Box Gutters and Warehouse Leaks

Ask anyone who has managed a warehouse for long enough where their worst roof leak came from and you'll hear the same answer more often than not. It wasn't the sheets. It wasn't the skylights. It was the box gutter.

Box gutters do a big job quietly. On most Melbourne warehouses and factories they collect the water off two roof slopes and carry it to the sumps and downpipes. When they work, nobody thinks about them. When they don't, the water doesn't spill harmlessly over the edge of the building. It runs back under the roof sheets and comes down inside, onto stock, ceilings, switchboards and floors.

Here's why that happens, what the warning signs look like and what actually fixes it.

What a box gutter is, and why it's different

An eaves gutter hangs off the outside edge of a roof. If it overflows, the water falls to the ground outside. Annoying, but rarely a disaster.

A box gutter sits inside the roof. It's a flat bottomed channel, usually running along the valley between two roof slopes or behind a parapet wall. Water that it can't carry has nowhere to go except back over the edge of the gutter, under the roof sheets and into the building.

That single design difference is why box gutters cause so many commercial leaks. An eaves gutter fails outside. A box gutter fails inside.

The parts of a box gutter system

It helps to know what you're looking at, because a leak can start at any one of these parts:

• The sole. The flat bottom of the gutter. It should have a slight fall towards the outlets so water doesn't sit. Where it doesn't, water ponds and rust starts.
• The sides. One side usually sits under the roof sheets and the other against a parapet or the opposite roof slope. Water that gets over the top of either side ends up inside.
• Joints and laps. Box gutters come in lengths that are lapped, riveted and sealed. Every joint is a potential leak.
• Stop ends. The closed ends of the gutter. They take a lot of movement and are a common failure point.
• Sumps and outlets. The deeper boxes that collect water and feed it into the downpipes. If these block, everything upstream backs up.
• Rainwater heads. Boxes on the outside of the building that take water from the gutter to the downpipes, often with an overflow built in.
• Overflows. The safety valve. Weirs, overflow outlets or rainwater heads that let excess water out of the building before it can come in.
• Flashings and cappings. The metal pieces that cover the joins between the gutter, the roof sheets and any walls.

On a typical warehouse, those parts have been sitting in the weather for decades. Any one of them can let you down.

Why Melbourne is hard on box gutters

Melbourne's weather does box gutters no favours. Long dry spells let leaves, dust and debris build up in the gutter. Then a summer thunderstorm drops more rain in an hour than the gutter was ever sized for. Autumn brings leaf fall from street trees and nearby parks. Winter keeps the gutter wet for weeks, which is when slow leaks at joints and rusted spots show up.

Add heat. A metal gutter in a Melbourne summer expands and contracts every day, and over the years that movement works sealant loose at the joints and stop ends. A gutter that was watertight when it went in will not stay that way on its own.

The five usual causes of a leaking box gutter

Most box gutter leaks come down to one or more of these.

1. Blockages

Leaves, sticks, sediment, dead birds, tennis balls and the occasional forgotten tool. A box gutter only has so many outlets, and once a sump or outlet blocks, the gutter fills behind it. Sites near trees or with flat, slow draining gutters block faster than most.

2. Rust at joints and low spots

Box gutters are made in lengths and joined. The joints, the stop ends and any low spots where water sits after rain are where rust starts. Once it gets through, every downpour drips straight through the hole.

3. Failed seals and laps

Joints are sealed and riveted when the gutter goes in. Over years of heat, cold and movement those seals dry out and crack. The gutter can look fine from a distance and still leak at every joint.

4. Outlets and sumps that are too small

Some gutters were never big enough for the roof area they drain, or the building was extended and the drainage wasn't upgraded to match. They cope with light rain and overflow in a heavy storm, which is why some leaks only ever show up in a downpour.

5. No overflows, or overflows that don't work

When a box gutter blocks or can't keep up, it needs somewhere safe for the extra water to go. That's what overflows are for, whether they're overflow outlets, rainwater heads or weirs that spill outside the building. Older warehouses often have none, or have overflows that are blocked, too small or sitting higher than the point where water gets into the building.

Warning signs you'll see from the inside

You don't need to get on the roof to spot a box gutter problem early. These are the signs to look for underneath:

• Water stains or drips along a straight line, often under the valley between two roof slopes
• Leaks that only appear in heavy rain, not light showers
• Rust staining on purlins, beams or the underside of the roof sheets near the gutter line
• Wet patches on the floor in the same spot after every storm
• Water running down columns or walls near the middle of the building rather than the outside edge

If you're seeing any of these, the box gutter is the first place to check.

Materials and what they mean for rust

Most commercial box gutters in Melbourne are galvanised steel, Zincalume or Colorbond. A few older or higher end buildings have stainless steel.

All of the coated steels do well for years, as long as water drains away and the coating isn't damaged. Problems start when:

• Water sits in low spots and never fully dries out
• Leaf litter and debris hold moisture against the metal for months
• Rivets, screws or offcuts of a different metal are left in the gutter, which can corrode the coating around them
• Someone walks in the gutter in boots and scratches through the coating
• Copper from old pipework or flashings drains into a zinc coated gutter

That's why a gutter can look sound from a distance and still have rust holes at every low spot and joint. The rust starts where the water sits.

Clear it, repair it or replace it?

