Extending basement outside

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Posted by: from
1/21/2012 at 1:26:51 PM

I would like to increase the space of my basement by extending it outside the existing perimeter. My worry is the new created joint between the old concrete and the new concrete of the wall. What is the best practice to prevent groundwater seeping through the cold joint in the wall?


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Date/Time1/23/2012 at 8:59:14 PM

Hi George,

It would be best to include best practices when installing an exterior waterproof system in addition to proper installation of new weeping tiles. If you are unable to connect the new weeping system to the existing or the existing is not functioning properly, you may wish to consider a sump pump.

As it pertains to waterproofing the foundation, first fill any visible cracks with hydraulic cement, then, apply liquid rubber to the wall and finally apply a dimpled sheet air-gap membrane which not only provides you with a further line of defense, but it also creates a hydrostatic pressure relief which is how water forces it's way through your foundation wall in the first place.

This should extend beyond the new foundation past the new joint.


Kingsway Construction Inc

Glenn Rosborough

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Date/Time1/31/2012 at 9:06:14 PM

Hi there,

Yours is a very valid question.

While there are a lot of things to consider when tackling and addition, water penetration is the bane of everyone's existence.

Here is how to do it correctly:

1) When excavating for the addition, excavate an additional 4' past the joint.

2) BEFORE waterproofing (or damp proofing to minimum code requirement) ensure that you have exposed your existing exterior drainage pipe (weeping tile).

3) Dig in with a shovel or by hand about a foot around the drainage pipe (an excavator will tear it up and lift it).

4) Place (in most cases) 3/4 clear gravel to a couple of inches in depth in the excavation along the new (stripped) footings. Only to the underside of the existing drainage pipe and ensure that this new bed is sloped to drain the way it needs to!

5) CLEANLY cut the pipe and ensure that no dirt, gravel, stone, or silt has migrated into the pipe end and have a pipe cap (the blue plastic ones are best as it is easier to find the next morning).

6) THIS IS VERY IMPORTANT! Drainage pipe must be buried in gravel or stone and when you work or manipulate the drainage pipe, gravel fills in any gaps when the pipe is lifted. This will cause a dam effect if it is not addressed before connecting to the new pipe! MAKE SURE that the pipe maintains a continuous slope to the storm connection or sump pit! Even a slight hump will cause problems particularly at the joint.

7) Once the connection is made lay the new pipe into the excavation alongside the new footings. This stuff is difficult to straighten out, so have help. Then spike the pipe with long spikes meant for this purpose. This will ensure that you have no kinks and more importantly that there are no "hills or valleys" to dam water.

8) If your new drainage pipe is being connected to a new sump system you will have to account for the penetrations through the footings. Make sure that these are not high spots, essentially creating a reservoir right outside your new basement!

9) Place more 3/4 clear gravel on top of the pipe and then stop and fold the landscape filter fabric over that lift of gravel and continue to place at least 2' of additional gravel on top (3/4 gravel is "non-compacting" this is important so as not to crush your new drainage pipe).

10) Waterproof the foundation to 4' past the joint (not peel and stick) spray on rubber is best

I'm out of characters so good luck.

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