Box gutter problems: why they leak into the house, and what to check

Box gutter problems cause some of the most frustrating leaks we see in Melbourne. The water does not drip from a gutter outside. It comes through a ceiling, often in the middle of a room, often in the front bedroom of a terrace or across a factory floor. Because the box gutter is hidden from the street, most owners have never seen theirs, and it is easy for the problem to go on for years.

This guide explains what a box gutter is, the signs that one is failing, the usual causes on terraces and factories, what a roofer checks, and why an overflow does so much damage.

What a box gutter is

A normal gutter hangs off the edge of the roof, outside the walls. A box gutter sits inside the roof line. It is a rectangular channel, usually lined with metal, set between two roof surfaces or between a roof and a wall. Water runs along it to an outlet, often into a sump or rainhead, and then down a downpipe.

You find box gutters in a few typical places:

• Behind the front parapet of a Victorian or Edwardian terrace, catching the front slope of the roof
• Between two roofs over a party wall, where neighbouring terraces meet
• Between the main roof and a later extension
• Along the valleys and edges of factory and warehouse roofs, sometimes running the full length of the building

Why an overflow ends up in your ceiling

When an external gutter overflows, the water usually spills outside, though it can run back into the eaves. When a box gutter overflows, there is nowhere outside for it to go. It spills over the side of the channel, straight onto the ceiling below or into the wall. A heavy downpour can put a lot of water into a building in a few minutes.

That is why box gutters need overflows: a planned escape route, such as a slot through the parapet or a rainhead with an overflow, so a blocked outlet sends water outside instead of indoors. Many older box gutters were built without one.

Signs of box gutter problems

From the ground or inside the house, look for:

• Ceiling stains near the front wall of a terrace, or along the line of a party wall
• Stains or drips that appear in heavy rain and stop when it eases
• Water staining or moss on the face of a parapet
• Rust stains running down the outside wall below a rainhead or outlet
• Leaves or plants visible poking above the parapet line
• On a factory, water appearing along a line under a gutter, or pooling on the floor beneath one
• A downpipe from a box gutter that barely flows during rain, which suggests a blockage above

The usual causes

Blocked outlets. Leaves, silt, moss washed off the roof, and on factories, litter and bird nests. The outlet is the narrowest point, so it is where a blockage forms.

Undersized gutters and outlets. Many box gutters were never sized for the area of roof draining into them. They cope with light rain and overflow in a summer storm. This is especially common on factories, and on terraces where the roof has been altered.

Not enough fall. A box gutter needs a slope toward its outlet. Over time, timber supports sag and the gutter can end up holding water in a low spot, which speeds up rust.

Rust and splits in the lining. Old galvanised linings rust through, especially where water sits. Joints can split as the metal expands and contracts.

No overflow. Without one, any blockage becomes an indoor leak.

Failed flashings at the edges. Where the box gutter meets a parapet or wall, the flashing has to keep water from getting behind the lining. Cracked or lifted flashings let it through.

What a roofer checks

When we inspect a box gutter, we get up to it and look properly. That means:

• Clearing it, so the lining can actually be seen
• Checking the lining for rust, holes and split joints
• Looking at the fall, and whether water sits anywhere
• Checking the outlet, sump or rainhead, and whether the downpipe below is flowing
• Checking for an overflow, and whether it would actually work
• Checking the flashings where the gutter meets the parapet, wall or roof
• Looking at the size of the gutter against the roof area it drains
• Photographing all of it, because most owners have never seen their box gutter

Often clearing the gutter and fixing the roofing around it, such as cracked tiles, slipped slates or a failed flashing along the roof edge, is enough to stop the leak.

When a licensed roof plumber is needed

In Victoria, building, rebuilding or relining a box gutter, and resizing sumps, outlets and overflows, is roof plumbing work. It needs to be done by a licensed roof plumber, who can issue the compliance paperwork that goes with it. A good roofer's inspection photos and notes are still the starting point, because they show exactly what is wrong and why.

Maintenance that prevents most problems

• Have box gutters cleared at least once a year, more often under trees
• Clear them before summer storms, when outlets are most likely to be overwhelmed
• Keep moss under control on the roof that drains into them, since it washes straight into the gutter
• Check rainheads and downpipes after big storms to make sure they are flowing

Get your box gutter looked at

If you have a terrace, an older home with an extension, or a commercial building, and you have never seen your box gutter, it is worth a look. We get up to it, clear it, photograph it and tell you what we found. The inspection is free. Call 0420 541 479 or send a quote request and we will call you back within 2 hours, Mon to Sat, 7am to 6pm. Emergency call outs run 24/7 if water is coming in now.