Frequently Asked Questions

Do you have a commercial roof drain question? Check out answers to our most commonly asked questions, or contact our Drain Wizard below.

Roof Drain Questions

How do I select a roof drain for my roof?

The size and type of roof drain needed is generally determined by a number of factors, including:

  • Total roof area in square feet.
  • The drain leader size (based on the roof area and the structure’s main storm drain).
  • The area’s average rainfall per year.
While there are standard formulas to calculate roof drain requirements, it is best to have the architect or engineer provide you with this information, since they must consult local plumbing codes before any drain changes or additions.

How do you repair a roof drain?

Have a roofer or a plumber inspect the leak. There are 3 common causes of roof drains leaks:

  1. The clamp ring has failed or broken. 
  2. The connection between the drain outlet and the drain leader—a rubber boot, gasket, threaded or lead joint, etc.—has failed. If this is the case, have the leak inspected by a plumber and fixed.
  3. The drain bowl has cracked or rotted through, which requires that the drain be replaced. Find a replacement roof drain.

While these are the most common causes of leaks, there are many others ways a drain can fail and only an inspection can determine the actual cause.

What is a roof drain sump pan?

A “sump pan” or “sump receiver” is a square, flat piece of steel (unless it’s a dual pan) approximately 1/8-in. thick. It fits a roof drain bowl, and secures the drain to the roof. Sump pans are manufacturer specific, so they cannot be used with other manufacturers’ roof drains.

Are roof drain sump pans required?

Building designs will normally specify whether or not a sump pan is necessary. Ask the engineer or architect if you’re unsure. Some roofs don’t require a sump pan: for instance, poured concrete roofs normally don’t need them. That being said, sump pans tend to make installation easier for most applications.

Which is better, a roof drain top set or underdeck clamp?

If the building plans do not specify that either is required, this decision is one of personal preference.

The Wizard recommends a top-mounting sump instead of an underdeck clamp, as this eliminates the need to attach a clamp inside the building. However, the underdeck clamp does provide a tighter connection to the roof and is preferred by some roofers.

What is a lead stack flashing?

A lead stack flashing is a pipe-shaped lead sheet with a flat, square base perpendicular to the main/vent stack pipe. It covers a building’s main or vent stack, and protects it from the elements.

A lead stack flashing slips over the main/vent stack, and then the top is folded over the top of the stack, draping over the inside of the pipe. The square base is then sealed to the roof in a manner consistent with the roof style.

There are other methods of stack sealing, or “flashing,” including aluminum or rubber boots. However, lead stack flashing is most common.

What is a drain insert?

The term “drain insert” is actually a misnomer; the proper term is “insert-style drain.” An insert-style drain is a drain that is inserted into an existing drain, completely sealing the interior of the drain leader/pipe by specially designed rubber seals or o-rings.

These are ideal for resolving leaks without removing the existing drain.

Olympic, Marathon and Zurn are the only insert-style drain manufacturers we offer. 

What connects to a 3-in. lead roof drain?

If the drain itself is made of lead, the Wizard is unaware of any connectors available. Some plumbing supply houses do carry transition fittings for lead pipes, but they’re rare and sizes over 1-in. even more so.

If you’re referring to a “lead joint” or an “inside caulk joint,” you’ll need to find a plumber that still pours lead joints. This is a very tedious and specific process that requires special tools and lead melting equipment. A No-Hub connection with rubber coupling is the preferred alternative, since it remains watertight despite the drain expanding and contracting with the weather.

Should I use lead on a roof drain?

When flashing, lead is a common material and perfectly acceptable.

When referring to the drain connection, there’s no reason you can’t use lead if you have a plumber with the equipment and knowledge to pour a “lead joint,” commonly called an “inside caulk joint.” Many roofers and plumbers still prefer this style as it lasts a long time and rarely leaks, but finding someone with tools and experience is difficult.

The recommended method is a No-Hub connection with a rubber coupling, since it maintains a watertight seal despite the drain expanding and contracting with changing temperature and weather.

