Gutters & Drainage
Box Gutter Repairs & Replacement in Sydney
Box gutters are unforgiving — we build in the falls, sumps and overflows they need.
Box gutters sit inside the roof structure, usually between two roof planes or behind a parapet, which makes them powerful but unforgiving. When they leak or overflow, the water goes straight into the building — not over the edge onto the ground. They demand correct falls, properly sized sumps, and an overflow provision, and they're one of the most common causes of internal water damage when they're done poorly.
Why box gutters fail
The usual culprits are insufficient fall so water sits and finds the joints, sumps and outlets that are undersized for the roof area, and — critically — no overflow provision, so a single blockage sends water backing up under the roof sheets and into the ceiling. Old soldered or riveted joints crack with thermal movement, and blocked outlets do the rest. Because the gutter is concealed, the first sign is often a stained ceiling.
Australian Standards require box gutters to have adequate capacity and an overflow measure precisely because the consequences of failure are internal. We build to that principle rather than hoping it never rains hard.
Repair, reline or replace
A sound box gutter with a localised split can be repaired or relined with a waterproof membrane system that bridges the joints. But if the fall is wrong, the sump is undersized, or there's no overflow, patching the leak won't stop it happening again. In those cases we rebuild the gutter with the correct fall, a properly sized sump and rainhead, and a dedicated overflow so a blockage spills outside, not inside.
Sumps, rainheads and overflows
A box gutter needs somewhere for the water to go fast. We size the sump and outlet to the catchment and fit a rainhead or overflow outlet so that if the downpipe ever chokes, the excess discharges safely to the outside of the building. This single detail is the difference between a blocked outlet being a minor nuisance and being a flooded ceiling.
What's Included
- Inspection of falls, joints, sump size and overflow provision
- Membrane relining of sound gutters with localised leaks
- Full rebuild with correct continuous fall where required
- Correctly sized sump and outlet for the roof catchment
- Overflow outlet or rainhead so blockages discharge outside the building
- Water testing to confirm capacity and no internal leaks
Frequently Asked Questions
Why is my box gutter leaking into the ceiling?
Usually the water is backing up rather than draining — a blocked or undersized outlet, insufficient fall, or a cracked joint. Because box gutters sit inside the roof, backed-up water has nowhere to go but into the building. We check the fall and outlet capacity, not just the joint you can see.
Does a box gutter need an overflow?
Yes — it's essential and required by the Standards. Without an overflow, a single blocked outlet sends water straight into the ceiling. A properly placed overflow or rainhead means excess water spills safely outside the building instead, turning a potential flood into a harmless trickle down the wall.
Can you reline my box gutter instead of replacing it?
If the gutter has good fall and adequate capacity but a leaking joint, a membrane reline is a sound, cost-effective fix. If the fall is wrong or there's no overflow, relining just delays the next flood. We assess the whole design before recommending reline versus rebuild.
How do I know if my box gutter is undersized?
Telltale signs are overflow into the ceiling during heavy rain despite the gutter being clear, and water marks high up on the sides of the gutter. If it copes with light rain but floods in a downpour, capacity — the sump and outlet size — is usually the issue, and we can calculate it against your roof area.
Request a Free Inspection
Book a free, no-obligation inspection with our local crew.
Ready To Protect Your Home?
Book Your Free Roof Inspection Today.
Whether you need emergency roofing, leak detection, storm damage repairs or simply peace of mind, our experienced roofing specialists are ready to help. No pressure. No hidden costs. Just honest roofing advice.