Bryan Avery, of Avery Associates, insists that a 'simple and cheap' solution could easily be installed to fix the roof.
The building's owner, the Commonwealth Institute, has regularly cited problems with the roof as part of its efforts to get the Grade II*-listed Holland Park structure demolished.
The institute, which is determined to win delisting, claims that the 1962 building - by Robert Matthew, Johnson-Marshall and Partners - is now completely unfit for its purpose.
Avery was brought in to deal with the building in 1999 when the institute, at least publicly, was still interested in maintaining its Modernist building.
'I have seen much written about this roof recently and I have to say that in my view the roofing contractors did a superb job,' Avery said.
'I am not aware of any leak in the new roof. What could be at issue here is the fact that the existing drainpipes under the building are probably a bit too small and there's always a danger that water could back up in to the building, if there's a blockage or an abnormal storm.
'A cure, if additional pipes cannot be introduced, would be to install simple overflow pipes which could be easily and cheaply fitted and would at least redirect any water back-up safely outside the building.'
'I recall there had already been sensors and warning alarms installed in case this should occur, but I imagine that with the building now empty they are not always acted upon,' he added.
And Avery's assessment has been backed by English Heritage. A spokeswoman for the quango said: 'The Commonwealth Institute was weatherproof, watertight and secure at the time of our visit in August 2005.'
The plight of the institute's building became increasingly public when it emerged earlier this year that two cabinet ministers - Culture Minister Tessa Jowell and Foreign Secretary Margaret Beckett - were preparing to force unprecedented legislation through parliament that would allow for demolition.
One supporter of this move was architect and academic Peter Carolin, who argues that the building is fatally flawed.
'I never said that the roof leaked, it's just that you cannot increase the size of the opening in the drainpipes, or at least it would be very difficult,' he said.
The Commonwealth Institute defends its policy of seeking demolition, pointing to an Arup report it commissioned into the drainage.
'The problem is with the drainage of the roof, which has a very large span, and the size of the opening in the pipes,' a spokeswoman said.
However, this claim was condemned by the Twentieth Century Society. 'The scandal
is that [the Institute] carried out the Avery work,' caseworker Cordula Zeidler said, 'and then they didn't look after it.'