I have water coming in through the ceiling when it rains heavily (like once a year or so). I have checked and can see no obvious holes, etc in the roof. My suspicion is that the downpipe is too small to carry the weight of water during heavy rain and that some is running over into the ceiling. Unfortunately it's a flat roof and so I can't get in and take a look to be certain. The roof area is approx 70 square meters, about two thirds on a very steep angle, the gutters are 165mm, but the downpipe, which is round, is only around 70mm in diameter.

I asked a plumber (here to do some unrelated work), he didn't think that was too small but I still have my suspicions.

Is this likely to be my problem, or should I be looking elsewhere.

Thanks in advance for your help on this!!!

It could be a problem.

What is likely to be the real problem is that when the gutter overflows, it should overflow outside, not inside.

Woodbe

If your gutter is 165mm D gutter then your down pipes should be 90mm dia or 100x50 according to the building code. Also check the fall, it should be at least 1:500. But it does sound like your down pipe is too small.

Andy

If you have only the one 70mm downpipe on 70 sq. m. of flat roofing, it is way too small. Should be 100mm minimum for the Melbourne ARI.
I would be very concerned about the competence of your plumber as displayed by his lack of knowledge in this area. Is he licensed? At any rate, l wouldn't be using him again for anything.
I have attended three properties recently where the gutters drained away from the downpipe. Get a `real' plumber to also check this.
Fitting another downpipe would probably be the easiest option if this is also an issue, the position determined after checking the slope.
Also check the front of the gutter to see if it has overflow slots. People sometimes paint over these. Although the front of the gutter should be lower than the back to prevent overflow into the building, high fronted gutters are becoming popular and they are legally sold. The PIC should be directing all plumbers not to fit these as they do not comply with regulations.

Thanks for all the advice. The plumber I talked to worked for one of the companies subcontracted to my water supplier, he was actually here to fit a loo I'd bought under the rebate scheme. I don't think I ever even got his name. He didn't strike me as especially savvy or knowledgeable which is I suppose why I still had doubts after talking to him.

I couldn't see any overflow slots on the guttering, but I'll take another look. Maybe if there are none there I can put a couple in.

Again, thanks for the advice, I've now at least got something to go on to try to fix this up.