Well, Reactors 1, 3, and 4 don't even have roofs on the reactor building. What difference would plugging the holes in the Reactor 3 turbine building make?
For this meaningless effort, 2 of the 4 human workers who installed the metal plate over the large, oblong hole on the roof received radiation exceeding 10 millisieverts. That's extremely high, considering the work was done outside, not inside the building, though there's no information as to how long the workers took to complete the task.
Now why would the rooftop be so radioactive for this particular turbine building? The holes look as if something heavy fell through the roof.
Only regional newspapers like Hokkaido Shinbun even had the radiation numbers for the workers. No such information in the national papers like Yomiuri and Asahi.
From Hokkaido Shinbun (7/18/2011):
東京電力は１８日、福島第１原発で台風６号の接近に備え、３号機のタービン建屋の屋根に事故で開いた穴などを鉄板でふさいで雨の流入を防ぐ作業を進めた。長径１４ｍの楕円状の穴をふさぎ終え、長径５ｍの穴と、事故でふたがなくなった換気口などを１９日までにふさぐ予定。
TEPCO continued the work on July 18 to cover the holes on the roof of Reactor 3's turbine building with metal plates in preparation for the approaching Typhoon No.6. The workers finished covering the 14-meter diameter oblong hole, and will plug a 5-meter diameter hole and vents that lost the lids in the accident by July 19.屋根の上で穴に鉄板をかぶせる作業中の４人のうち１人が当初計画の１０ミリシーベルトを超える１２ミリシーベルトと１１ミリシーベルトの放射線量を被ばくした。東電は「思ったよりも作業に時間がかかったため」と説明している。
Of the 4 workers who did the work on the roof, two received radiation exceeding the planned 10 millisieverts. One received 12 millisieverts, the other 11 millisieverts. TEPCO explained the reason as "the work taking longer than expected".
３号機のタービン建屋ではこのほか、屋根の雨水を落とす二つの穴も土のうでふさぐ。
In Reactor 3's turbine building, TEPCO plans to plug two holes with sandbags. (The holes used to have pipes that collected rainwater from the roof.)