Bristol Central Library will be shut on weekdays for four weeks in March as work is carried out to prepare the basement for the Cathedral Primary School.
The school got planning permission last year for its controversial move underneath the library on College Green.
As part of the move, glass blocks in the library floor are being restored to bring more light into the school below.
The planned works, which will run from February 29 to March 24, will also be used to redecorate the first floor and make improvements to access to the toilets. Full works converting the basement begin this month.
During the closures there will be additional weekday hours at the three nearest library branches in Redland, Bedminster and at Junction 3.
Fi Hance, assistant mayor for neighbourhoods, said: “This was not a decision that was taken lightly, but the safety of our staff and library users must come before anything else.
“Although inconvenient, this approach will mean the noisy intrusive work can be carried out more quickly, without compromising the comfortable environment we provide for library users.
“We will be taking a number of steps to try and minimise the disruption during this period, with extended opening hours at nearby libraries and additional access to the Central Library on a Sunday.
“We would like to thank everyone for their patience at this time.”
Reference books previously kept in the basement were moved to B Bond, home of the Create Centre, last year.