Roof goof: City Hall covered in clover

When Mukilteo's city design team was working on plans for a new City Hall a couple of years ago, it was decided to install an environmentally friendly vegetated green roof that would reduce the building's carbon footprint and provide better water management.

Native perennial herbs and leafy sedum were chosen because of their low growth and ease of maintenance. Invasive weeds were not in the plan.

City offices moved into the $9 million, two-story building at the corner of Harbor Point Boulevard and Cyrus Way a year later, just before last Christmas.

Invasive clover moved in some time after that.

Now workers are busy removing all the vegetation from the new City Hall's weed-infested roof, along with the soil and the contaminated seed-filled mulch. All the clover has killed most of the greenery the city intended to be up there.

When the roof is replanted, it will be with noncontaminated materials.

"Unfortunately, the mulch that was used had clover in it, and it shouldn't have," said Jim Niggemyer, Mukilteo's assistant city engineer. "Clover grew up and overtook everything else."

"You plant things that won't get real tall and are low maintenance, and that gives you the benefit of an environmental roof," Niggemyer said. "Clover is not the material you want on a roof like that. It squeezes out other plants."

Subcontractors goofed with the original roof, he said. The wrong mulch was used.

Niggemyer said the city decided replacing the roof plantings was the best fix. Spraying the original roof with weed killer a few times was ruled out.

"Between the contractor and the city, we discussed the options, and this is the option that we went with," he said.

Workers began removing the old roofing material early last week. They expect the replacement to be finished in a couple weeks.

How much is the replacement costing the city?

Not a dime, said Niggemyer. It's all under warranty.

Charles E. Brown: 206-464-2206 or cbrown@seattletimes.com