What type of plumbing to use under a concrete slab


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Old 01-19-06, 07:12 AM
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Question What type of plumbing to use under a concrete slab

I have done alot of plumbing but never anything quite like this. The slab is going to be 6" thick and needs drains, p-traps (for the showers) and vents plumbed in before I pour. The building will have two showers, two toilets, and many smaller drains and vents which I plan two run through the walls. For the main drain stub-ups I planed to use 3" and 4" abs and put insert plugs in for the pour, then I am going to use insert type flanges for the toilets and insert reducers to plumb in the showers and the rest of the fixtures. Am I on the right track here? Is one 4" main enough to handle two showers, two toilets, three sinks, and four washing machines? any help greatly appreciated!!
 
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Old 01-19-06, 09:10 AM
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handy666, Welcome to the DIY Forums.
Drains are sized by "fixture units". Each fixture has a number of these units designated. The total number of these units decides what size pipe you must use. Example: A typical bathroom with toilet, tub and sink equals 5 fixture units. (the total of these fixtures seperately would be 6 but many areas deduct one unit for a grouping of fixtures) If you are getting a permit to do this work, I would think you need plans to submit before doing the work. Best idea is to call the local inspection office and find out what their requirements are. Codes vary from area to area. Good luck.
 
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Old 01-19-06, 10:19 AM
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Originally Posted by handy666
Is one 4" main enough to handle two showers, two toilets, three sinks, and four washing machines?
As far as physical capacity it is plenty. Make sure you also have adequate venting capacity. If you have radon gas, also vent the cavity under the slab into your vent system. The four washers is curious. Is this residential?
If it is commercial, you might need 6" where two or three 4" lines are combined.
I would try to defer joining them until they exit the building and tie together for the sewer lateral or only where a cleanout is installed.
You don't want pipe larger than necessary because water flows much faster in a 3" pipe than in a 6" pipe. And you want that speed to assure that solids are washed away rather than settling out. Smaller lines are also easier to snake.
If you put in cleanouts, you won't need them. Omit them and some little fellow will flush 50 baby wipes with 500 bobby pins and maybe a few sponges and rags.
 
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Old 01-19-06, 05:59 PM
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thankyou for the advice all

This is not going to be inspected and I have alot of freedom in my design since I work for a tribal government agency, I think I will take your advice and run a 4" line for the toilets and showers and a 3" line for the washing machines and join them outside the building before they enter the septic tank , The building is a public laundry/ shower/restroom, So I guess it would be considered commercial. One more question, Why would radon gas venting be nessasary? I thought that was only for subterrain basements and crawlspaces. This building will be slab on grade. Again, A big thanx for the advice. I am sure I will be returning for more as I am becoming the foreman for alot of projects like this one.
 
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Old 01-19-06, 07:39 PM
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One 3" (12 fixture units) and one 4" (20 fixture units) sounds plenty adequate for future expansion. Be sure to make a good diagram and take pictures as a reference.


Why would radon gas venting be necessary? I thought that was only for subterrain basements and crawlspaces. This building will be slab on grade.
Radon, if present, is not stopped or dissipated by the few feet of ground cover that you did not remove. 3" perforated sewer & drain pipe in the 4" of course gravel under your slab and tied into your rain drain (not the septic) is a better way to vent in my opinion as radon is a very heavy gas and prefers to travel down hill.
 
 

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