# Sewer backup prevention Part 2 of 3

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One other danger of a sewer plug or standpipe is if you have clay tile pipes under the floor instead of cast iron. They are found in older houses (40 or 50 years or older). During a backup the joints will leak causing hydraulic pressure under the floor. The floor will try to float and could break up. As far as back pressure causing the pipe to rupture, it's not likely. The pressure, even from a 8' head, is not that great, still far less than 1 psi.
Artfd wrote:

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Art Todesco wrote:

Art beat me to it. <g> There is indeed danger of lifting the entire basement slab when a drain plug or a standpipe is used.
The joints in clay drain pipe are notoriously leaky and even a small backpressure will cause many substantial leaks under the slab. If you are there at the time of the backup, you can actually hear the joints spitting water.
Assume a slab which is 10' X 10'. That's 100 sq ft, which is 14,400 sq in. Now, lets say that there is only 1/2 PSI of pressure in the backed up sewer. If the leaky tile has flooded under the entire slab (quite possible), the force trying to lift the slab is over 7,000 Lbs !!! While that may not actually float the thing, it usually results in very nasty cracks.
(BTW, the pressure developed by a head of water is closer to 1/2 PSI per foot of head. So, 8 Ft of head will give you roughly 4 PSI.)
Jim
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Actually, I had it on my calculator but didn't post the number; I thought under 1 PSI was sufficient. But for the purists, it is 0.29 PSI for an 8' head, water, of course. But, as Speedy Jim points out, all those square inches add up real fast.
Speedy Jim wrote:

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Nah! <bg> You have a typo. It's 8 *inches* of water, not 8 feet, which produces .29 PSI.
8 foot of head gives 3.46 PSI.
See this instant chart: http://www.amerstain.net/reference/feetwater.html Jim
Art Todesco wrote:

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I'm still working on part 3 of 3. The references I found about standpipes in floor drains did mention the possibility of leakage under the slab due to leakage from the sewer pipe joints, including heaving of the floor. I noticed my floor drain is part of a cast iron pipe system & I can only assume the rest of the drain to the city sewer is the same material. I wonder how common basement floor heaving is versus the damage caused by sewage backflow, which is in our local media many, many times annually. The concrete basement floor must weigh several thousand pounds on its own. Other references advised not to use standpipes higher than 3 feet from the basement floor. The reasoning was that concrete block walls that make up most basements can endure up to 3 feet on their outside surface, above that level, they tend to cave in, which could result in a collapsed house. So, it would be better to allow sewage to pour into one's basement when the backup pressure exceeds 3 feet of water (and equalize the pressure from inside to outside) rather than have the basement walls collapse. I'm a doctor, not an engineer, but that makes sense. This is the reasoning that also supports the gradual pumping-out of a totally flooded basement, since too-rapid draining could also cause the basement walls to cave in. If the basement is <= 3 feet of water in it, it's OK to pump it out as fast as possible. My 3 foot standpipe would, if filled, mean a back pressure of 1.3 psi, per your reference. for a 100 sq. ft. slab, the underpressure could be as much as 18,720 lb. If my concrete floor weighs 145 lb/cu ft and is 3 inches thick, a 100 sq ft slab weighs 3625 lb. When I learned that the Flood-Guard wouldn't fit my cast iron floor drain, I used a standpipe instead: Home Depot has variety of rubber donuts one of which fitted precisely into the bell of my cast iron floor drain, and allowed me to leave the iron grate inside the mechanism to keep solids out of the drain. The inner diameter of the donut is a precise fit for 4" Sch. 40 PVC DWV, which is gently pounded in with the help of a 2 x 4 and a hammer. The fit is very tight, appears leakproof, and the mechanism can be disassembled for servicing by wiggling it free. My washing machine drain hose hooks over the top of the standpipe, and is held in place with wire to prevent it from whipping free. I understand some modern washing machines drain themselves extremely rapidly, but mine isn't one of those. The cost of my setup was \$11. Now a Flood-guard would fit in the PVC pipe, but the structure would start to resemble a water-propelled rocket, so I will leave my standpipe as is. I had to disconnect my laundry tray to install the standpipe, but it was a small price to pay for peace of mind. I am considering reattaching the laundry tray drain to the standpipe using a Y of some sort and a rubber sleeve, and putting a gate valve into the laundry drain, which would be left normally closed unless the tray was actually in use. The function of the standpipe would be unchanged. I could also install a laundry tray electric sump system to pump the tray out to the top of the standpipe, or even to a higher level, HD's price was \$165 and a 4-week wait for the special order. The other two items that used to go into the floor drain are the central A/C condensate, and the basement dehumidifier condensate, but my plumbing inspector told me it was OK to drain these into the sump pit for pumping into the street. I understand there are small pumps available to put the A/C condensate anywhere desired, but gravity is OK for me, and the sump pit will be very close to where the floor drain/standpipe is now.

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Water will take the path of least resistance.
Its not gonna raise the basement floor. Its just gonna go somewhere else.
Have a nice week...
Trent
Dyslexics of the world ... UNTIE !
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