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 ROOF GARDEN ON CHICAGO'S  CITY HALL

 

BY BRUCE ZIMMERMAN

Recently a few members of the Garden Writer Association were given a brief tour of the Roof Garden of the Chicago City Hall. Here is some of what I saw. Roof Gardens have the advantage that they can keep our buildings cooler, save energy, extend the useful life of the roof, and add beauty and usable space often in very inhospitable spaces.

Roof Garden Chicago City Hall The design of the  City Hall Roof Garden required many months of work by environmental engineers, structural engineers, architects, and landscape architects. The plan for the City Hall Roof Garden was

designed so that any of the standard Roof Garden systems could be adapted to this project. Structural considerations, water storage and irrigation, and green roof system layers were specified in the design. The structural capacity

Roof Garden City Hall of the existing roof was one of the most important considerations in the design of this Roof Garden system. The average structural capacity of the roof is approximately 30

pounds per square foot. There are also structural columns that can locally support a considerable amount of additional weight.
The City Hall roof is able to support an extensive system overall, a semi-intensive system over the former skylights, and an intensive system localized over the support columns. The framing of ten former
skylights remains on the roof. These frames are 

Roof Garden Rolling Topography raised 19 inches to 24 inches above the surface of the roof and remain intact. The frames are reinforced with shear studs to allow for additional structural capacity. They will be 

covered entirely with a water proof membrane and will be planted with a semi-extensive roof system. The remaining green areas of the roof are extensive systems tapering to semi-intensive systems, except for the small, localized intensive area over each of the support columns. Varying the amount of an installed lightweight fill/insulation will transition the Roof Garden system thickness and will ameliorate the possibility of kinking the membrane. This will result in a slightly rolling topography.

Roof Garden Raised Wall Retaining walls are used to contain the Roof Garden. The service paths and access to doorways and the parapet wall remain at the existing roof elevation. The existing drain system is incorporated into the design and used for

overflow during heavy storm events, both for the green roof system and the remaining impervious surfaces.
The Chicago City Hall Roof Garden system was designed to store as much precipitation in the growing medium and drain layer as the structural loading capacity will allow. This water storage will slow down and reduce direct discharge into storm sewers resulting in less pressure on the sewer system and improved water quality. On the average, the Roof Garden system will be able to store 1inch of rain.

In Chicago, precipitation averages slightly more than 37 inches per year, which is also approximately the same amount that annually evaporates.Given the wind conditions on the roof, it is likely that there will be periods during the growing season when the plants will be in water Roof Garden Hardy Cactus and Drip Irrigation

deficit. This may stress the plants resulting in decreased functioning in the ability of the Roof Garden to mediate its climate.

Many of the plants specified for the Roof Garden are native to the Chicago region, and therefore adapted to the local climatic conditions. The cultivated plants are bred for dry soil and sunny conditions. Although the plants were chosen for drought-tolerant characteristics, a drip irrigation system has been incorporated into the design to provide a supplemental water source during establishment and dry periods.
Additionally, water from one-third of the penthouse roof is collected in water tanks. The water tanks are located near the penthouse down spouts on the south and north portions of the roof and serve the drip irrigation system. Overflow from the fan rooms is directed into the Roof Garden through down spouts.
Filter Layer. A thin mat filter layer is necessary to prevent the growing medium from leaching into the drain layer. The mat filter also needs to allow roots to grow through the filter layer and into the drain layer 

below.The growing medium consists of a specialized soil mix. The growing medium layer must be able to retain moisture and nutrients to be absorbed and used by the plants. However, the  Roof Garden Rudbeckia

growing medium should be porous enough to avoid build-up of excess water. The growing medium must provide firm anchorage for the plant roots and must supply the plants with essential nutrients - nitrogen, phosphorous, calcium, magnesium and oxygen. This layer should not become muddy or easily compacted, which could lead to acidification of the soil and ultimate degeneration of the plants.

