A street grate takes water from above; a curb inlet takes it from the side. When the first flush of a storm floats leaves and litter over the grate, the opening in the curb face keeps draining the gutter. DomCast Metals supplies the cast iron pieces that make a combination inlet: side-inlet frames, two-piece side-inlet covers, and backarch curb castings that convert a standard square frame. Canadian-owned, Windsor, Ontario.
How a curb inlet works with the grate
FHWA's HEC-22 urban drainage manual treats the combination inlet, a curb opening plus a grate, as the workhorse for sags and shallow grades, and recommends the curb opening sit upstream of the grate as a sweeper so it intercepts debris before the grate is blinded. In Ontario practice the same result comes from a standard OPSD square frame with a backarch casting forming the curb-face opening, or from a purpose-made side-inlet frame.
Curb inlet castings in the catalog
- Side inlet catchbasin, 2-piece cover — the cover splits so one section lifts for cleaning while the curb opening stays in place.
- Side inlet frame and cover, Ottawa 522/523 — the City of Ottawa pattern, supplied to Ottawa part numbers.
- Catchbasin backarch — the arched curb casting set behind a standard frame; it turns an existing dished or flat grate into a combination inlet.
- Side Inlet Catchbasin 2 Piece Cover · 400.081
- Side Inlet Catchbasin Frame and Cover (Ottawa Part Number 522/523) · 400.082
- Catchbasin Backarch · 400.090
Specifying a curb casting
- Name the curb profile: barrier, mountable, or rolled. The backarch must match it.
- Name the grate it pairs with (dished, flat, or V-type) and the OPSD pattern number.
- State the load rating for the travelled lane, normally AASHTO M306 / H-20. See load ratings explained.
- For sag locations, consider a two-piece side-inlet cover so crews can clean without pulling the whole frame.
All patterns above are part of the catch basin line; dimensions, open area, and weights are confirmed per SKU at quote.
Frequently asked questions
What is a curb inlet?
A curb inlet is an opening in the curb face that lets gutter flow enter the catch basin from the side, so water still gets in when leaves or trash blind the street grate. In Ontario practice it is built with a side-inlet frame or a backarch casting behind a standard square frame.
When should a side-inlet catch basin be specified?
At sag points, at the bottom of long grades, on streets with a ponding history, and anywhere heavy leaf drop is expected. FHWA's HEC-22 recommends curb openings in sags and on shallow grades, with the curb opening placed ahead of the grate as a sweeper.
Does the backarch have to match the curb profile?
Yes. The backarch forms the curb-face opening, so it must match barrier, mountable, or rolled curb. Confirm the curb profile with your order.
Is a curb casting the same as a curb inlet?
In tender language the two are used interchangeably for the cast iron piece that forms the curb-face opening. The catalog lists them as side-inlet frames and backarch castings.