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Community and Q&A

Structural considerations when cutting into a CMU foundation wall

marleyandbowie | Posted in General Questions on

Hey everyone. I’d like to cut into our cinder block foundation in a few places to enable access to our three crawl spaces. I have attached some photos of the two areas where I would like to create large enough entrances that I can move in and out without having to squeeze through a tiny hole. As you can see, there is already a small entrance in one of the walls but I need it to be bigger. I have outlined in red where the entrances would be. Should I hire a structural engineer and a mason or other specific contractor? Or is this something that I could carefully do myself or with the help of a carpenter? I’m guess this is beyond DIY but just wanted to see what the community had to say…

Link to photos:


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  1. user-5946022 | | #1

    Door 1: What is on the other side of this wall? It does not appear to be an exterior wall because there is a duct going through it. There appears to be a water infiltration problem as you have water at the bottom of that wall. What is at the top of this wall? It sort of appears to stop, but I would guess this CMU wall is supporting something part of the structure above and or has some other function. In that case cutting a hole in that CMU is rather more complicated. It can be done but you must install a header to carry the load across the top of your hole where there will no longer be CMU, and you have to make sure the remaining CMU on either side can carry the additional load.

    2. Door 2. In this case you are proposing to expand the existing access DOWN ie for those who did not go to the photos, you are proposing to cut below the current sill. Again, what is on the other side of this wall? This entrance may be easier to deal with, depending on what is on the other side of that wall. You need to maintain the structural integrity of the remaining CMU. I would be inclined to install vertical resteel in the cells closest to the new jambs, then pour the cells closest to the new jamb solid, and form up an enclosure where I busted the CMU to get the reinforcing in. I would do one jamb at a time and let it cure. But it is all dependent on what is on the other side of that wall.

    1. marleyandbowie | | #3

      Thanks for the thoughtful reply, CL. "What's on the other side of Door #1?" Sounds like a line out of fun yet terrifying gameshow about buying old houses. Yes, behind door #1 is an INACCESSIBLE CRAWLSPACE. Yes, you heard that right. Whatever idiot(s) built this house decided it was a good idea to add a ~500 square foot poured crawlspace addition to an existing CMU foundation but instead of expand the basement they built it on a very shallow crawlspace and sealed it up with absolutely no access from anywhere in the house. I am dying to gain access to this space because we're getting water, weird smells and rodents in this space. I may have to rip up the floors above the space anyways but either way I want permanent access (obviously) to every foundational component of the house.

      What's behind door #2? Lucky for me this is an accessible crawlspace but I'd like a bigger door to be able to properly clean it out and keep it healthy moving forward.

  2. Expert Member
    NICK KEENAN | | #2

    The correct expert to talk to is a structural engineer. However, an experienced mason could probably look at it and tell you whether the wall is holding up anything other than itself, and what kind of reinforcement is needed over the opening to make it as strong as the wall it replaces. It's kind of what they do all day.

    1. marleyandbowie | | #4

      Good advice DCC, thank you

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