Do I need a riser or extension collar on a grease interceptor?

You need one whenever the tank's own access opening is below the surface you want the lid to sit at. That is the case for most buried tanks. A shallow tank may reach grade with just its frame and cover. A deeper tank needs risers, also called grade rings, extension collars or neck extensions, to close the gap.

Here is the honest catch. The codes we read do not say the word riser as a requirement. They say access must exist, and they say a few things about how that access is built. Your plan reviewer and the tank's instruction sheet fill in the rest.

### What do the words mean?

Names vary by maker, so match the word on your plans to the part on the sheet.

• Manway. The opening people and pump hoses use to reach a compartment.
• Grade ring or riser. A ring stacked on the tank's neck to raise the opening. Precast concrete tanks use concrete rings. The National Precast Concrete Association (NPCA) has a drawing of an interceptor that lists cast iron frames and covers with grade rings as required.
• Extension collar or cover adapter. A part that slips over or into the neck and can be raised or lowered a little. One maker's small buried interceptor has a cover adapter that adjusts up or down by a couple of inches. Anything deeper takes risers.
• Frame and cover. The visible lid and its ring, set at the surface.

### Why does this matter to the owner?

Because an interceptor that cannot be opened cannot be cleaned or inspected. It also matters for safety. The NPCA says many gravity interceptor access lids are brought to grade, where they are visible, and that this raises the risk of falling into the tank if a lid is not secured or is damaged. A tall riser stack with a weak or missing lid is a hole in your parking lot.

Short version: access is required everywhere we looked. Phoenix is the only one of these texts that names grade rings. None sets a riser height.

### What does the model plumbing code say?

The 2024 International Plumbing Code (IPC), Section 1003.10, says access shall be provided to each interceptor and separator for service and maintenance. That is the whole rule. It does not mention risers, depth or lid height.

### What does Phoenix say?

Phoenix amends the code for gravity grease interceptors in Section 1003.3.7. We read the city's amendment sheet, including how it marks its changes. Struck text is removed, and underlined text is added. The grade ring sentence is underlined, so it is new city text and not a deleted one. It reads:

The grade rings (risers) of gravity grease interceptors shall be grouted with shrink proof grout.

The same underlined text says:

• A 500 gallon interceptor has at least two compartments and two manways.
• An interceptor of 750 gallons and above has at least two compartments and three manways.
• All manways have a minimum 20 inch inside diameter.
• Gravity interceptors are installed in line with the manufacturer's instructions and the Authority Having Jurisdiction.
• Gravity interceptors are installed outside unless the city approves otherwise.

The Phoenix text adds a grout rule. It does not set a maximum ring stack or a maximum depth. The city's FOG manual, which reprints City Code 28-13, says interceptors must be located free from obstructions and readily and easily accessible for cleaning and inspection. A lid buried under dirt or pavement does not meet that.

### What does Mesa say?

Mesa City Code 8-4-22 says interceptors must have easily removable covers and must be readily and easily accessible for cleaning and inspection. Where bolted covers are required, they must be gastight and watertight. The code lists a traffic rated cover among the construction requirements. It does not mention risers.

### What does a sewer district in another state say?

Arizona cities we read are short on detail here, so it helps to see how one utility writes it. The Metropolitan Sewer District in Louisville, Kentucky, is not a Valley rule, and its manual was revised in 2013. It says access to a grease interceptor is by at least one manhole per chamber, of 24 inch minimum size, ending 1 inch above finished grade with a cast iron frame and cover. It calls for an 8 inch thick concrete pad extending at least 12 inches past the frame. It also says access manholes should never be paved over, and that covers must withstand vehicle loads, at a minimum HS20 loading.

Treat that as an example of what a plan reviewer may ask for, not as your city's rule.

### What do the codes leave open?

Three things. No text we read gives a maximum burial depth, a maximum riser height or a required number of rings. None says which riser material to use, apart from Phoenix's grout rule for grade rings. And none says how a pump hose reaches a deep tank. Those answers come from the tank maker and the plan reviewer, so ask both early.

Makers publish riser tables sized to burial depth, and each one caps how deep a tank can go. The two guides below are for specific products, so they show how a maker thinks, not a rule for your tank.

