Why roof plumbing works hard in Sydney
Designed for a five-minute deluge
The National Construction Code sizes eaves gutters for a five-minute burst of rain with a 5% chance of happening in any year. For Sydney that's 201 mm an hour, with overflow measures expected to cope with 262 mm an hour. Sydney also averages 20 to 25 thunder days a year, so those bursts aren't rare.
One downpipe for every 12 m of gutter
The code lets each downpipe serve no more than 12 m of gutter, and wants downpipes as close as possible to valley gutters. Older houses with one downpipe at each corner can fall short of that, and the first sign is usually water sheeting over the front of the gutter in a storm.
Days of rain, not just bursts
Between 7 and 10 February 2020, 391.6 mm of rain fell on Sydney. Long soakings like that find tired flashings, rusted valley irons and blocked rainwater heads that a short storm never tests.
Mixed metals on older roofs
Woollahra Council notes that Paddington's roofs moved from slate to corrugated galvanised iron and zinc-coated steel over the years. BlueScope says lead, copper and stainless steel don't mix with its steel, and that galvanised gutters can corrode early under COLORBOND or ZINCALUME roofs. On an older roof, ask what metals are meeting where.
Box gutters on the bushland edge
The NSW RFS says box gutters and flat roofs should be avoided on bush fire prone land, because leaves and embers collect in them. If you're replacing one in a bushfire area, ask whether the roof can drain to an eaves gutter instead.
What counts as roof plumbing in NSW
NSW defines roof plumbing as work on guttering, downpipes, roof flashing and roof coverings. It doesn't cover tiles, slate, glass, concrete or membranes, which belong to roof tiling, roof slating or other trades. It does include putting in skylights and ventilators, and protective coatings that don't change the structure.
A contractor licence is needed for roof plumbing worth more than $5,000 including GST. Ordinary plumbing and drainage are separate licences, so a general plumber isn't automatically licensed for your roof. Unlicensed work can draw a fine of $22,000 for an individual or $110,000 for a company under the Home Building Act 1989. Verify NSW shows the categories each licence covers.
Questions to put to a Sydney roof plumber
- Does your licence on Verify NSW show the roof plumbing category?
- Are there enough downpipes for the length of gutter I have?
- Where will overflow go in a downpour, and will it stay outside the house?
- What metals are already on my roof, and will anything you add react with them?
- Will the new flashings be sealed with neutral cure sealant, and is any swarf (cutting filings) cleaned off before you leave?
If we were booking a roof plumber
We'd watch the roof from inside a dry window during the next big storm and note exactly where water spills or drips, then give the plumber that list. We'd ask for a downpipe count against gutter length, because adding one is often cheaper than fighting overflow forever. The national roof plumbing page explains the trade in more depth, and the Sydney hub covers what else is local.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.