How to Replace a Flat Roof Drain

Flat roofs are practical and durable, but when the drain fails, you’ll know it fast — puddles form, leaks appear inside, and water doesn’t flow where it should. Most of the time, it’s because the old drain cracked, pulled loose, or wasn’t installed right in the first place.

The process shown here uses a cast-iron drain on a modified bitumen roof, but the same steps apply to most flat roofs.

Understanding the Problem

The original roof in this project had a PVC drain that failed. The pipe inside had broken loose and couldn’t be repaired — once that happens, the drain loses its seal and water starts leaking inside the building.

On top of that, the roof showed signs of mixed materials. A section of TPO roofing was patched next to the modified bitumen surface. Those two materials don’t bond well together, so that repair was never going to hold up.

The plan was simple: remove the damaged drain, install a solid cast-iron system, and re-seal the roof with modified bitumen so it’s watertight again.

Tools and Materials You’ll Need

Tools

• Utility knife or roofing knife

• Circular saw or reciprocating saw

• Drill and screwdriver bits

• Torch (for modified bitumen roofing)

• Trowel and small roller

• Caulking gun

• Measuring tape

• Wrench set

Materials

• Cast-iron roof drain

• Clamping ring and strainer

• Sump receiver plate

• Coupling (metal-band or flexible type)

• Modified bitumen base sheet

• Lead flashing sheet

• Roofing screws

• Asphalt adhesive or mastic

• Plywood or deck patch if needed

Step 1: Understand the Parts of a Roof Drain

A flat roof drain has four main components:

1. Sump receiver – The base plate that sits into the roof opening and supports the drain. It prevents the assembly from falling through.

2. Drain body – The main cast-iron bowl that connects to the plumbing pipe below.

3. Clamping ring – Holds the roofing membrane tightly to the drain body so it can’t pull loose.

4. Strainer – The top grate that keeps debris and leaves out of the drain.

These parts work together to keep the connection watertight. The clamping ring is especially important; without it, the roof material could shrink and pull away from the drain over time.

PVC drains often skip that step or rely on glued joints that eventually fail. Cast iron, on the other hand, uses a bolted clamping system and lasts for decades.

Step 2: Remove the Old Drain

Cut around the old PVC drain to expose the layers underneath. Be prepared for surprises — you might find insulation, multiple roofing layers, or odd patch jobs. In this case, there was a mix of roofing materials and some uneven repairs. Everything in the drain area had to be cut out down to the plywood deck so the new assembly could sit flush. Remove the old drain body, clean the opening, and check that the decking underneath is solid. Replace any rotten wood before proceeding.

Step 3: Cut the Opening for the New Drain

Once you know where the new drain will sit, mark the outline for your sump receiver. Cut through the insulation and roofing until you reach the plywood deck. The hole doesn’t have to be perfect — the receiver plate will cover the edges — but it should be wide enough for the drain body to fit comfortably. Dry-fit the plate and drain together to make sure everything lines up before fastening anything down.

Step 4: Bolt the Drain and Receiver Together

The sump receiver and drain body bolt together using four bolts through the back flange. Tighten them evenly so the connection is snug. After bolting, flip the assembly upright. It should now be one solid piece. The receiver supports the drain from the top side, and later, the pipe connection underneath will support it from below.

Step 5: Secure the Drain to the Roof Deck

Place the assembled drain into the hole. Make sure it’s sitting level and centered. Screw the sump receiver to the rafters or plywood deck using roofing screws so it’s firmly held in place.

This support is crucial — the drain has to stay perfectly still while the roofing layers expand and contract through the seasons.

Step 6: Add the Base Sheet

With the drain in place, begin waterproofing. Lay a modified bitumen base sheet around the drain area and secure it to the deck. This layer forms the foundation for the roofing system. Spread asphalt adhesive or roofing mastic where needed to ensure a tight bond.

Step 7: Install the Lead Flashing

This is one of the most important steps. Modified bitumen doesn’t bond directly to metal, so you need an intermediate layer — lead flashing — to create a strong waterproof seal.

Cut a sheet of lead large enough to cover the drain flange and extend several inches onto the roof surface.

• Fit it into the drain opening, tapping it down so it curves neatly inside.

• Extend it outward so it forms a smooth flange.

• Secure it with screws around the perimeter to hold it flat.

The lead gives you a wider bonding surface and prevents water from sneaking under the roofing.

Step 8: Apply Adhesive Over the Lead

Next, coat the lead sheet with a thin, even layer of asphalt adhesive. This is required because modified bitumen can’t be torched directly onto bare metal — it needs a bitumen base to stick. Spread the adhesive with a trowel, making sure to cover every inch of exposed lead.

Step 9: Torch in the Modified Bitumen

Once the adhesive has set slightly, roll out the modified bitumen cap sheet over the area. Use a roofing torch to heat and bond it to the base sheet and the lead flashing.

Work slowly and evenly:

• Start at the edges and move toward the drain.

• Watch for a glossy finish — that’s how you know it’s hot enough to bond.

• Press it firmly into place with a roller or the back of your trowel.

The goal is to create one continuous, sealed surface that ties into the existing roof membrane.

