12/9/2004;Multnomah County Green Roof 1

 

Multnomah Building Green Roof 

501 SE Hawthorne Blvd. 

Project Summary 

Introduction 

Multnomah County considered a number of options in 2000 when it became clear the leaky roof on the 

Multnomah Building was in need of replacement. With the encouragement of Portland’s Bureau of 

Environment Services (BES), the County evaluated the potential for an ecoroof and ultimately decided to cover 

most of the west-facing roof with a vegetated system. 

The County Commissioners endorsed the project because of the longevity of the green roof, the predicted costeffectiveness over the long term, and the environmental benefits. They also supported the project because it 

offered a unique opportunity to promote the technology. The 12,000 sq. ft. ecoroof is accessible and open to 

the public and is visible from many nearby offices. 

The project’s demonstration value led to strong financial 

support from outside parties: the Department of Environmental 

Quality (DEQ) contributed $75,000 of Federal funds for nonpoint source pollution control, and BES contributed $50,000 

from the Willamette Stormwater Control Program1

. Tremco 

Roofing provided certain components at no cost, and BES and 

Portland State University committed resources for monitoring 

the performance of the roof. 

The project goal was to create a landmark demonstration 

ecoroof with amenities such as a stone terrace and educational 

signage. The focus of this report is limited to the design, 

construction, and cost of the vegetated ecoroof. 

 1

 Portland’s Bureau of Environmental Services implemented the Willamette Stormwater Control Program in 2001. The Program 

offered financial grants and technical support for a series of projects to retrofit existing commercial properties with stormwater 

controls incorporating green technologies. The Program recruited these demonstration projects in order to research the feasibility, 

cost and performance of commercial stormwater retrofits in the area served by the combined sewer. 

Project Type: Commercial / Industrial retrofit - demonstration project 

Technologies: Green roof (ecoroof); 12,000 sq. ft. (area); 6-in. deep growth medium (soil) 

Major Benefits: • Roof longevity - the ecoroof should double the life of the roof. 

• Stormwater management - there will be a substantial reduction in runoff from the roof, with 

improvements in the quality of the runoff (reduced pollutant loads; lower temperature). 

• Improvements to the urban environment - the new landscape adds green space to the urban 

environment and enhances the aesthetic appeal of the property. 

Cost: • Total project cost: $343,000 (includes design, terrace construction, etc.) 

• Ecoroof construction cost: $179,801 ($15 per sq. ft.) 

• BES provided a $50,000 grant for the project. 

Constructed: April - July 2003 

Looking north across the newly planted roof

(taken from the upper roof); 2003
12/9/2004;Multnomah County Green Roof 2

 

Figure 1. Diagram of the ecoroof system components 

 

Overview of the Stormwater System 

The roof absorbs rainfall that falls directly on it. The roof also receives runoff from a flagstone terrace 

(1,660 sq. ft.) and pavers along the perimeter of the roof (1,780 sq. ft.). The average depth of the growing 

medium (soil) is 6 in. Although the performance of the roof will vary depending on a number of factors - 

storm size, soil moisture content, and temperature - researchers hope to confirm that the roof retains more 

than 50% of the total annual runoff. 

Stormwater Capacity and System Components 

Stormwater Management Goal

The overall stormwater management goal was to reduce the peak runoff and volume that would otherwise 

contribute to combined sewer overflow (CSO) events in the adjacent Willamette River. Although the project 

did not trigger the requirements of the City’s Stormwater Management Manual (SWMM), it meets the general 

standards for ecoroofs. 

System Components

Planted Roof portion 

(See Planting Plan for more details, pg. 10) 

• The footprint of the vegetated portion of the ecoroof is approximately 11,900 sq. ft. The catchment 

is 15,420 sq. ft., including the planted area, stone terrace, and the pavers along the roof’s perimeter. 

• Overflow drains to internal downspouts that are connected to the combined sewer system. 
12/9/2004;Multnomah County Green Roof 3

 

Refer to Figure 1 for the following description: 

a. Structural roof support (previously existed) 

b. 5-ply roofing system (previously existed): A built-up 

roof (3-ply polyester and 2-ply fiberglass membrane) set 

in hot asphalt atop three inches of polyisocyanurate and 

fiberboard insulation. 

c. 1⁄4 in. protection board over existing roof: laid on top of 

the existing roof, provides roof protection during 

construction as well as additional root barrier protection. 

d. 1⁄2 in. drainage mat with root barrier: A hollow core 

layer of TremCo GR with a top layer of fabric that 

allows water to pass through while restricting the 

movement of soil. The fabric is treated with copper 

hydroxide to restrict root penetration. 

e. Stainless steel edging: Retains the soil around the 

perimeter of the roof and prevents it from entering the 

drains. 

f. 6 in. growth medium (soil): see “Landscaping” section 

g. Green roof vegetation: see “Landscaping” section 

h. Drip irrigation system: see “Irrigation” section 

i. Gravel ballast: Surrounds the roof drains. Secures the drainage mat and allows free movement of runoff 

around the roof drains. 

j. Roof drain: Provides overflow drainage for runoff that is not absorbed, evaporated, or evapotranspired. 

k. Monitoring equipment: Measures the rate and volume of runoff. 

l. Concrete pavers: Provides access for maintenance personnel. 

