After $4 million in building costs, three years of construction delays, two new floors and the expiration of a one-year warranty on the construction work, the new Gerritsen Beach branch of the Brooklyn Public Library will have to close this month for massive reconstruction of its water-damaged floors.

Library officials had resisted accepting the new building for a year after construction was completed because of worries about excessive moisture in the 10,000-square-foot facility perched at the tip of the Shell Bank Channel.

The floor slabs were repoured twice before library officials finally signed off on the project in 1997. But even so, the contractor had to come out several times over the one-year warranty period to make patchwork repairs to the buckling floor tiles, library officials said.

Now, continued problems with moisture seepage are forcing the Brooklyn Public Library to shut down the branch for months and completely replace the flooring - at a cost of at least $180,000.

"The installation of the floor slabs did not meet Brooklyn Public Library standards and need to be replaced," said library spokeswoman Valerie Geiss.

The branch will close April 23, she said, and will remain shut for two to three months.

"Here we see the stupidity of government in action," said state Sen. Carl Kruger (D-Canarsie) outside the Gerritsen Beach branch yesterday.

"To their credit, the people at the Brooklyn Public Library saw the deficiencies in the construction all the way though the project," Kruger said. "That should have been a warning flag to start a lawsuit to preserve our rights.

"But we did nothing," he said, "and now it's our responsibility."

Kruger blasted the corporation counsel's office for not filing suit before the warranty expired, and the Economic Development Corp. - which oversaw the project - for approving the work in the first place.

"During the construction we had done moisture tests," said EDC spokesman Bruce Brodoff, "and the measurements were within the architectural design specs, and they met all the building codes."

Brodoff also said that about a year after the library opened, the Economic Development Corp. contacted the Brooklyn Public Library's chief of capital projects and offered to pay for retiling the floor, but library officials never followed up on the offer.

Kruger also had harsh words for the Brooklyn Public Library for its decision to shut the branch down in the months leading up to final exams.

"The timing of this was done in a vacuum," he said. "Why not wait until students are done with their exams and end-term projects and then close for repairs during the summer?"

Geiss said patrons could use facilities in adjoining neighborhoods, such as the Kings Bay, Sheepshead Bay and Kings Highway branches, during renovations of the Gerritsen Beach branch.