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French Drain Question

Hey folks!

Last year I bought a 70yr old house with a French drain installed in the basement. Both my parents and in-laws have had terrible water problems over the years so I was VERY happy to see it.
There are no real problems, but I am concerned because it is an older 'open back' design. There is a gap between the new concrete and the wall from 1 to almost 2 inches along the perimeter of the basement. It seems this would be nice for drainage, but I never get any water that would enter the system through the gap. I've done a little research lately and it seems that nearly all new systems are closed. This would prevent bugs and more importantly in my area - radon, from entering the basement.
Does anyone have DIY suggestions for filling this gap? Does it make sense? My inclination is hydraulic cement, but I am hoping someone else can either back me up or make another suggestion.
The basement will not be finished so the solution does not need to be elegant.
I appreciate any help you can offer and promise to offer up advice in the future (this is my 1st post, but I'm a HUGE fan of TOH). Thank you.

Re: French Drain Question

I have the same problem and am looking for the solution (sorry I can't help). I don't get water in my basement either since the french drain was installed but the 1-3 inch gap is ugly.

I also think that I get moisture (not water) coming into the room since it's not covered and in the winter it's really cold in the basement and I can't help but think that having this trench open lets cold ground air in.

Half of my basement was finished by the previous owner with a media room and full bath but the trench was not covered or closed. So whatever I decide to do will mean pulling some of the drywall down.

(BTW - We have had 23 days of rain and the sump is completely dry. The only time I have seen water in the sump was in the winter after a heavy snow melt.)

Any thoughts on if or how we should cover up this gap ????

Re: French Drain Question

Well considering that both your systems are working, any change you can call the company who installed the french drain?

Modern internal drain tiles come with a special baseboard that will look better than the ugly gap and will also prevent debris from falling into the drain and clogging it. As it is in the picture, there is nothing there preventing someone from accidentally sweeping pebbles and small objects into the gap.

Maybe the waterproofing company has some sort of upgrade that can be fitted in the system you have in place?

As I understand, most reputable companies installing these systems offer a lifetime warranty and service plan.
These systems need to be periodically maintained and checked.

Last resource: Install a thin plastic baseboard a few inches from the wall, hiding but not covering the gap.

Re: French Drain Question

Thanks Cyfree for your reply.

I don't know who did the drain work, I didn't own the house then....but I guess I could call any company and ask for ideas.

I found some interesting information here:


Basically reconfirming my initial thoughts that moisture and cold air are pouring in through this gap all round the perimeter of the basement floor. I've done a radon test and don't have an issue there. So no I have to come up with an idea to close this gap.

I found a product called Flexi Flange that seems can be used with or without a vapor barrier to close the gap.


If anyone else has a "open back" french drain and has sealed it up or closed it off I would love to hear how you did it.


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