Step 10: Install the Clamping Ring

After the bitumen has cooled, set the clamping ring over the drain. Align the bolt holes, then tighten each bolt gradually in a crisscross pattern so the ring presses evenly against the roofing membrane. This compression keeps the waterproof layers locked to the drain body and prevents them from lifting or shrinking later. When properly installed, the clamping ring should feel solid — no gaps or loose edges.

Step 11: Attach the Strainer

The strainer (the top grate) snaps or bolts onto the clamping ring. It prevents debris, leaves, and small objects from entering the drain and clogging the pipe. Choose a strainer that sits high enough to allow free water flow but low enough to avoid tripping hazards on the roof.

Step 12: Connect the Plumbing Below

Now that the top is complete, it’s time to go inside and connect the pipe underneath.

Because you’re joining cast iron to PVC, you can’t use glue — it won’t hold. Instead, use a metal-band or flexible rubber coupling. These couplings tighten with stainless-steel clamps and create a watertight seal between different materials.

If the existing pipe size doesn’t match, install a reducer or adapter before coupling. In this example, a three-inch PVC line was adapted to four inches to match the new drain.

Once connected, check that the pipe is secure and sloped slightly downward so water flows away freely.

Step 13: Double-Check Waterproofing

Before wrapping up, inspect all around the drain. Look for:

• Smooth, continuous seams in the bitumen

• No visible gaps around the lead flashing

• Proper bolt tension on the clamping ring

If everything looks clean and solid, the drain area is fully waterproofed.

Step 14: Handle Rain or Test with Water

Sometimes you’ll finish right before bad weather. If rain is expected, set up a temporary tarp over the area to protect it while you finish interior plumbing. In this project, the customer had already rigged a clever tarp system with a small hole leading to a hose that drained out a window — a simple but effective temporary fix. Once the new pipe is connected and sealed, remove any coverings and let the rain test your work. You’ll be able to see immediately whether water is draining correctly.

Step 15: Clean Up and Inspect

When the weather clears, go back up to check your finished work. A proper installation should show:

• No pooling or standing water around the drain

• A firm, secure strainer

• Smooth transitions between old and new roofing

• Even bonding of the modified bitumen

At this point, the roof is ready to handle heavy rain again.

Why Cast-Iron Drains Are Worth It

Replacing a lightweight PVC drain with cast iron may take a little more effort, but the benefits are clear:

• Stronger and more durable – Handles temperature changes and heavy loads better.

• Better seal – The bolted clamping ring holds roofing materials tight.

• Less movement – Cast iron expands and contracts less than plastic.

• Longer life span – With proper installation, these drains can last the life of the roof.

PVC drains tend to crack, pull loose, or leak where they’re glued. Cast iron, properly installed, virtually eliminates those issues.

Safety Tips

• Always wear gloves, long sleeves, and eye protection when cutting or torching materials.

• Keep a fire extinguisher nearby when using a roofing torch.

• Never torch near insulation or open framing without proper shielding.

• If you’re unsure about connecting plumbing, hire a licensed plumber for that portion.

Troubleshooting Common Issues

1. The membrane won’t stick to the lead. Make sure the lead was coated with asphalt adhesive before torching. Modified bitumen won’t bond directly to bare metal.

2. Water still ponds near the drain.Check that the deck around the drain slopes slightly toward the opening. If it’s flat or reversed, you may need to build up the surrounding area with roofing compound.

3. Leaks show up below the roof.Inspect the coupling connection — loose clamps or an uneven cut can cause small leaks. Tighten and reseal as needed.

4. The strainer clogs frequently.Clean it every few weeks during heavy leaf season. A clogged strainer slows drainage and can create new ponding spots.

Maintenance After Replacement

Once your new drain is in, take a few minutes each season to keep it working properly:

• Clear debris. Sweep away leaves, dirt, and gravel around the drain.

• Check the bolts. Make sure the clamping ring stays tight.

• Inspect seams. Look for cracks in the bitumen and reseal with mastic if needed.

• Flush the drain. Pour a few gallons of water to ensure it flows freely.

A few minutes of maintenance each year will keep your roof watertight for decades.

Signs Your Drain Needs Replacement

If you’re not sure whether you need a new drain, look for these warning signs:

• Water leaking around the drain after storms

• Cracks or loose joints in PVC piping

• Roofing pulling away from the drain flange

• Rusted or missing strainer

• Frequent blockages or slow drainage

If you see any of these, replacing the drain sooner rather than later will prevent much bigger repairs.

Summary of the Process

1. Inspect and remove the damaged drain.

2. Cut a clean hole to the roof deck.

3. Install the sump receiver and drain body.

4. Bolt the assembly together securely.

5. Add base sheet and lead flashing.

6. Coat the metal with adhesive.

7. Torch in the modified bitumen.

8. Install the clamping ring and strainer.

9. Connect the pipe below using a coupling.

10. Inspect, clean, and test for leaks.

Follow these steps carefully, and you’ll end up with a watertight, professional-quality result.

Final Thoughts

Replacing a roof drain isn’t the most glamorous project, but it’s one of the most important. A solid, properly clamped drain keeps your entire roof system safe from water damage.

With a little patience, a torch, and the right materials, you can do this repair yourself and avoid the frustration of recurring leaks. Now when the rain comes, instead of dripping through the ceiling, it’ll flow exactly where it should — right down your new, durable drain.