Torrents of water brought down a suspended ceiling and damaged stock in the Victoria Centre store at about 22:40 BST on Tuesday.
Managers had hoped for a weekend reopening but it is now closed "until further notice".
Staff have been helping with the clean-up operation.
Water poured through from a rooftop room, leaving the top floor under three inches of water and stock "significantly" damaged.
A spokeswoman said: "Our teams are working around the clock to get the shop open as quickly as possible and we're sorry for the inconvenience this has caused to our customers."

The medal - won as a teenager at his first World Cup - was estimated to be worth between £70,000 and £140,000.
The three-time World Cup winner is selling all three of his winner's medals at auction, along with more than 2,000 items of personal memorabilia.
His winning medal from the 1962 World Cup in Chile was sold for £140,800.
The memorabilia, which includes shirts and trophies, is being sold over three days and his winner's medal from the 1970 World Cup, where he was part of the Brazilian team regarded as one of the greatest in history, has yet to go under the hammer.
A one-off Jules Rimet trophy made for the former striker after Brazil's triumph in Mexico in 1970 is the most expensive item being auctioned, with an estimated price tag of £281,000 to £420,000.
On the first day of auction, the boots worn by Pele in the classic football film Escape to Victory sold for £8,025.
Other items sold include L'Equipe athlete of the century trophy with book (£20,480), his 2007 Fifa presidential award (£30,720) and a 1,000th goal tribute crown with book (£162,500).
Pele is the only player to have won the World Cup three times and is regarded by many as being the greatest footballer in history, scoring 1,281 goals in 1,363 matches and appearing 91 times for Brazil.

Stamp duty would raise less money per head in Wales than the rest of the UK, the Wales Governance Centre claims.
The Cardiff University experts fear the UK Treasury could cut the block grant based on revenue predictions inflated by the London property market.
They have called for the figures to be adjusted to set Wales "a more realistic and achievable target".
Control of property and landfill taxes passes to Wales in April 2018, with the prospect of power over income tax rates to follow.
Until now, the Welsh Assembly and Welsh Government have largely been funded by a block grant from the UK Treasury.
Negotiations are due to begin in the autumn over how much the grant will be cut as Wales raises more of its own income.
On the subject of stamp duty, a Wales Governance Centre report said there was a "very significant difference" between revenues per head in Wales and the UK as a whole.
It claimed it could result in a "large negative impact" on the Welsh budget - about £50m a year - if the higher UK figure was taken as the benchmark for income.
Report author Ed Poole said changes to grant funding "exposes the Welsh Government to the deep-rooted differences in UK housing market conditions".
He added: "In many ways, the property market of London and south east England is detached to that of the rest of the UK and is heavily influenced by international factors totally outside the control of the Welsh Government.
"And that distinct nature of London's property market has further amplified since the UK's vote to leave the European Union.
"Excluding revenues from London and south east England from the adjustment calculations would still provide an incentive for the Welsh Government to grow the Welsh tax base, but with a more realistic and achievable target."
John Lewis's Nottingham store is to remain closed longer than expected after 80,000 litres of hot water leaked from a ruptured heating pipe.
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Pele's 1958 World Cup winner's medal has been sold at auction in London for £200,000 - at least £60,000 more than its estimated price tag.
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Wales could suffer a budget shortfall if devolved taxes do not make up for grant cuts, academics have warned.