Downpipe size

Very suspicious that you don't see any flow down that downspout. If I were you I think I'd go up on the roof and have a look. In any case,

My house has a roof area of about 200 m2, and three 80mm downspouts, which seem to be adequate (we do have quite big gutters).

Approved documents, Part H - official UK government guidance on the design and sizing of rainwater systems (page 40 onwards)

But if each 150mm downpipe is handling an equal 60-ish sq metres, then you're well inside the UK spec.

I'd love to look at the roof but it's not my job to do that. It's not my workshop - I just work there and clean up the water on the floor and on machinery. An "expert" plumber has been hired to fix the leak. His 6 metre extension ladder looks very dangerous.

I found a New Zealand site that suggests a single 150mm downpipe is OK for 240 square metres. This particular pipe probably has more water directed at it than the other three pipes.

Well then I'd advise the owner to get a second opinion from someone who's willing to go up there.

I discovered too late that there was an inspection cover near the bottom of the suspect downpipe. All I have to do is have a look in there next time it is raining very hard. What annoyed me about the plumber is that he said that he'd been a plumber for 30 years, as if that trumps the need for common sense and logic. Mind you most of the tradesmen I see working are doing a lousy job. I have not hired one for 30 years.

You are. It's a PDF. It's free.

I own a factory unit that has a low pitched roof of about 500 sq m, which is adequately drained by six 150mm pipes. I, therefore, am not inclined to believe your plumber.

You may find this site useful, although it may need adjustment if you get more rain than we do:

"We do not have any products in our database suitable for the roof dimensions you have entered."

In message , Matty F writes

My grain barn is 100'x45' (450m2) it has 4 x 100mm down pipes and meets the requirements for BS 5502 class 2 buildings. Mind you this is fine for *British* rain:-)

regards

I must admit I haven't tried breaking it :-)

Colin Bignell

The diagram shows that the height refers to the distance from the bottom of the roof to the top. The workshop roof is almost flat.

Oops!

You read the instructions? Strange person :-) I just saw it said roof height.

Colin Bignell

The plumber hass finished putting in the new pipe. The roof leaks like a sieve, all over expensive machinery as usual.

Have you asked him how this can possibly still happen, with his 30 years of experience?

He says that while he was working on the joob his pipe was not connected to the spouting and there was a heavy downpour. He apparently didn't think to cover the machinery underneath his hole so it got wet and went rusty.

My common sense trumps his 30 years of experience. I would not do roofing work in the rain. I would cover machinery if water is pissing all over it.