[puzled]

I have a challenging problem. I have a ranch home with a completely internal atrium in the center that is surrounded by home on all four sides but exposed to the elements, the square footage of the atrium is approx. 700 sq ft. Currently there is an old deck over soil that I would like to change to pavers. My question is about water drainage. I have 3 downspouts in the atrium collecting roof water connected to an internal drain that goes under the home to the sewr. If I place pavers, I will need to drain the ground water too and connect into the same line as the downspouts. How do I know how much water the system can hold when heavy rain falls. The amount of roof that comes into the drain in the atrium is perhaps 2000 sq ft. So I would be adding about another 700 sq ft of surface water, for a total of 2700 sq ft going into one pipe. I would guess the diameter of the pipe going under my house to the sewr to be 6 or 8 inches.

How do I know if this will be possible? And what precautions do I need if there is a severe rainfall? Should I do pavers or just redo a deck so as not to add more water to the system?
Your professional expertise is much appreciated.

[EpicInterlock]

I've never actually encountered this challenge before, but common sense would tell me that the builder (of your house) would have compensated/accounted for the water run off from your roof and ground by putting in the right size pipe.
I would also think that whatever water goes into the atrium would be the same amount regardless of whether you use pavers or a deck. In fact, pavers may be a better option, because you would be able to direct all the water that enters the atrium, whether from the downspouts or from the ground, into the drain. This would prevent water from getting into your foundation and perhaps causing a water leak/mold in your house.
Now, if you find out the pipe isn't the right size, then you have other problems to worry about, like replacing all the piping, which would mean you'd have to go underneath the house.....I hope you don't need to do so.

Good luck

[agla]

I would not bank on common sense of builders anticipating a patio.

The correct way to calculate if the pipe can handle the increase in water is not so easy.

First you need to know how much water you are going to be dealing with. You can find that out by looking it up in the USDA Soil Survey bookk for your county which can be found in the library(you may find this info on line or in other publications as well). These rain events are described by expected frequency of occurance (ie, 25 year storm). Usually drainage like this is designed for a 25 year storm.

Some ways of designing drainage are difficult to learn and nearly impossible to describe, but here is a simple one that can get you by. The storm will be described in inches per hour and with a duration time. Take that number of inches multiply by duration. Multiply that number by .0833 (feet per inch). Multiply that by the area of surface that actually drains to the pipe. That gives you cubic feet (There are 7.48 gallons per cf) of water that you need to handle.

There are charts and slide rules that can helpyou out with calculating pipe size. That is also more complicated than most would think. Pitch of the pipe effects this as well.

My opinion is that you simplify the project by creating a detention well in the middle of your area that you can connect to the drain pipe. That would be a leach pit that can accept the first flush of water from the patio and slowly leach it into the soil (all the rainfall landed here before, but dispersed). The leach pit would ideally be able to hold the entire amount of patio runoff (probably about 600 gallons in a 25 year storm if you are in the northeast).

You have to realize that no more water is entering the courtyard. The only difference is that it is not absorbed evenly over the entire surface. A drywell (or leach pit) in the center should not cause any water problem with the basement, assuming there is one, than already exists.

If you tie the roof drains and the patio drain, you run the risk of some roof runoff backing up into the courtyard if the pipe can not drain quickly enough. That would put more water than you currently have into the courtyard. I'd keep the systems separate.

[Lanelle]

Great post agla!

If you go with the leach pit as agla suggests, you might consider doing permeable pavers such as Techo-Bloc Permea.
These actually look like a patio/driveway paver. You can use polymeric sand in the joints instead of #8 so it is nicer for foot traffic and still drains.

[Matt Kulp]

I second Lanelle with the permeable pavers. I would install over clean aggregate and install perforated pipe in the base under the pavers and tie into the sewer pipe.

Another security measure would be to install rubber liner against the foundation from finished excavation to the top of the base to prevent any water from entering the house foundation if you are running the pavers up to the foundation.