15 years ago we added an extension to the old cinder block foundation which is now 60 years old. The new extension is also cinder block. Where the old and new meet, we have a leak. I put hydraulic cement in the inside seam (first picture) but that did not stop the leak. Ideally I'd just dig up the outside and waterproof the wall or pour cement on the flat earth to seal it, but my three A/C units sit there and I was quoted $2,000 to move then re-attach the units (this is New York so it would be expensive). The second picture shows where the new and old foundations meet, at the end of the siding (new foundation continues the wall). I've tried to grade the slope with gravel which didn't help.

I'm thinking about chiseling away the inside seam this time and forcing cement into the bigger crack. My concern is that since both walls are cinder block, the water is inside the block and unless I fill the block completely, chiseling will make the problem worse.

Any suggestions? Thanks in advance




  • 3
    If by "grade the slope with gravel" you mean that loose rock around the A/C units, that will make NO difference to water flow. You need the soil under that to slope away from the house, then you can put that back as a decorative top layer. Any proper gravel is far too permeable to water to do a very good job of solving a drainage grading problem simply by dumping it on top of the problem area.
    – Ecnerwal
    Apr 25, 2022 at 1:17

2 Answers 2


Air conditioning condensers aren't tremendously heavy (likely under 200 pounds each) and the plumbing likely has a small amount of give in it. You can probably raise the condensers an inch or two off the ground. They could be supported in the raised position with boards or planks beneath, or suspended from above with straps hanging from a temporary scaffolding.

With the condensers raised you'd be able to adjust the grade of the soil, excavate a little more for better access to the joint in the wall, pour a concrete slab beneath the condensers, etc.

I note that there's a down spout attached to some kind of drain pipe in one of the photos. One presumes the drain is doing its job, but -- have you verified it? One reliable way of stopping a leaking foundation is to get rid of the water before it reaches the foundation. Especially if the water leak occurs during or following rain, have a look at whether the gutters and drain are doing their part to move water away from the foundation.

  • All that black tape on the down spout indicates it probably isn't doing it's job properly. That screams "band-aid!" to me. That'd be the first thing I'd check. That and making sure the underground portion hasn't cracked/had roots growing into it, and is properly running downhill to wherever it goes.
    – FreeMan
    Apr 25, 2022 at 12:01
  • Both good suggestions, Greg and FreeMan. The underground part of the down spout has very low grade (maybe 2%) and the tape does not leak except for a minor drip. How can I tell if it's cracked underground? Thanks
    – Adammmmm
    Apr 25, 2022 at 14:54
  • Buy some "non-toxic septic dye" (aka water tracing dye) and apply to your gutter/downspout. If you get color in the basement, there's a problem involving the downspout leaking into the basement.
    – Ecnerwal
    Apr 25, 2022 at 15:08

For the inside basement end of things, I'd scrub the surface down with a grooved silicon carbide block to get the sticking-up bits of previously applied patches off, and apply a cove corner (rounded radius fill) of mortar or hydraulic cement, limited by that inconveniently located pipe, then apply a masonry-waterproofer (thick paint-like substance) to the entire area well beyond the patch.

It's not guaranteed to work, as it still on the wrong side, but it's also not messing with the structural integrity of your cinderblock walls by chiseling into them, and sometimes it will work better than expected for long enough to be worth it. I've got one such patch going on 20 years which I put on fully expecting failure in a year or two, but outside access was impractical. It's still holding.

  • to keep it simple, could I use a vertical, narrow, flat board against the corner (like the hypotenuse of a triangle) and then fill in with hydraulic cement? What is the benefit of a concave radius fill in this case (newbie here)?
    – Adammmmm
    Apr 25, 2022 at 17:43
  • I did as you suggested @ecnerwal and it did reduce the water signifcantly but not 100%. A contractor friend suggested applying Xypex mixed in mortar mix and applying it all over. Just did that a week ago and waiting for the next rains. First time I've heard of Xypex, very interesting stuff.
    – Adammmmm
    Jun 24, 2022 at 20:07

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