MSPs have been told it could be another two months before they can return to the Holyrood debating chamber.
The wooden beam came loose above MSPs in the chamber
The structural engineers, Arup, said the problem with the collapsed beam was entirely due to the failure of one bolt and the absence of another.
They confirmed that there was no design fault but said the whole structure would have to be inspected.
Chairman Philip Dilley said the damage was likely to have been done during construction work.
This meant the beam may have been unsupported since before the parliament opened.
The company's report said two of the bolts were of different lengths, and the shorter of the two may not have engaged fully in its socket.
"This casts doubts on the condition and adequacy of the bolt groups elsewhere in the roof and it is readily concluded that all bolts in load-critical connections will need to be checked and perhaps replaced," it said.
It said one of the bolts was broken and had damaged threads.
"Clearly there has been some difficulty in installing this particular bolt," the report said.
It said the head of the bolt had sheared off through twisting anti-clockwise, and had not been found.
The report said this was only possible by the bolt having become jammed and someone then trying to remove it, twisting the head off or coming close to doing so.
This was the first time the company had spoken publicly since the heavy oak beam collapsed two weeks ago.
MSPs have been using The Hub for the past two weeks
Mr Dilley defended the company's silence on the matter, saying it was important that it waited until it had the facts.
Presiding Officer George Reid said: "After two weeks of behind the scenes effort, we now have real progress.
"We now know the likely cause and have discussed with Arup their favoured option of how to resolve it - additional components around the bolts."
Mr Reid said this could be done in two stages - a temporary installation first, followed by a permanent stainless steelwork later in the year.
This would avoid having to remove the existing bolts.
Arup said it would come back with firm proposals within two weeks. Scaffolding will be installed in the meantime.
For the past fortnight, MSPs have been holding the full sessions of parliament in The Hub, a converted church near Edinburgh Castle, at a weekly cost of £20,000 plus broadcast costs unofficially estimated at £40,000 a week.
With The Hub unavailable to them next week, two Holyrood committee rooms are to be turned into a makeshift parliament chamber for the next two weeks, at an estimated cost of £16,000.
There is then a two-week Easter recess.
Mr Reid told MSPs that after that options could include the continued use of the committee rooms or finding a new temporary venue.
Suggestions included the Church of Scotland Assembly Hall, the old Royal High School and the old Scots Parliament Hall in the Royal Mile.