An ordinary gutter hangs off the edge of the roof, so when it overflows, water spills outside. A box gutter is built into the roof itself, usually behind a parapet or between two roofs. When it overflows, there is nowhere for the water to go except into the building. That is why box gutter leaks tend to appear suddenly, in heavy rain, and in places that seem nowhere near a gutter.
Where are box gutters found?
They are everywhere in central Sydney. Terraces in Surry Hills and Paddington often have a box gutter behind the front parapet and another between the house and its neighbour. Warehouse conversions and older apartment blocks have them running across wide, low roofs. Many modern homes use them behind parapet walls for a flat-roofed look. If your roof has a parapet or a valley between two roofs that drains to a single point, there is a good chance a box gutter is doing the work.
What makes a box gutter overflow?
- A blocked outlet, sump or rainwater head. Leaves, tennis balls, nesting material and silt all end up at the one point everything drains to.
- An outlet or downpipe that is too small for the area of roof feeding it. This shows up in heavy downpours rather than steady rain.
- A gutter that has sagged or was laid without enough fall, so water pools instead of running to the outlet.
- Rusted or split joins in the gutter lining, letting water out underneath long before it reaches the outlet.
- No overflow. Older box gutters were often built without any way for excess water to escape safely outside.
- Debris washed in from a neighbour's roof, which is common on terraces that share a gutter line.
How do I know if my box gutter is the problem?
Box gutter leaks have a pattern. They tend to appear only in heavy rain, they often show along the line of a parapet or a party wall, and they can stop as soon as the rain eases. In a terrace, water often shows on the top floor ceiling near the front or back wall. In an apartment building, it is usually the top-floor units that notice first. If your leak behaves like that, the box gutter is a strong suspect.
Every box gutter needs somewhere safe for water to go when the outlet cannot keep up. Without an overflow, the next place it goes is inside the building.
What should I do while it is overflowing?
- Do not go onto the roof in the rain. Wet metal and parapets are dangerous, and storms are when people get hurt.
- If you can reach the rainwater head or downpipe safely from the ground, check whether it is visibly blocked.
- Inside, put buckets under the drips, move furniture and electrical items, and turn off power to any light fitting that has water near it.
- Keep away from any ceiling that is bulging or sagging.
- Take photos and note the time, which helps us trace it and helps with any insurance claim.
- Call us for a make-safe if water is coming in faster than you can contain it.
How is a box gutter fixed properly?
First we clear it and look at it dry, to see where water sits and where the lining has failed. Then the fix depends on the cause. A blocked outlet needs clearing and often a leaf guard or a larger rainwater head. A gutter with no overflow needs one added, typically a rainwater head with an overflow slot or an overflow spitter through the parapet, so excess water goes outside where it can be seen. A rusted or badly sagging gutter usually needs relining or replacing, with the support underneath corrected so it falls to the outlet.
Current plumbing standards expect box gutters to be sized for the roof area they drain and to have a way of overflowing safely. Many older gutters in the inner city predate that thinking. When we replace one, we size the outlet and the overflow for the roof, not just copy what was there.
Is box gutter maintenance worth it?
Yes, more than any other gutter on a building. A blocked eaves gutter makes a mess. A blocked box gutter can bring a ceiling down. Keeping the outlet and rainwater head clear, especially under street trees, is the single most useful thing you can do. If you are in a strata building, box gutters are usually common property, so raise it with your strata manager.
We repair and replace box gutters on terraces, apartment buildings and commercial roofs across the CBD and Surry Hills and right through our service area. See gutter repairs and replacement for how we approach it.
