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A couple of the concrete block in my basement wall are cracked (diagonal crack on the bottom right of the photo in the "unfinished" blocks).

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The house is a 1920s duplex in Washington DC. It originally, I think, had a two story sleeping porch which has since been fully enclosed. These cracks are under the former porch on the corner of the common wall and the outside. The outside wall has concrete steps so I cannot see if the crack goes all the way through. The grade is right about at the top of the extra row of blocks on the right.

This area we think was a former laundry room and may have been dug out and under pinned by the former owner. I can see two piers supporting the porch, and I am assuming there is a third pier somewhere near this wall.

Is this crack something to worry about or does it just need to be sealed? We are planning on turning the space into a mechanical room for a new HVAC system.

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  1. That looks like settlement cracks.
  2. Whether you should worry about it depends on several things:
    • what is that supporting?
    • Is it still settling?

This is 100 yr old construction, so there will have been settlement. The question is whether that settlement is done or you have a new problem. Water can be a source of new settlement, and it can become a new problem on old construction.

The pictures look like there is a water problem in that area so even if it isn't causing settlement issues, it will create a mold problem. Look for easy to spot, easy to fix water issues (gutters that need cleaning, gutters that have pulled away from the house, clogged downspouts, downspouts discharging near the house, poor grading around the foundation, etc.).

That corner is supporting something. A typical first step is to stick an indicator on the crack. You can get simple measuring tools like this that just lets you measure and record the size of the crack: https://www.amazon.com/CRACKMON-224R-Comparator-224R-01-Compliant/dp/B00K5HXJB0

A more reliable indicator is a simple gauge that is glued across the crack and shows the current alignment. For example: https://www.amazon.com/CRACKMON%C2%AE-4020A-Heavy-Duty-Building-Foundation/dp/B00QWFYA7G You leave it in place and check it every few weeks. If there is any additional movement, it is easy to see. You could just watch it. If there is no movement in say half a year, that indicates that it's old settlement and you don't need to worry about it.

Another possibility: if there are existing piers, this may be a problem that has already been fixed.

If you do see new movement, it would be important to take remedial steps. I would have a foundation repair specialist evaluate it (often no charge for an initial look and recommendations on how to proceed).

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In DC, especially with 1920's buildings, these kinds of cracks are basically inevitable.

Are they moving? Probably not much.

Are they leaking? Not according to your photos.

What would it take to fix it? Lots of disruption of the current integrity of that section and the tie-in to the rest of the foundation, and maybe cooperation and cost-sharing with neighbors (which is always fun....not).

Your question hits the nail on the head.

I believe you should seal it then see what happens.

If any problems develop then you can deal with them as they come.

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