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Re: Pouring a garage floor

Subject: Re: Pouring a garage floor
From: nick brearley <>
Date: Fri, 08 Nov 2002 20:56:12 +0000
At 10:27 08/11/02 EST, you wrote:
>My question is about a floor drain.  I would like to have one.  Can I dig a 
>hole, and make a French drain for it?  Install a 4" plastic pipe down
>How deep?  I have no experience with pouring a concrete floor!  (Yes, I know 
>a French drain is probably not code. But this is a hypothetical question, 
>regarding historical accuracy.  How's that?)  About how much lower should
>top of the drain pipe be than the edge of the floor (the long wall is 18 
>feet) to promote drainage?  How do you keep concrete from filling the drain 
>pipe when they are floating it?
>Also, I am just getting plain concrete, not fiberglass reinforced.  Is a
>of chicken wire good for strength, or is it not required for this small 
>Allen Hefner

Hi Allen,

It's worth considering some form of a gully trap for your floor drain then
lay a pipe from that to some point outside the shop where you can construct
the outfall soakaway. That way if you feel the need to install an oil trap
before discharging to ground at some time in the future  you don't have to
break up the shop floor. The gully trap is also useful because at some
future date you will drop a spanner or head bolt down the drain and it will
just sit in the trap waiting for you rather than disappearing down the
pipe. Lay your outfall pipe to a grade of  2 percent and you'll be alright.

In practice you would setup the gully at the lowest point in the centre of
the floor then screed down to it. As long as you plug it with cement bags
or a ball of polythene you needn't worry about concrete filling the drain,
just make some method of pulling the plug out at the end of the job.

If you're only going to have foot traffic I can't see the need for
reinforcing unless you are going to exert some very high point loadings.
1:2:4 mix, half or threequarter inch aggregate (half inch better for
floating), will give you a lifetimes floor. FWIW I've had good results
laying a polythene membrane under the concrete, particularly if the base is
very porous. It helps the concrete cure and provides a slip membrane for
future expansion/contraction. One of the list members runs (ran?) a
concrete plant so he can probably comment from a position of some authority
on this one.


Nick Brearley

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