Q. Our new home has two downpipes six metres either side of a roof valley. The gutter slots
overflow during heavy rain but the builder has told us that the gutters are compliant. It is worst at
the valley. The area is now concreted and fitting another downpipe would be expensive and
unsightly. Would a Gutter Pumper stop the overflows?
A. The slotted gutters may have been fitted as a “provision for overflow” because a downpipe was
not fitted within 1.2 metres of the valley as required by the Building Code of Australia (BCA).
Eaves gutter cross sectional areas, downpipe sizes and roof areas harvested are calculated in
accordance with a region’s 1:20 Average Recurrence Interval (ARI) which is based on a 5 minute
An eaves gutter cross sectional area is the area calculated not less than 10 mm below the
overflow. For slotted gutters, the overflow is the bottom of the slots and slotted gutters
consequently have a smaller calculated cross sectional area that must drain a smaller roof area
than the same profile un-slotted gutter. Manufacturer’s specification sheets show the two different
cross sectional areas when a gutter is optioned with slots.
A slotted gutter’s effective carrying capacity can be between 12-55% less than the un-slotted
version. When standard guttering is substituted with slotted guttering as a ‘provision for
overflow’, the maximum roof area drained needs recalculating for compliance. This is not always
If the roof area drained is found to be compliant, the low slots could be overflowing due to the
rush of concentrated water flowing down the valley during heavy rain. This often causes hydraulic
jumps that push the water to a level higher than the slots, causing them to overflow.
Roof valleys are required by AS/NZS 3500.3 (Plumbing and drainage) to drain a maximum 20
square metre roof area but no provisions are made in the regulations to reduce this when smaller
capacity slotted gutters are substituted for standard gutters.
A Gutter Pumper would drain a lot of water from the area but roof valleys also attract a lot of
debris. Fitting leaf mesh to the valley and gutter may be necessary to ensure trouble free
operation. Fitting a Gutter Pumper and suitable gutter mesh is often neater and cheaper than
plumbing another downpipe.