Rainwater Systems

Rainwater systems collect water from the roof and carry it to the stormwater drainage: eaves gutters, box gutters, valley gutters, sumps, rainwater heads, scuppers, overflows, downpipes and internal stacks, siphonic systems, and the connections at ground level. On large metal roofs, box gutters run the length of the building between bays or behind parapets, inside the building envelope; on apartment and office buildings with flat roofs, outlets feed internal downpipes in risers.

Every system is designed for a rainfall intensity and a catchment. Its weak point is what happens when it is overwhelmed. An eaves gutter that overflows spills outside the building. A box gutter or internal outlet that overflows spills inside it, unless it has an overflow sized to take the full flow. Buildings extended, re-roofed or fitted with larger plant decks often have rainwater systems sized for the original roof, and rainfall intensities used in design have increased in many regions.

What shortens life:

• No cleaning regime, especially with trees over the roof: the single largest determinant of gutter life.
• Flat or back falls in box gutters, keeping them wet.
• Dissimilar metals and copper or lead run-off.
• Foot traffic in box gutters used as walkways to plant.
• Undersized outlets and missing overflows, which turn every blockage into an internal flood.
• Ground-level damage to downpipes from vehicles and trolleys.

Inadequate capacity

Gutters, outlets and downpipes sized for a smaller catchment or lower rainfall intensity overflow in heavy storms. Box gutters without overflows, or with overflows smaller than the outlet they protect, overflow into the building. Siphonic systems depend on their design flow; a blockage or an added catchment changes their behaviour.

Blockage

Leaves, sediment, bird nests, balls and debris block outlets and sumps. Leaf guards that are not maintained make it worse. A blocked outlet holds water in the gutter, which then overflows, ponds and corrodes the gutter sole.

Corrosion

Steel gutters corrode along the sole where water and debris stand, at laps and at sumps. Box gutters perforate and leak inside, often first at sumps and stop ends. Cast iron cracks at joints and from frost; its joints fail where the paint has gone.

Joint and connection failure

Joints in internal stacks fail inside ceilings and risers; outlet clamping rings loosen from the membrane; downpipes disconnect from the drain at ground level and discharge against footings.

Ponding load

Water held on a roof by blocked outlets adds load. On light steel roofs, deflection holds more water, and the process can end in collapse. Codes in all three jurisdictions now treat secondary drainage and rain loads explicitly for this reason.

Cast iron and lead goods

Cast iron gutters and downpipes crack from frost and impact and leak at joints once the paint has gone; lead rainwater heads and gutters are stolen, leaving water discharging against the wall.

From the street and from aerial imagery

• Vegetation and debris in gutters and box gutters; ponding at sumps.
• Staining on walls below gutter lines and at downpipe joints; overflow marks on fascias.
• Downpipes disconnected, damaged or discharging onto walls, paths or lower roofs.
• Trees overhanging the roof.
• Overflows missing from box gutters and parapet roofs.

On the roof

• Corrosion along gutter soles, at laps and at sumps; perforations.
• Falls in box gutters: water standing after rain.
• Outlet size against the gutter and catchment; overflow presence and size.
• Leaf guards blocked; outlet grates missing.

Inside the building

• Water staining along the line of box gutters on purlins, ceilings and walls; leaks during heavy rain only.
• Staining and leaks at internal stacks in risers and ceilings.
• Damp at the base of walls where downpipes discharge.

In the records

• Gutter cleaning contract and log.
• Leak register, especially leaks in heavy storms.
• Drawings or hydraulic report showing design rainfall, catchments, outlets and overflows.
• Records of roof extensions, re-roofing or added plant decks.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A roof plumber inspects condition; a hydraulic engineer checks capacity where the roof has changed or storm leaks occur.

• Visual inspection of every gutter, sump, outlet and downpipe, with falls checked after rain or by level.
• Capacity check of box gutters, outlets and overflows against the catchment and current design rainfall intensity.
• Water testing of internal outlets and stacks.
• CCTV of internal stacks and the connections to stormwater.
• Hazardous material check on asbestos cement goods before removal.

The report should list runs needing replacement, any capacity shortfall and the fix, and the cleaning cadence the roof needs.

• Box gutter measurement: falls, sole width and depth against the catchment.
• Rainfall data: current published design intensity for the location, which may exceed what the system was designed for.
• Riser inspection of internal stacks through access panels.

• Cleaning regime with leaf guards maintained; quarterly where trees overhang.
• Repairs: reseal joints, replace sections, refix brackets.
• Replacement of gutters and downpipes, usually with the roof covering.
• Box gutter upgrade: replace with correctly falling, larger gutters; add sumps and rainwater heads.
• Overflows: add or enlarge overflows to discharge outside the building.
• Additional outlets and downpipes where capacity is short.
• Spreaders and diverters where downpipes discharge onto lower roofs.

• Rainwater heads to collect and redistribute flow into downpipes.
• Trace heating of gutters and outlets in snow regions.
• Licensed replacement of asbestos cement gutters and downpipes.
• Secondary drainage on parapet roofs where codes require it.

Replacement is rate-based on linear metre and profile, with sumps, rainwater heads and overflows itemised. Capacity upgrades are lump-sum design-and-construct items. Cleaning is a recurring maintenance contract. Gutters are often replaced with the roof covering and may sit inside that register line.

Replacement is lifecycle; capacity upgrades are lifecycle or code compliance where overflows are missing; cleaning and minor repairs are repairs and maintenance.

Where trees overhang a roof, cleaning may need to be quarterly; it is still the cheapest protection the roof has.

• Australia. Roof drainage design follows AS/NZS 3500.3, called up by the National Construction Code, with box gutters and overflows sized to the design rainfall intensity for the location; SA HB 114 guides the design of eaves and box gutters. A change of roof area or plant deck requires the system to be rechecked.
• United Kingdom. Approved Document H (H3) covers rainwater drainage; BS EN 12056-3 sets roof drainage layout and calculation. Sustainable drainage requirements can apply where discharge arrangements change.
• United States. International Plumbing Code Chapter 11 covers storm drainage, including secondary (emergency overflow) drainage for roofs where parapets or other construction would trap water; ASCE 7 sets rain loads, including ponding.

Gutters and downpipes on common property are owners corporation items in Australian strata schemes and service-charge items on British leasehold blocks.

• Gutter cleaning contract and log.
• Leak register, noting which leaks occur in heavy storms.
• Roof drainage drawings or hydraulic design report.
• Records of roof extensions, re-roofing or plant decks added.
• Roof plumber or leak investigation reports.

• Tree management records.
• Records of leaks from internal stacks.