What you describe is a "combined" sewer, but an 8" diameter sewer pipe is not sized for combined sewer operation.  It could be that the people who built the building routed the roof drains into the sewer when they shouldn't have. Storm runoff should not go into the sanitary sewer, and many municipalities with historic combined sewers try to separate them whenever they get the opportunity.

Designing for 2" of rain will help with frequent rainfall events (multiple storms a year), but won't help with infrequent events.  When designing for volume, you need to design for a sustained rainfall event (24 hours is typical) and if your detention system takes longer than 24 hours to drain, you might fall very short on a 2" design storm.  Your client will not be happy to spend all this money and still get flooded out every year.

I see you're listed as a structural engineer.  If possible, find a drainage engineer to look at the plans with you, including contours and near-by storm sewers. It shouldn't take long to see if it's viable to separate the storm water runoff from the sanitary sewer.

Is this a single story building?  If it is, you will stop the back up to the sanitary drains by installing a backwater valve upstream of where the storm connects to the combined sewer.  This will keep the water from going back to the sanitary system.  Of course, you won't be able to use those fixtures until the water drains.

But be careful in doing this.  If your storm piping is cast iron and using standard no hub couplings, these only hold about 10' of head.  If the water in the storm backs up past this - which it could if the backwater valve is installed, then your storm downspouts might leak - which is probably worse than getting back up on the floor.

Do what Francesca says. Get a drainage savy engineer involved. Call the City about the sanitary and available storm drains. Start looking at seperating the drains. You may have to spill onto the ground for a while.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

Thanks all for your responses. This is a one story building with a basement. As for contacting the City, well, that would be a little difficult and time consuming. Anyway, I am considering a check valve between the U-trap and the connection to the sewer line. Since all pipes are connected to the main drain in the basement, I’ll recommend leaching basins around the bldg such that the rain water can overflow into them once the check valve has stopped the flow due to full sewer pipe. I’ll also size the leaching basins for 2 to 5 year storm which means there will be occasional overflow into the parking and the street. It is obvious that we can not economically prevent flooding but this will be a definite improvement over no current protection.

You are in over your head, get the city involved prior to dumping storm water into the sewer or be liable for fines. In the US, EPA mandates the timetable to separate sewer flows from storm with monetary penalties, and the cities must both pay the penalties and build new sewers.  A clandestine connection to the sewer or plumbing modification is prohibited, (get a permit).

I'll second civilperson's statement.  You need to to talk to the city before you start these plans, no matter how difficult and time consuming it is.  Everything you are proposing affects the public and requires a permit.  You and your client are heading for trouble.

I understand your concern but perhaps I was not very clear. This is a 70+ year old bldg and the storm water from the flat roof and the parking already drain into the city sewer which is a combined system. The area is over developed and apparently during the heavy rain storm, the sewer system which is also conveying the storm water runoff does not allow for quick drainage of the subject site and causes some backup due to head difference. What I am proposing is to prevent the city sewer to cause a backup by installing a check valve. I will then have a few leaching pools for the roof water to overflow to. Once the check valve opens as a result of less head pressure on the sewer side, the bldg continues to drain into the city sewer as it has been doing for the past 70 years. Of course a city permit will be obtained.

Any modifications to the existing setup makes you liable and REQUIRES a permit. Everything said by kxa and myself still apply. New construction or existing facilities modifications are included in the law.  The sewer is not yours and you need the sewer's owner to give permission for your changes.  If the back up due to over pressure is a concern it is the city who should specify the remedy.

It sounds like the OP is proposing a check valve on his own sewer line, not the City line and this is a legitimate way to solve the problem.  I have seen this done quite frequently to prevent flooded basements.  However, I agree that a permit may still be needed.