Stormwater surcharge on a new roof
You are roofing a new two-storey dwelling with a 35-degree skillion roof of 180 m2 plan area. The roof drains to two 90 mm PVC downpipes at the low corner. The site falls away from the house, but the stormwater connects to a street kerb outlet that the council has told the owner is undersized for the street. During the first heavy rain after handover, water backs up the downpipes and ponds 40 mm deep against the fascia, and a bedroom ceiling below shows a damp patch. The builder says the roof is fine and the problem is the council's drain. The owner wants you to add another downpipe at the same low corner.
1. How do you work out whether the roof drainage itself is adequate, and what does the catchment area tell you?
Read the worked answer
Start with the catchment, not with the complaint. The plan area is 180 m2, but a pitched roof has a larger actual surface, and the effective catchment for a skillion includes the slope factor. For a 35-degree roof the slope factor is roughly 1/cos 35 degrees, about 1.22, so the effective area is around 220 m2. That is the number to use for gutter and downpipe sizing, not the plan area.
Two 90 mm downpipes may be adequate for 220 m2 in normal design rainfall, but adequacy depends on the design rainfall intensity for the location and the gutter capacity, not just pipe count. You need the local design intensity, the gutter profile and fall, and the outlet capacity. If the street connection is undersized, the system must still handle the design storm without surcharging into the building, which usually means an overflow relief or a charged system designed for the head, or a detention/absorption arrangement.
Adding a third downpipe at the same low corner does not fix a downstream capacity problem; it may just move the surcharge. The judgement is to separate roof capacity from outlet capacity and design for the weaker one.
2. The builder blames the council drain. What is your position, and what do you recommend?
Read the worked answer
Both things can be true, but your obligation is the roof and its drainage up to the connection point. If the street outlet is undersized, the roof system must be designed so that a surcharge does not enter the dwelling. That means providing overflow paths, such as a raised outlet or overflow weir at the low point, so water escapes externally rather than through the fascia and ceiling.
Recommend: verify the design rainfall and gutter/downpipe sizing; add an overflow relief at the low corner discharging clear of the building; check the downpipe and connection capacity; and raise the street capacity issue with the council and builder in writing so the owner has a record. Do not simply add a downpipe and call it fixed. The damp ceiling indicates water has already entered, so the detail at the fascia and the ceiling penetration also needs review.