### Example one: a 2,000 gallon gravity interceptor

Schier's GGI-2000 guide, dated 2020, says the unit is ready for a burial depth of 77 inches from finished grade to the bottom of the tank. Deeper burials require extending the cover adapters and possibly adding risers. The guide has a table by riser height needed:

• 0 to 4 inches: none, use the adapter.
• More than 4 up to 34 inches: three FCR2 risers.
• More than 34 up to 64 inches: six.
• More than 64 up to 94 inches: nine.

It also says a concrete slab to finished grade with rebar is required when the tank goes below grade. The slab is 4 inches thick for pedestrian areas and at least 8 inches thick for vehicle areas.

### Example two: a 100 gpm buried interceptor

Schier's GB-500-B guide, from July 2025, says the cover adapter can be raised or lowered up to 2 1/2 inches from its factory position. Deeper burials require Fast Pitch FP24 risers to extend the adapter to grade. Its chart lists up to 17 risers per manway. The same guide sets a maximum burial depth of 96 inches from the shoulder of the unit to grade. Deeper burials mean calling the maker.

### What else do the guides warn about?

• Risers are not water tanks. The GB-500-B guide says risers are not designed to retain water and should not be water tested. The GGI-2000 guide describes a riser system test to finished grade if one is required, with the risers supported first. The two makers differ, so follow the sheet for your model.
• Groundwater. Both guides say the interceptor and risers are not designed for a water table above the top of the unit. If that can happen, the tank goes in a watertight vault or is encased in concrete, per the maker.
• Riser material. The GB-500-B guide says corrugated pipe is not allowed for riser use.
• Backfill and surface. The GB-500-B guide says do not compact backfill mechanically, do not use native soil at grade, and do not use an asphalt pad. It asks for a concrete pad at grade.
• No pressure tests. Both guides say not to air test the unit or the riser system.

The point is that a riser is part of the tank system with its own limits, not a piece of pipe from the yard.

Get the maker's sheet first, then measure, match the lid, seal it and keep the surface clear. Use this order.

1. 1Get the instruction sheet for the exact model. It has the burial depth, the riser table and the slab detail. Phoenix requires the tank be installed in line with the manufacturer's instructions.
2. 2Measure from the right point. The GB-500-B guide has you measure from the centerline of the inlet or from the top of the temporary covers to finished grade, then read the chart. The GGI-2000 guide has you measure a dimension X to find riser height. Use the maker's reference point, not your own.
3. 3Match the lid to the traffic. The frame and cover carry the wheel loads. See the lid traffic rating page.
4. 4Check the seal. Mesa says bolted covers must be gastight and watertight. The NPCA says gastight manhole covers and seals help keep odors in the tank. See the odor control page.
5. 5Grout the rings on a Phoenix concrete tank. Use shrink proof grout, as the amendment says.
6. 6Keep the lid clear. No dumpster pad, planter or paving over it. Louisville's sewer district says never pave over a manhole.
7. 7Write it on the plan. Show the riser count, the frame and the finished grade elevation, so the reviewer and the pumper see the same thing.

The NPCA also says to inspect access covers for damage or missing parts, and that a compromised lid means secure the area right away. It also says do not enter the interceptor to perform maintenance. It suggests a secondary safety apparatus in each opening: a removable device installed below the cover that can support enough load to prevent accidental entry if a cover is missing or damaged.

### What if the lid is already too low or too high?

A lid that has sunk, cracked or come loose from its riser stack is a repair. So is a lid that ends up below the pavement after a repaving job. Someone has to raise the frame with rings or an adjustable adapter, using the part the maker's sheet lists for your model. If the work opens the tank, that is a good time to look at the outlet tee and baffle. See what happens when a baffle or tee breaks.

Lids get opened at every service visit, which is one more reason to keep them easy to reach. The pump-out schedule page shows how often that happens in Phoenix. For the full sequence of a new install, see the installation steps page, and for permits, the Valley permit page. Vent routing is on the venting page.

Call one before a new interceptor goes in, when a lid has sunk or cracked, or when a repaving job is about to change the grade over a tank. The riser plan ties into the plans the city reviews, and it has to match the maker's sheet. HQ plumbers are licensed and insured. Estimates are free, and the price is set up front. If a grease line is backing up, we offer 24/7 emergency service. Start on our commercial plumbing page.