Not every box gutter problem needs a new gutter. The right fix depends on what's actually wrong.

Clearing a blocked gutter fixes today's leak. It doesn't fix the reason it blocked. If the same gutter keeps blocking, it usually needs leaf guards, bigger outlets or a gutter with more fall, otherwise you'll be back on the roof after the next storm.

Box gutter design and installation in Australia is covered by the roof drainage section of the plumbing standards (AS/NZS 3500.3), which sets out how gutters, sumps and overflows should be sized. Replacement and resizing should be done by licensed roof plumbers who work to it.

What a proper box gutter inspection covers

A quick look from a ladder won't tell you much. A proper inspection walks the full length of every gutter and checks:

• Debris, sediment and anything sitting in the sole
• Every joint, lap and stop end for rust, gaps and failed sealant
• The fall of the sole, and any spots where water ponds after rain
• Each sump and outlet for blockages and size
• Downpipes for blockages, cracks and connections
• Overflows: are there any, are they clear and do they discharge outside the building
• Flashings and cappings where the gutter meets the sheets and walls
• The underside, from inside the building, for staining, drips and rusted purlins

You should get photos of all of it, so you can see exactly what was found and track how it changes over time.

What affects the cost

Box gutter work ranges from a few hours of clearing to a full replacement over several days. The cost comes down to:

• Length and access. Long gutters, high roofs and tricky access take more time and safety equipment.
• Condition. A gutter with a few bad joints is a repair. A gutter rusted along its length is a replacement.
• Drainage changes. Upsizing outlets, adding overflows or replacing downpipes adds to the job but often fixes the actual cause.
• What's underneath. Working over live production lines, cool rooms or high value stock may need extra protection or out of hours work.
• Damage inside. Ceilings, insulation and stock damaged by past leaks are a separate repair, but often worth fixing at the same time.

We quote after we've seen it, because the same leak can be a half day repair or a full replacement depending on what's actually there.

Scenarios we see all the time

A logistics tenant in the west kept buying new pallets of stock to replace the ones ruined under one corner of the warehouse. Three different contractors had sealed the roof sheets above it. The leak was coming from a box gutter twenty metres away, where a blocked sump was backing water up under the sheets and the water was running along a purlin before it dripped.

A manufacturer in the south east only ever had leaks in summer thunderstorms. The gutter was in good condition. It simply wasn't big enough. The building had been extended a decade earlier and the new roof area drained into the original gutter. Bigger outlets and a new overflow fixed it.

A property manager arranged a gutter clear every autumn for years and still got tenant calls every winter. The gutter was clear. The joints had rusted through. Clearing was the right maintenance, but it couldn't fix a gutter that had reached the end of its life.

Box gutters and your lease

If you're a tenant, read what your lease says about gutters. Many commercial and industrial leases ask the tenant to keep gutters and downpipes clear during the lease. If that hasn't happened, blockages and the damage they cause can come back to you at handover.

On the other hand, a box gutter that has simply rusted out with age is usually a landlord issue, not a tenant one. It's worth knowing which is which before the final inspection. We cover the roof side of lease handovers in more detail in our guide to whether the roof is part of your make good.

How to keep box gutters out of trouble

Box gutters reward a little attention. A few habits go a long way:

• Clear them before winter. Autumn, after the leaves have dropped and before the heavy rain, is the most important time.
• Check after big storms. A storm can drop a season's worth of debris in one night.
• Look at the joints, not just the debris. A clean gutter can still leak at every lap.
• Make sure there's an overflow. If water has nowhere safe to go, it will find somewhere unsafe.
• Photograph it. A photo record of the gutter's condition makes it much easier to see what's changing year to year.

If you look after several buildings, putting the roofs on a schedule saves a lot of reactive call outs. Our roof inspections and maintenance service covers gutter and sump clearing, small repairs on the spot and a photo report after every visit.

Box gutter questions we get asked

Can I just run silicone over the leaking joints? It might stop a drip for a while. It won't last. Sealant over rust or dirt doesn't bond properly, and the joint keeps moving. A proper repair cleans back the joint, treats or replaces the rusted metal and reseals it with the right product.

How long does a box gutter replacement take? It depends on the length and access, but most replacements are done in days, not weeks. We replace in sections so the roof stays watertight overnight.

Do we have to clear the warehouse underneath? Usually not. Most of the work happens from the roof. We might ask you to move stock or cover equipment directly under the gutter line while we work.

Why does it only leak in heavy rain? Usually the gutter or outlets are too small, or there's no overflow. Light rain drains away. Heavy rain overflows back into the building.

Can you work on the gutters while we're operating? Yes. Box gutter work is roof work, so your operation carries on underneath. We plan around deliveries and anything sensitive below.

Getting it fixed properly

Box gutter leaks are usually fixable without replacing the whole roof. The key is finding the actual cause before anyone starts sealing things. Our box gutters and roof drainage team repairs and replaces gutters, sumps, rainwater heads and downpipes on warehouses, factories and commercial buildings across Melbourne. If you're not sure the gutter is the problem, our roof leak repairs service traces the water back to where it starts.

And because roofing sits with our make good work, the same team can repair the ceilings, walls and floors the water damaged. One quote, roof and all.

Send us the address, photos of the gutter and any water marks inside and tell us when it leaks. Get in touch.