Guy Harrison, who was fined for his involvement in the powder-bombing of Tony Blair in the Commons chamber a year ago, claimed he was a reluctant protester.
"The last thing we want to be doing is scaling this massively high, extremely windy place to get our stories across," he told the ITV News Channel.
The security breach was timed to coincide with the Prime Minister's address to the Labour Party conference in Brighton.
Fathers 4 Justice leader Matt O'Connor said the protest was staged "to highlight the fact that they are not addressing the issues".
The protester could be seen sat astride the roof of Westminster Hall. He tried to unfurl a banner but was frustrated by the brisk wind.
Dozens of police were monitoring him below, but there seemed to be no initial attempt to get him down.
Parliament is in recess, so there were few MPs around.
Mr O'Connor said Mr Harrison and another man had gone into the main entrance of the Houses of Parliament and had taken a tour of the building.
They had then climbed out of a window, and unfurled a banner which proclaimed "Does Blair Care?"
Mr O'Connor said: "Basically they went into the main entrance to the Houses of Parliament which is a bit of a surprise because we thought obviously he would be a familiar face to them from last year.
"Obviously, security was ... a bit lax to put it mildly.
"They just walked into the main door, climbed out of a window, shinned down a drainpipe and he basically climbed on top of the roof."
He said the protest was also a " birthday present" for Mr Harrison's young daughter, whom he had not seen for years.
He added the group had been worried about the risk of somebody getting shot and so had warned Scotland Yard.
Talking from the top of the Houses of Parliament on his mobile phone, MrHarrison told the ITV News Channel: "We choose to do this today on top ofParliament and not in Brighton as we did not want to be seen as a terroristthreat or anything like that.
"We are not out to undermine the police at all, we just want to get our message across.
"We've been struggling with various government bodies for nine months, trying to negotiate things, but as they are not taking it seriously again, we felt we had to return to this."
He added: "I've got about five people below me trying to talk me into getting down, but apart from that, I did not meet much security getting here, because we've found yet another loophole in the security here. It has gone relatively smoothly so far.
"This is my birthday present for my daughter, to tell her how much I love her and miss her. It is my birthday tomorrow, which I will probably be spending in custody."
A Scotland Yard spokeswoman said: "At approximately 15.35 a man gained access to the roof of Westminster Hall, part of the Houses of Parliament."The man remains on the roof. Police are at the scene."Reuse content