The physical and chemical properties of the growing medium satisfy relevant FLL guidelines for the chosen material (Forschungsgesellschaft Landschaftsentwicklung  Roof Garden Heating Vent and  Climbing Hydrangea

Landschaftsbau E. V., Ausgabe 1995:  The physical     and chemical properties of the growing medium satisfy relevant FLL guidelines for the chosen material (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau E. V., Ausgabe 1995:

Research Association for Landscape Development and Landscape Construction. Guidelines for Planning, Performance, and Maintenance of Vegetated Rooftops). Materials used in the growing medium include: mixture of mineral and  organic soil  Roof Garden Butterfly

components (composts and mulches); soil mixture of humus; and mineral bulk material mixture with a high or low proportion of organic matter. The growing medium weighs approximately 65 lb/cf , whereas regular soil weighs approximately 110 lb/cf.

Roof Garden Monarch Butterfly Roof Garden Bird House

The surface of the growing medium is covered with a biodegradable, one-quarter inch mesh. This material holds the growing medium, prevents loss of water through evaporation, and enhances nighttime condensation during the initial growing season. Once well established, the plants themselves serve this purpose.
In addition, an infrared thermometer is used to 
measure surface temperatures. On August 9, 2001, at 1:45 pm, when the temperature was in the 90's, the following measurements were obtained:

City Hall Roof (paved) 126 - 130°F
City Hall Roof (planted) 91 - 119°F
County Roof (black tar) 169°F

That's at least a 50°F difference between the Roof Garden and a black roof!

Direct Cooling Savings from a Roof Garden

The key features that affect the energy use of this Roof Garden include: an increased layer of insulation under the main roof, an irrigation system to ensure adequate water throughout the cooling season, and a combination of plants and walkways that cover 20,300 square feet of the roof.

Roof Garden Wildflowers The Roof Garden generates direct energy savings through a combination of shading, evapotranspiration effects, and insulation. Direct savings are measured by changes in

City Hall energy use and costs that are attributed to the Roof Garden system.

Planting and higher density ballast are estimated to decrease the amount of direct sun on the total roof by 25%. The design of the Roof Garden is intended to retain and utilize the maximum amount of water needed for the plants. In order to ensure plant viability throughout the summer, the Roof Garden's semi-intensive and intensive zones will be irrigated as necessary to ensure that  Roof Garden Echineacea

the vegetation receives enough water. The projected avoided energy cost is $3,600 per year. The total direct savings are estimated to be 9,272 kWhr per year and the corresponding savings in natural gas for heating are 7,372 therms per year.

Indirect Energy Savings from a Green Roof

Indirect savings are due to the cumulative effect of additional Roof Gardens throughout the city and their effects on ambient outdoor temperature and humidity.  If Roof Gardens eventually cover a significant portion of the urban landscape, reduced air temperatures would be expected in the area immediately above and downstream of the Roof Garden.

Air Quality Benefits

Through a citywide program of green roofs and "greening" of other areas throughout the city, one should expect to see measurable improvements in Chicago's air quality by the reduction of ozone and smog.

The 20,300 square foot garden has 20,000 plants of more than 150 varieties including 100 shrubs, 40 vines and 2 trees. The plants were selected for their hardiness on a roof, where wind and watering are two Roof Garden Wildflower

challenges. The plants require little water and can survive Chicago's seasons. Most are native to the region. This low-maintenance garden relies on a special blend of compost, mulch, and sponge-like ingredients that weighs less than regular topsoil and retains more water.  

Here Is the Plant Material used in this Roof Garden:

QTY. BOTANICAL NAME    COMMON NAME
45

 Achilea millefolium ‘Paprika’

Yarrow
20  Achillea millefolium ‘Heidi’ Yarrow
35  Achillea sp. Schwelienburg’
40  Allium canadense Wild Onion
30  Amorpha canadense Leadplant
125  Anctropogon scopanus Little Bluestem Grass
20  Anemone canadensis Meadow Anemone
60  Anemone patens wolfgangiana Pasque Flower
10  Anemone virginiana Anemone
35  Aquilegia canadensis Canada Columbine
24

 Arabis caucasica ‘Flora Pleno’

Arabis Flora Pleno
24  Artemisia schmidtiana ‘Powis Castle’
30  Artemisia sp. ‘Silver Mound’
24  Artemisia stelleriana ‘Silver Brocade’
30   Asclepias tuberosa Butterfly Weed
37  Asclepias verticilata Whorled Milkweed
50  Aster azureus Sky Blue Aster
25  Aster encoides Heath Aster
25  Aster laevis Smooth Blue Aster 
35  Aster novae-angliae New England Aster 
30  Aster oblongifolius Aromatic Aster  
30  Aster parmtcoides Upland White Aster
20  Aster senecio Silky Aster
20 

