Roof Gully: What It Is, How to Unblock It and When to Replace It

A roof gully is the outlet that lets rainwater leave a flat roof or a roof with an internal gutter. When it works, you never notice it. When it blocks, water builds up on the roof, spills over the edge, soaks the walls or leaks into the building. It is one of the smallest parts of the roof, and one of the most common causes of flat roof problems.

This guide explains what a roof gully is, the types, signs of a blockage, how to clear one safely and when it needs replacing. For the bigger picture on flat roofs, see our flat roof guide.

What Is a Roof Gully?

A roof gully, also called a roof outlet or rainwater outlet, is the opening in a flat roof where water drains into a downpipe. It is usually set into the roof at the lowest point, with the waterproof membrane dressed into it. A grid or dome guard over the top stops leaves and debris from entering the pipe.

The word gully is also used for ground drains. On a roof, it means the rainwater outlet that carries water from the roof surface into the drainage system.

Types of Roof Gully and Outlet

Flat Roof Outlet (Round or Square)

The most common type, set into the roof deck, with a spigot that connects to the downpipe below. The membrane is dressed into a flange around the outlet, so water flows in and not under the covering.

Hopper and Downpipe at the Edge

Water runs off the edge of the roof into a hopper, which then connects to a downpipe. It is often used where the roof meets a parapet or wall.

Scupper

An opening through the parapet wall that lets water drain directly outside. It is often used as an emergency overflow, as well as a primary outlet.

Siphonic Outlets

Used on large commercial roofs, where a special design fills the pipe to move water faster. They are specialist designs and not normally found on houses.

Signs of a Blocked Roof Gully

• Water standing on the roof for more than a day or two after rain.
• Overflowing water from the edge of the roof or the parapet.
• Leaves, moss or debris around the outlet.
• Damp or staining on the ceiling below the outlet or the wall near the downpipe.
• A gurgling sound or no water flowing during heavy rain.
• Algae or vegetation growing in the area of ponding.

Standing water ages a flat roof faster and adds a lot of weight to the structure, so a blocked outlet should be dealt with quickly. If you see water dripping inside, our guide on dripping roof causes helps you work out whether it is the outlet or something else.

How to Unblock a Roof Gully

If you can reach the roof safely, for example from a low roof or a balcony, you can often clear a blocked gully yourself. If the roof is high or difficult to reach, use a roofer.

1. Stay safe. Use a proper ladder and never work on a roof in wet, icy or windy conditions.
2. Remove the grid or guard. Take off the cover, if it has one.
3. Clear debris by hand. Wear gloves and remove leaves, moss and silt from around and inside the outlet.
4. Flush with water. Run a hose slowly into the outlet to check that water drains freely. If it backs up, the blockage is further down the pipe.
5. Clear the downpipe. A drain rod or a plumber’s snake can free a blockage in the pipe. Do not use sharp tools that could puncture the membrane.
6. Replace the guard. Check that it sits properly and is not damaged.

Avoid pressure washers, wire brushes and sharp tools around the outlet. They can damage the membrane and cause a leak that is worse than the blockage.

When Does a Roof Gully Need Replacing?

• The outlet is cracked or corroded.
• The seal between the membrane and the outlet has failed.
• Water leaks around the outlet into the building.
• The outlet is too small for the roof, and it overflows in heavy rain.
• The outlet sits above the roof surface, so water ponds around it.
• The roof is being re-covered, and the old outlet is not compatible with the new membrane.

The new outlet has to match the roof covering. Our guide to flat roof materials explains the different coverings, and each one uses its own outlet or sealing kit.

How a Roof Gully Is Replaced

1. Isolate and protect the area. The roofer checks the downpipe and cuts back the membrane around the old outlet.
2. Remove the old outlet. It is cut or lifted out of the deck.
3. Check the deck and fall. Rotten boards are replaced, and the surface is shaped so that water runs towards the new outlet.
4. Fit the new outlet. It is fixed into the deck and connected to the downpipe.
5. Dress in the membrane. The covering is bonded or welded to the outlet flange, using the manufacturer’s method.
6. Test. Water is run through to check that it drains freely and does not leak.

How Many Outlets Does a Flat Roof Need?

The number and size of outlets depend on the area of the roof, the expected rainfall and the position of the walls and upstands. A small extension might have a single outlet, while a large roof needs several. Good design includes an overflow route, such as a scupper or an overflow pipe, so excess water leaves outside the building if the main outlet blocks. Ask your roofer or designer how the drainage has been sized for your roof.

Roof Gully Problems at the Roof Edge

Where the roof drains into a gutter, the roof edge matters just as much as the outlet. A short or badly fitted trim can send water behind the fascia. Our guide to EPDM roof edge trim explains how the edge should be finished, and our article on roof valleys and guttering shows what to check on the gutter itself.

Maintaining Your Roof Gully

• Check the outlet in autumn after the leaves fall, and again in spring.
• Keep a grid or dome guard in place.
• Remove leaves, moss and silt promptly.
• Look for water that stays for more than two days after rain.
• Have the outlet and the surrounding membrane checked by a roofer every few years.
• Trim overhanging trees that drop leaves onto the roof.

When to Call a Roofer

• The outlet is high up or hard to reach.
• Water is leaking inside the building.
• The membrane around the outlet is torn, lifted or blistered.
• Clearing the gully does not fix the drainage.
• The roof keeps ponding even when the outlet is clear, which suggests a problem with the falls.

For other common problems and how to choose a roofer, see our roof repair guide.

Frequently Asked Questions

What is a roof gully?

A roof gully is the rainwater outlet in a flat roof, where water drains into a downpipe. It normally has a grid or guard to keep out leaves.

How do I unblock a roof gully?

Remove the guard, clear leaves and debris by hand, flush with a hose and clear the downpipe with a drain rod if needed. Avoid sharp tools that could damage the membrane.

Why is there water standing on my flat roof?

The most common reasons are a blocked outlet, poor falls or a sagging deck. Clear the outlet first, then ask a roofer to check the slope.

How often should I clean my roof gully?

At least twice a year, in autumn and spring, and more often if you have trees nearby.

Can a blocked roof gully cause a leak?

Yes. Standing water can find its way past seams and edges, and it can overflow into walls and ceilings.

How much does it cost to replace a roof gully?

It depends on the roof covering, the access and whether the deck needs repair. Ask for a written quote that includes the outlet, the membrane work and the testing.

Final Thoughts

A roof gully is a small part with a big job, and a short check twice a year can save you a lot of money. Keep it clear, avoid sharp tools and have a roofer look at any signs of ponding or leaks. If the outlet needs replacing, make sure the new one is matched to the roof covering and sealed correctly.