. AESTHETICS: flat roof is not visible.  Aesthetic choice is the coping.

For comparison of conventional and protected roofs see the entire slide show on the DOW Chemical Website.

http://building.dow.com/styrofoam/na/pro-canada/tools/benefits/slideshow.htm

OPTIONAL / ADDITIONAL COMPONENTS:

pre-cast conc. deck pavers as ballast.

LINKS: 

 insulation: www.dowbuildingmaterials.com

membrane technology: www.hydrotechusa.com    or   tremco or   www.sopremaworld.com

2.     Green Roofs 

ADVANTAGES of GREEN ROOFS: 

1)    energy efficiency:

             soil acts as insulation reducing heat loss therefore lowering energy costs due to heating

             absorb and or reflect solar radiation reducing heat gain therefore lowering energy costs due to cooling

2)    manage storm water  on the roof surface therefore reducing storm water run-off in municipal watermains

3)    adds valuable green space open space amenity area

4)    creates habitats for plants, birds, insects

5)    air/vapour/moisture membranes are one and the same

6)    wet soil helps to slow the process of spread of fire

7)    membrane is protected from:

            A) wear due to pedestrian traffic,

            B) ultraviolet degradation due to exposure to sunlight

            C) temperature fluctuations so it's life is extended

8)    Soil acts as a sound insulator for quieter buildings

9)    do not underestimate enhancement of resale value and marketability

10)    low profile extensive gardens can replace the ballast on a typical inverted roof

11)    green roofs filter the air and provide habitat for some species

DISADVANTAGES: 

1)     more expensive than conventional roofs

2)    gardens need continual maintenance until plants are established

3)    intensive planting is complex and may require irrigation

4)    wet soil is heavy and may require more substantial structure

5)    in retrofit situations there are limitations of access and existing structural capacity

ENVIRONMENTAL IMPACT AND SUSTAINABILITY

TORONTO, June 3, 2006 (GLOBE-Net) - Researchers at the University of Toronto have completed a ‘life-cycle assessment’ of a green roof in order to confirm that ecological rooftops can provide significant environmental benefits. In the case-study undertaken, annual energy savings from a simple green roof amounted to 1.2%, summer cooling needs were reduced by 6%, and water runoff and pollution were reduced.
 

• environmental impacts of the building were reduced by between 1.0 and 5.3%
• benefits afforded to wider environment included 5% decrease in eutrophication (the excessive accumulation of nutrients) and abiotic depletion (loss of natural environmental resources and minerals)
• 2.5% drop in potential human and aquatic toxicity

Conversion of available 50 million sq.m. of rooftop in Toronto to green roofing would mean:

• reduced storm-water runoff = infrastructure savings of $79 million
• erosion control = savings of $25 million
• pollution avoidance = $13 million
• an additional three ‘beach days’ in the city each year
• reduced energy demands during peak cooling periods = save $68 million
• reduced urban heat island effects
• reduced air pollution
• city beautification

Extensive vs Intensive Roofs

Top is an extensive roof

Extensive and intensive roofs can be differentiated by the depth of the growing medium, the choice of plants and their respective costs.

Lower roof is an example of an extensive roof

Extensive Roofs

Shallower soil ( up to 150 mm), therefore lower weight, lower cost, lower plant diversity, minimal maintenance

Extreme desert -like microclimate requires very hardy plant material (alpine dryland or indigenous varieties)

Tend to be less attractive as a result

Intensive Roofs

Deeper soil (200mm- 600mm) therefore more weight, greater plant diversity therefore more cost

Can develop a more complex ecosystem

Requires watering / irrigation

With careful design and expert consultation these spaces can be very attractive.

Accessible vs Inaccessible Roofs

If a green roof is to be used as an amenity space by the general public then a number of issues come into play. 

Firstly, the designer must consider the method of bringing the public up there.  If there is a fire alarm sounded while it is occupied the people must be safely exited to the ground plane.  The access therefore must be designed also as an exit.  If the roof is unoccupied then a service hatchway will suffice.  Our roofs will be accessible. Begin to plan the access/egress....

Secondly, if the people using the roof can walk to edge then the roof parapet must be designed as a guard. ( ie 1070mm high)

Special design considerations:

roof must be designed for exiting

roof must be designed to carry the extra dead and live loads (increased structural cost)

vegetation layer

soil / substrate layer

filter fabric

drainage layer

moisture retention mat

rigid insulation

root barrier protection course

roof membrane

structural roof deck

FUNCTIONAL COMPONENTS:

1. STRUCTURAL: 3 assemblies assigned: site cast concrete, pre-cast concrete, steel deck on OWSJ

2. AIR BARRIER, VAPOUR BARRIER, WATER BARRIER: monolithic rubberized asphaltic membrane (eg. Hydrotech's MM6125-EV)

3. THERMAL BARRIER: 50mm rigid polystyrene insulation

4. FIRE PROTECTION: not required if building is sprinklered

5. AESTHETICS: attractive garden amenity spaces can be landscaped beautifully to create an amenity space

OPTIONAL / ADDITIONAL COMPONENTS:

vegetation layer: hardy plants with shallow root systems, resistant to direct solar radiation, drought frost wind resistant, regenerative.

soil / substrate layer: mixture 75% mineral soil 25% organic soil

root barrier: Root Stop WSF 40 is loose laid over Hydroflex 30 barrier embedded into warm tacky MM6125-EV rubberized asphalt membrane

filter fabric: non-woven, non-rotting polypropylene geotextile filter sheet to keep the soil out of the drainage layer

water retention mat: non-rotting polypropylene fibre (Hydrotech SSM45)

drainage layer: Floradrain FD40

LINKS: 

Garden roof

Canada Green Building Council

The City of Chicago Experience

The City of Toronto Experience

Green Roof Designers, Installers, Consultants ZinCo or Tremco

Green Roof Industry Association with links to suppliers, articles case studies

www.greenroofs.ca

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