This topic is covered quite extensively on the Roofing professionals website.
The image gallery above shows some typical box gutters found on tile and metal roofs in Sydney:
A box gutter may look like a box because of its ‘boxy’ shape. But the main reason it is a box gutter is because it is ‘boxed in’ on all sides. The picture below shows the layout of a typical box gutter on a metal roof.
Typically, a box gutter is ‘trapped’ between two roofs that feed rainwater into it. The water is drained via downpipe nozzles in the box gutter or via a sump built into the box gutter. A good box gutter design will have falls along the length of the box gutter, sumps to collect water before it is fed into large downpipes. There will also be an overflow facility on the side or the end of the box gutter – so that water will overflow externally to the building, rather than flood into the roof cavity.
I recommend that colorbond should always be used for a box gutter. Some severe situations may call for colorbond “ULTRA” or even colorbond stainless. Box gutters get a lot of moist leaves and debris and these can shorten its life – so the rule of thumb is to have the box gutter material as good or better (in the corrosion department) than the roofing material. Sometimes I see architectural specifications nominating a colorbond roof with a zincalume box gutter. This combination can result in much higher maintenance costs for the roof. Why?
…Because, the box gutter will need replacement a lot sooner than the roof. And a box gutter replacement is quite expensive and time consuming – because it usually involves removal of the roof sheeting.
Since the box gutter is originally installed before the roof – the roof has to be removed to remove the old box gutter and to allow the new box gutter to be inserted.
Unfortunately, with flat roofs, larger complex roofs, parapet walls and other architectural building features, rainwater does not always drain out directly to the external perimeter of the roof. In these instances, box gutters are the only viable option. So, if your roof is not ‘straight forward’ – then you probably have a box gutter somewhere on it.
It pays to have box gutters maintained and regularly cleaned out – simply because a box gutter leak has much higher consequences that the eaves gutters along the edge of your roof leaking.