Additional Information: 

• The saturated soil weight is approximately 28 pounds per sq. ft.

• The planted area is gently mounded in the center due to the roofing structure. The slightly western 

aspect adds visual interest. 

Landscaping 

• The growth medium (soil) is a proprietary aggregate from a local company. The 6-in. deep soil is 

comprised of pumice, perlite, digested fiber, and paper pulp (reclaimed); it has a higher proportion 

of mineral content than organic content. The composition meets the structural requirements of the 

roof, and is free draining to promote plant growth 

• A seed mix was broadcast by hand. The mix includes various wildflower species and fescue grass. 

• The vegetated portion directly adjacent to the building was densely planted with plugs and 

containers of sedum species, perennials, bulbs, and ornamental grasses. 

• The project designers selected the plants with advice from a green roof consultant from Toronto, 

Canada. The selection criteria included adaptability to roof conditions, ecological function, local 

availability, drought tolerance, seasonal interest, aesthetics, and maintenance requirements. 

Irrigation 

Portland’s extensive summer dry periods made an irrigation system a requirement. The system includes: 

• A permanent drip irrigation system, approximately 3 in. below the soil surface, that delivers water 

directly to the plant roots. 

• Soil moisture sensors to control the drip irrigation system, delivering water only when needed. 

View of the terrace with the green roof in the 

background.; note educational signage and 

fence preventing public access to the roof; 

summer 2004.
12/9/2004;Multnomah County Green Roof 4

 

• A temporary above-ground, spray irrigation system, to ensure that the new seeds received 

sufficient water for germination. The system also helped the new plantings survive the 

establishment period while their roots were still fairly shallow. The temporary system was 

removed in late spring 2004. 

Stone Terrace Planter 

• The footprint of the terrace planter is approximately 200 sq. ft. It is filled to a depth of 6 in. with 

the same proprietary soil mix used on the roof. 

• The raised planter, featuring a built-in bench seat, is incorporated into the patio hardscape. 

• The planter receives precipitation and some supplemental water from a drip irrigation system. 

• It is planted with perennial flower species and varieties of sedum. 

Budget 

The final budget for construction of the Multnomah County building ecoroof was $179,807. This cost 

covers all of the construction activities for the vegetated ecoroof; it does not include any design activities 

or costs associated with construction of the terrace. 

Over $125,000 in grant funds and in-kind donations of materials and services were provided by interested 

professionals, businesses, and agencies. BES contributed $50,000. The in-kind costs are included in the 

costs summarized below. 

 Item 

 Item Cost 

or Value Total Cost 

Construction: 131,462.00 $ 

Protection board (material & labor) 8,644.00 $ 

Ecoroof components (drain mat, root 

barrier, filter fabric, soil) 48,517.00 $ 

Ecoroof installation (labor) 38,121.00 $ 

*Mobilization/demolition/cleaning 4,800.00 $ 

*Crane 12,480.00 $ 

*Miscellaneous supplies 2,400.00 $ 

Rock removal (labor) 16,500.00 $ 

Landscaping (11,900 s.f.) 44,418.00 $ 

Plant /seed material 10,475.00 $ 

Installation (labor) 15,712.00 $ 

Irrigation:

Permanent drip 16,362.00 $ 

Temporary overhead spray 1,869.00 $ 

Irrigation booster pump

Permitting 

Construction bonds/permits 3,927.00 $ $ 3,927.00

TOTAL 179,807.00 $ 

Multnomah County 

Budget Summary for Ecoroof Construction

* Mobilization costs for the ecoroof were estimated by pro-rating the 

mobilization costs for the entire roof project (including the terrace). Eightypercent of the costs were attributed to the vegetated portion of the ecoroof 

because it occupies 80% of the roof area. 
12/9/2004;Multnomah County Green Roof 5

 

I. Budget Elements 

Non-Construction Activities 

The cost for management, design, and permitting was not fully 

documented and therefore is not included in the budget. 

Construction Activities 

The total construction cost was $179,807. The activities include 

general construction, landscaping, the value of in-kind materials and 

labor, and the cost of the permits. 