 Astragalus canadensis

Milkvetch
40  Baptisia leucophaea Cream Wild Indigo
10  Blephilia ciliate Ohio Horse Mint 
60  Bouteloua curtipendula Side-Oats Gramma
580  Bouteloua gracilis Blue Gramma
485  Bouteloua hirsute Hairy Gramma 
725  Buchloe dactyloides Buffalo Grass
35  Callirhoe involucrata Wine Cups  
60

 Campanula poscharskyana

Serbian Bellfower
25  Campanula rotundifolia Harebell
445  Carex Bicknellii Bicknells Sedge 
105  Carex cephalophora Woodbank Sedge 
100  Carex gravida Sedge
100  Carex grayi ‘Morning Star' Sedge
1060

 Carex pennsylvanica

Pennsylvania Sedge
32  Cassia fasciculata Partridge pea
10  Ceanothus americanus New Jersey tea 
12  Cerastium tomentosum ‘Silberteppich’
15 ChrysanthemumLeucanthemum Ox-Eye Daisy
40  Coreopsis auriculata’Nana’ Coreopsis
15  Coreopsis lanceolata Sand Coreopsis
190  Corydalis flexosa 'Blue Panda" Blue Corydalis
230  Corydalis lutea Yellow Corydalis
440  Danthonia spicata Poverty Oat Grass
15  Desmanthus illinoensis Illinois Sensitive Plant
32  Dianthus allwoodii Allwood Pink 
32  Dianthus alpinus Dianthus
400  Dianthus carthusianorum Dianthus
370  Dianthus deltoides Dianthus
70  Dianthus gratianopolitanus 'Tiny Rubies’
35  Dianthus gratianopolitanus 'Spotty’
20  Dianthus gratianopolitanus Cheddar Pinks 
240  Dianthus plumanus Dianthus
15   Dodecatheon meadii Shooting Star Purple 
85   Echinacea purpurea Purple Coneflower 
20  Echinops bannaticus ‘Blue Glow’
85  Elymus canadensis Canada Wild Rye
85  Elymus villosus Silky Wild Rye
10  Eryngium yuccifolium Rattlesnake Master
35  Euphorbia polychroma Cushion Spurge
400  Festuca amethystina ‘Bronze Giant’
400  Festuca glauca ‘Elijah Blue’ 
32  Gaillardia ‘Kobold’ Blanket Flower
55  Geranium sanguineum Cranesbill  
20 Geranium sanguineum ‘Max Frei’ Cranesbill
30 Geranium sanguineum var. striat Cranesbill
40  Geum  x ‘Borisii’ Geum 
60  Geum triflorum Prairie Smoke 
12  Gypsophila repen‘Rosea’ Creeping Baby’s Breath   
25  Helianthus mollis Downy Sunflower
20  Helianthus rigidus Showy Sunflower
110  Helictotrichon sempervirens Blue Oat Grass 
45  Hemerocalis ‘Anzac’ Daylily
45  Hemerocallis ‘Little Wine Cup’ Daylily   
20  Hemerocallis ’SteIla d’Oro’ Stella d’Oro daylify
20  Heuchera brizoides ’Huntsman’ Coral Bells
45 Heuchera brizoides ‘RodSpangif’ Coral Bells 
10  Heuchera richardsonii Prairie Alum Root
435  Hieracium piloselia Hieracium
100  Hystrix patula Bottlebrush Grass
157  Iberis sempervirens Candytuft  
497  Juncus tenuis Path Rush
45  Knautia macedonia Knautia 
445  Koeleda glauca Large Blue Hair Grass
20  Lavandula angustIfolia 'Munstead’ Munstead Lavender
60   Lavandula angustifolia ‘Hidecote’ Hidecote Lavender
10  Lespedeza capitata Round-Headed Dover
75

