Box Gutters

Replace or Repair and Restore ?

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. 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 from the building, rather than flood into the roof cavity.

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. Some architectural specifications nominate a colorsteel 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 than an external gutter leak. Apart from the misery they cause, leaking box gutters cost New Zealand Industry millions of dollars every year in production disruption, stock loss and maintenance costs. Box gutters discharge water into a building predominantly due to old perished roofing membranes that line the gutters and joint failure but also due to substrate failure and severe corrosion in unlined metal gutters. Gutters can also become overwhelmed during storm conditions.

The long term repair of leaking gutter joints is extremely difficult and most repairs, which consist usually of bandage and bitumen applications, can only be considered as an emergency contingency whilst a more permanent solution is sought, or at best, one which will last for 1 to 3 years until the process needs to be repeated.

A solution to perished membranes and leaky joints is to line the existing gutter, however, effective access is a common problem due to roof sheet overhang and other obstructions and most rigid gutter lining systems, are unable to capitalise correctly on the available space within the existing gutter cavity, leading to a reduction in gutter size with the resultant loss of valuable water capacity. The solution to this problem is seamless liquid applied waterproofing membranes as they have no joints and they are able to reach the extremes of the existing gutter profile.


When box gutters leak, deterioration spreads rapidly. Soon, complete gutter replacement becomes inevitable. If your building is over 10 years old you need to have your box gutter system checked!