The phrase "a floor under which" is correct and usable in written English.
It can be used to describe a physical space or metaphorically to indicate a foundational level or condition beneath something else.
Example: "The basement serves as a floor under which the entire structure is supported."
Alternatives: "a base beneath which" or "a level below which".
Rats were placed into the test chamber, which had a transparent floor, under which an angled mirror reflected an image of the rat's ventral face and mouth into a digital video camera.
A large majority of estate agents are optimistic that 2013 will see a steady rise in the number of home sales and the improving situation will put a floor under prices, which have fallen on average for the last two years.
He said there should be a minimum floor under which no school should go, but said he opposed targets.
In one of three landscaped courtyards, Wesley Stout Associates installed a striking fountain consisting of a huge vertical iron pipe with a curved piece at the top, broken at the end where water spills out continuously onto a stone floor under which lies the reservoir that stores and supplies the water.
"We want to put a floor in, under which no one can fall.
Showing an unusual degree of discipline, members of the Organization of the Petroleum Exporting Countries have slashed their output by more than three million barrels a day in recent months as they sought to put a floor under oil prices, which have fallen by $100 a barrel since last summer.
That dividend tends to put a floor under the preferred, which doesn't exist for the common.
Be sure that the rough opening and the subsill (the section of the floor under which the sill will rest) is level.
The Fed and the Bush administration have been scrambling to shore up the banking system or, at the very least, put a floor under the housing market, which the banks' fate hangs on.