 Leymus arenarius

Blue Lyme Grass
30  Liatris aspera  Rough Blazing Star
32  Linens vulgaris  Butter-and-Eggs
47  Liatris cylindracea Dwarf Blazing Star 
25  Lobelia inflata Indian Tobacco 
25  Lychnis’Flottbeck’ Campion
20  Monarcla ‘Cambndge Scarlet Bergamot
10  Monarda fistulosa Wild Bergamot
47  Opuntia humifusa Prickly Pear Cactus
20  Organum vulgare Basil 
110   Panicum leibergli Panic Grass
110

 Panicum leibergii

Prairie Panic Grass
65  Panicum virgatum Switch Grass 
45  Papaver orientate Poppy
20  Papaver orientale ‘Brilliant' Poppy 
10  Parthenium integrifolium Wild Quinine 
10  Penstemon digitalis Foxglove Beard Tongue
10  Penstemon paladus Pale Beard Tongue
65  Penstemon ‘Prairie Dusk’  
70  Penstemon sp. ‘Utahensis’
162  Petalostemum candidum White Prairie Clover 
255  Petalostemum purpureum Purple Prairie Clover 
295  Petrorhagia saxifrage Petrorhagia
112  Phlox bifida Sand Phlox 
165  Phlox sp. Emerald Cushion Blue’ 
12

 Phlox sp.

‘Emerald Pink’ 
15

 Polemonium reptans

Jacobs Ladder
15  Potentilia argute Prairie Cinquefoil
10   Prenanthes alba Lion’s Foot 
25  Ranunculus rhomboides Prairie Buttercup
20   Ratibida columnifera Mexican Hat 
15   Ratibida pinnate Yellow Coneflower
10   Rosa carolina Pasture Rose
45  Rudbeckia subtomentosa Sweet Black Eyed Susan
215  Ruellia humilis Hairy Ruellia 
75  Salvia nemorosa ‘Mainacht’ Purple Salvia
25  Scutelana parvula Small Skullcap 
320  Sedum acre Sedum
655  Sedum album White Sedum
540  Sedum floriforum Sedum
620  Sedum hybrida Sedum
95  Sedum kamtschaticum Sedum 
380  Sedum ‘Mochren’ Sedum
450  Sedum reflexum Sedum 
720  Sedum sexangulare Orange Stonecrop 
185  Sedum spectible ‘Matrona’ Matona Sedum 
520  Sedum spurium Sedum
185  Sedum Vera Jameson’ Vera Jameson Sedum 
190  Sempervivum arachnoideum Hens And Chicks 
450  Sempervivum-Hybrid Sempervivum 
15  Smilacina racemosa False Solomons Seal 
15  Solidago flexicaulis Broad Leaf Goldenrod 
20   Solilago juncea Early Goldenrod 
25  Solidago nemoraiis Old-Field Goldenrod 
20  Solidago speciosa Showy Goldenrod 
15  Solidago ulmifolia Elm-Leaved Goldenrod 
110  Sporobolus heterolepis Prairie Dropseed
25

 Stachys byzantina

‘Helene von Stein’ 
125  Thymus praecox ‘Coccineus’ 'Coccineus'
112  Thymus praecox ‘Albiflorus ‘Albiflorus’
625  Thymus serpylium Thymus
10  Tradescantia  Common Spiderwort
30  Tradescantia  ‘Zwanenbi’ Spiderwort
432  Trifolium arvense Rabbitfoot Dover 
50  Viola saroria Common Blue Violet
1  Crataegus crusgalli Cockspur Hawthorn
1  Malus ioensis Prairie Crabapple
5  Diervilla Ionicera Dwarf Bushhoneysuckle 
24  Juniperous chinensis Sea Green’ Sea Green Juniper
11  Kerria japonica Japanese Kerria 
24  Microbiota decussata Russian Arborvitae 
26  Miscanthus sinensis  var.gracillin Maiden Grass
11  Rhus aromatica’Gro-low’ Gro-Low Sumac 
11

Symphoricarpus alba 

Snowberry
11  Celastrus scandens  American Bittersweet
22   Parthenoassus tricuspidata Boston Ivy 
 Clematis virginiana Virgins Bower

This wide variety of plant material is a good indication that we all have access to the plants that are suitable for Roof Gardening just about anywhere we live and work.

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