WINDHAM — The roof of a horse arena under construction on Nash Road collapsed Wednesday morning, causing about a dozen trusses to tumble to the ground.

Three construction workers, one who fell from the roof with the trusses and two more who were on the ground, were injured in the accident. Each was taken to Maine Medical Center in Portland but their injuries were not considered serious. Their names were not immediately available.

The horse arena and the adjacent home are owned by Gordon Chibroski, a Portland Press Herald staff photographer, and his wife, Robin Chibroski. The 72-foot by 64-foot structure, which was less than a week from completion, is going to be used as an equine training center.

Gordon Chibroski said the accident is likely to delay the $150,000 project, but he said it would still be finished by this fall.

“We’re very lucky no one was seriously hurt,” he said.

The couple was outside doing work when they heard a loud crash and saw the trusses tumble, almost like dominoes. Most of the structure remained intact but about half of the roof will need to be rebuilt.

By mid-morning Wednesday, the remaining construction workers were sent home. The general contractor, Marc Gagnon declined to comment on the accident, but Chibroski said the horse arena was insured and the contractor had insurance as well.