Discharge and local rules
Where the water is allowed to go
Sizing a pump is a hydraulics problem with a clean answer. Deciding where the water is allowed to go is not — it is a local regulatory question, it is answered differently three towns apart, and it is the constraint that most often forces a route the hydraulics would not have chosen. It is also the item that gets discovered last, usually after the concrete is poured.
The one rule that is not local
Everything about discharge is jurisdictional except this: putting sump water into the sanitary sewer is a problem everywhere, and a great many municipalities prohibit it outright and inspect for it.
The reason is capacity. A sanitary sewer is sized for what comes out of fixtures, and clear water entering it is called inflow. The United States Environmental Protection Agency's guidance on sanitary sewer overflows lists, among the causes:
Inappropriate connections – Connections of sources of water such as sump pumps, roof leaders, foundation drains and area drains
The same page notes that these connections "can overload a sanitary sewer". The practical consequence is that during exactly the storm when everyone's sump is running, a system taking that clear water can surcharge and discharge untreated sewage — sometimes back into the basements it came from.
If your discharge currently goes to a floor drain, a laundry tub, or anything that leaves via the sanitary line, treat rerouting it as a required item rather than an improvement. That is why the install scope tool flags it as required rather than optional when you select it.
How much water you are actually asking someone to take
It helps to have a sense of the volume, because "the sump discharge" sounds small and is not. The United States Geological Survey publishes the conversion directly: one inch of rain on one acre is about 27,154 gallons. Divided by the 43,560 square feet in an acre, that is 0.6234 gallons for every square foot, per inch of rain.
| Contributing area | Water from 1 inch of rain | From a 3-inch storm |
|---|---|---|
| 1,000 sq ft (a modest roof) | 623 gal | 1,870 gal |
| 2,500 sq ft (roof plus drive) | 1,558 gal | 4,675 gal |
| 5,000 sq ft (a large lot draining in) | 3,117 gal | 9,351 gal |
Only a fraction of that reaches a footing drain, and what fraction is the weakest assumption anywhere on this site. But the table sets the scale: the water arriving at a residential foundation during a serious storm is measured in thousands of gallons, and every one of them has to end up somewhere that can absorb it.
The terminations, and what each one demands
| Where it ends | What makes it work | How it fails |
|---|---|---|
| Daylight on your own lot | Enough fall to move water away, a termination far enough from the foundation that it does not return, and ground that absorbs it. | Discharging onto a slope that drains back toward the house, so the same water is pumped repeatedly. Also freezing at the outlet. |
| A dry well or infiltration pit | Soil that actually percolates, and enough volume to buffer a storm rather than a single cycle. | Saturated ground during the storm when it is needed. A dry well in clay is a hole that fills once. |
| Municipal storm system | A connection the jurisdiction permits, usually with specific fittings and an inspection. | An unpermitted tie-in discovered during a sale, or a storm line that surcharges and backfeeds. |
| Sanitary sewer or floor drain | Nothing. This is the one to get off. | Overloads the sewer, is prohibited in many places, and is inspected for. |
The recurring theme in the failure column is that discharge routes fail during storms rather than in fair weather, which is when nobody is testing them. A route that works in April is not evidence about the route in a saturated November.
Far enough away is further than it looks
A discharge that terminates a few feet from the foundation is, functionally, a very expensive recirculating pump. The water lands on ground that drains toward the house, works back down through the backfill — which is looser than undisturbed soil and therefore preferentially takes water — and arrives at the footing drain again.
You can detect this without any instruments. If the pump cycles at a steady interval that continues long after the rain stops, and the interval is suspiciously regular, some fraction of what it is pumping is water it has already pumped. A properly routed discharge produces cycling that decays over hours as the ground drains.
The fix is length and grade rather than diameter: get the outlet past the backfill zone, onto ground that falls away, and make sure the last few feet cannot pond. Where the lot does not allow that, a dry well positioned away from the house does the same job by giving the water somewhere to go that is not your footing drain.
The failure mode that only happens when it matters
In a freezing climate the discharge termination is the single most likely point of total system failure, and it fails in the worst possible way: the pump keeps running, the line is blocked, and the water has nowhere to go but back into the basin and then over the floor.
Three things make it likely. Water standing in a horizontal run between cycles has time to freeze. A long flexible corrugated extension lying on the ground freezes across its whole length. And an outlet buried in snow or ice at the end of a run is a plug that forms quietly during a cold snap and is discovered during the thaw — which is also when the ground is releasing meltwater and the pump is busiest.
Countermeasures are all about not leaving water in the pipe: enough slope that the horizontal run drains after each cycle, an oversized termination that is harder to block, and an air gap or relief fitting close to the house so that if the outside line does freeze the pump can still discharge somewhere rather than deadheading. That relief fitting is an ugly thing to have in a basement and it has saved a lot of floors.
What to ask your jurisdiction, in one call
- May a sump discharge connect to the storm system here, and if so does it need a permit?
- Is connection to the sanitary sewer prohibited, and is it inspected at point of sale?
- Is there a minimum distance from the property line or from a neighbouring foundation?
- Are there requirements about where it may terminate relative to a sidewalk or a public way, particularly regarding winter ice?
- Does any of this require an inspection, and who is expected to pull the permit — the homeowner or the contractor?
Related pages
- Head and discharge — once the route is legal, how much head it costs.
- Install scope — rerouting appears there as a required line when it applies.
- Check valves — the device that stops the line draining back between cycles.