Construction 

The core construction activities include removal of the existing rock 

ballast, mobilization and demobilization of a crane to transfer the 

rock and import soil, and installation of the ecoroof components. 

The cost of these activities was $131,462, which is approximately 

73% of the overall construction budget. 

Mobilization costs for the ecoroof were estimated by pro-rating the 

mobilization costs for the entire roof project (including the terrace). 

Eighty-percent of the costs were attributed to the green roof because 

it occupies 80% of the roof area. Permit costs for mobilization, 

including contract bonds, total $3,927. 

Landscaping 

The cost of the landscaping, excluding the soil matrix but including 

irrigation, was $44,418. The associated unit cost was 

approximately $3.73 per sq. ft. Landscaping costs comprised 

approximately 25% of the total construction budget (excluding soil 

matrix). The costs for the soil matrix are included in “ecoroof 

components” under construction activities. 

The irrigation system was valued at $18,231, including both the 

permanent drip system and the temporary overhead spray system. 

II. Cost Components

Construction – Mobilization and installation of the structural 

components comprised approximately 75% of the total construction 

budget; landscaping costs were about 25% of the total budget 

(excluding soil matrix). 

Labor vs. Materials – Labor accounted for approximately half the total 

construction budget. 

Installing the drainage mat

Installing drip irrigation system

Bringing the soil up from the street

Installing the soil

Plants ready for installation
12/9/2004;Multnomah County Green Roof 6

 

Irrigation – The County simply expanded the existing irrigation 

systems, which included valves, a backflow preventer, and a 

controller. Construction of a new system would have been 

substantially more expensive. 

Pavers – The County saved money by re-using the pavers from the 

previous roof. The pavers were placed around the perimeter to 

provide access for maintenance staff. 

III. Cost Comparisons 

Construction costs for the eco-roof project may be low, relative to 

other similar projects, for retrofitting existing roofs with ecoroofs. 

Although mobilization costs to raise materials to the 5th floor were 

substantial, the project saved on other activities that sometimes 

account for higher costs on similar retrofit projects. For example, 

the roof was sound and required no structural alterations - its load 

bearing capacity had been previously certified; the existing 5-ply 

roof was in good condition and did not necessitate removal or 

replacement. 

Maintenance and Monitoring 

Multnomah County owns the facility and is responsible for all 

maintenance activities. 

BES, the County, and PSU, are jointly funding hydraulic 

monitoring. Led by PSU, the team will intensively monitor the 

performance of the ecoroof facility for at least five years, and 

perhaps longer. BES will also regularly evaluate the level of effort 

required to maintain the facility, the success and viability of the 

planting regime (which may change over time), and any comments 

received. 

A detailed Operations and Maintenance Manual includes site plans, 

operation and performance requirements for vegetative care, natural 

pest/weed reduction, safety precautions to protect workers and 

visitors, erosion & litter control measures, and a detailed schedule 

and timeline for maintenance of the irrigation system. 

SE corner, planted with a variety of 

sedum species, bulbs and purple 

coneflower; early summer, 2004.

Looking east across the ecoroof, 

toward the terrace, early summer, 

2004.

View of the ecoroof with wildflowers 

in full bloom; spring 2004.
12/9/2004;Multnomah County Green Roof 7

 

Successes and Lessons Learned

Multiple Benefits - The project is a great example of how an ecoroof can be a central feature in the urban 

environment, with multiple benefits: 

• Many nearby offices enjoy views of the green roof and it provides an appealing picnic spot. It 

should continue to attract visitors interested in ecoroofs. 

• The roof will provide ongoing benefits for the County and the City in terms of stormwater 

management and moderating temperatures on the roof (including reductions in heating and cooling 

costs for the building). 

• The roof provides habitat for urban life – birds, butterflies, etc. 

Roof Longevity and Cost Effectiveness – In 2000 the County conducted a life cycle cost analysis 

comparing a conventional roof with an ecoroof. The study concluded the ecoroof would be more costeffective than a standard roof over a 60-year period. The analysis assumed that the ecoroof would double 

the life of the impermeable seal on the roof (a common assumption in the industry). More information 

about the study is available from Multnomah County or Allen Lee at allenl@quantecllc.com. 

Construction Schedule - The project confirmed the crucial influence of seasonal weather on installation 

schedules: in Portland planting should occur in early spring or early fall when the temperatures are cool 

and precipitation is most likely to assist with plant establishment. Seeds are especially vulnerable to 

prolonged dry periods. 
12/9/2004;Multnomah County Green Roof 8

 

Multnomah County Ecoroof 

Planting Plan