Do you need a waterproofing ring on a Zurn roof drain?

Zurn roof drains have no dedicated “waterproofing ring:" instead, you must use either a clamp ring, waterdam ring or extension ring, all of which create a waterproof seal around the drain bowl.

How do you connect a cast-iron roof drain to PVC pipe?

When purchasing a drain, the customer determines the connection type. Common connectors include:

  • No-Hub Connection (recommended) — A rubber boot that allows for building and drain movement caused by weather and temperature changes.
  • Threaded Joint — A common connector, a threaded joint requires a PVC male adapter of the appropriate size to be screwed to, and then glued onto the drain.
  • Inside Caulk (or Lead) Joint — An inside caulk joint is another common connector, but it is rare to find a plumber with the required knowledge and tools. An inside caulk joint also requires a PVC caulk adapter leaded to the drain.
A push-joint gasket is also available, but is not recommended for any manufacturer’s roof drains.

When should I use roof drain underdeck clamps?

The building’s architect or engineer will specify when roof drain underdeck clamps are required.

If not specified, they aren’t always necessary: certain types of sump receivers attach directly to the drain bowl, eliminating the need for underdeck clamps. These are generally referred to as “top mounting sump receivers,” though each manufacturer calls their own particular pan something different. (Among other names, some manufacturers may call it a “top set deckplate” or a “clamping drain receiver.”)

How do I install deck clamps and deck plates for roof drains?

An underdeck clamp is attached to the drain’s underside, inside the building. Each manufacturer’s method of attaching underdeck clamps differs slightly, but generally they attach with a provided bolt or mounting hardware—drawing the drain bowl towards the roof and sump receiver when tightened.

Standard sump receivers are attached directly to the roof deck, and the drain is placed into the receiver and attached with clips, if provided. Some drains “free float” in the pan and are secured by the underdeck clamp.

Top mounting sump receivers are bolted directly to the drain bowl’s bottom, and then the entire assembly is attached to the roof deck. No underdeck clams are required, or can be used.

Where do I find my Zurn drain number?

Most manufacturers do not place the actual model number on their drains—Zurn included—because many parts in the drain’s assembly are interchangeable and used in other products.

You may find identifiable casting or part numbers on the drain’s ring or dome, but this is not always the case. The only way to identify the drain otherwise is by the drain bowl’s dimensions (including the drain ring dimensions). This is what our Drain Wizard is intended to do.

What is a Fernco coupler?

“Fernco” is the brand name for a type of rubber coupling, or boot. Another brand name of rubber couplings is “Mission.”

“No hub” and “shielded couplings” are similar to rubber couplings and may serve as replacements, but the are not recommended for roof drain connections.

How do I install a Josam No-Hub roof drain rubber coupling?

The Josam roof drain rubber coupling is placed over the outlet on the drain’s bottom and secured by tightening the SS clamp on the coupling. The drain leader is then attached to the other end in the same manner.

What are the different roof drain manufacturer colors?

  • Blake roof drains are black
  • Froet roof drains are yellow-green
  • Josam roof drains are green
  • MIFAB roof drains are red
  • Smith roof drains are yellow / mustard
  • Wade roof drains are burnt orange
  • Watts roof drains are grey
  • Zurn roof drains are blue

Are there any parts available for Blake roof drains?

Blake is a manufacturer that has been out of business since the late 1950s / early 1960s. There are no Blake-brand manufacturer parts available for their drains, although the parts of some other manufacturers have been known to work. None, however, are a perfect fit.

Unfortunately, the most common repair solution for Blake drains is to remove and replace them, or use an insert-style drain. For more information, contact Hy-Tech at 800-635-0384 or

Ask the Drain Wizard - Robed Wizard Holding a Wand in one hand and a drain part in the other

Still Have Questions?

Contact the Drain Wizard or submit a photo of the drain or part you want to replace. Our Drain Wizard will follow up with you in the next 1-2 business days with an identified solution.